Testing for SQL Injection (OTG-INPVAL-005)

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Brief Summary
A SQL injection attack consists of insertion or "injection" of an 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 shutdown the DBMS), recover the content of a given file present on the DBMS filesystem and in some cases issue commands to the operating system. For an introduction to SQL Injection, please refer to the references at the bottom of the page.

Description of the Issue
SQL Injection attacks can be divided in the following three classes: Independently from the attack class, in order to perform a SQL Injection attack it is necessary to craft a syntactically correct SQL Query. If the application returns the 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 case is known as "Blind SQL Injection".
 * 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: there is no actual transfer of data, but the tester is able to reconstruct the information by sending particular requests and observing the resulting behaviour of the DB Server

SQL Injection Detection
The first step in this test is to understand when our 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: 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. The very first test usually consists in 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 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 comments (--) 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 A full error message like the ones 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.
 * Authentication forms: when the authentication is performed with 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, ...) are very likely to be stored in a relational database

Standard Sql Injection Testing
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 credentials exists, then the user is allowed to login to the system, otherwise the access is denied. The values of the input fields are inserted from the user generally through a web form. We suppose to 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 return 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 parenthesis and one due to the use of MD5 hash function. First of all we resolve the problem of the parenthesis. That simply consist of adding a number of closing parenthesis 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 as a comment. Every DBMS has the own symbols of comment, however a common symbol to the greater part of the database is /*. In Oracle the symbol is "--". Saying this, 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')))

The url request will be:

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

Which return a number of values. Sometimes, the authentication code verifies that the number of returned tuple 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 to 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 command "LIMIT ", where is the number of the tuples that we expect to be returned. The value of the fields Username and Password regarding the previous example will be modified according the following:

$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

Union Query Sql Injection Testing
Another test to carry out, involves the use of the UNION operation. Through such operation it is possible, in case of Sql Injection, to join a query, purposely forged from 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 fields of other tables. We 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 CreditCarTable

We will have the following query:

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

which will join the result of the original query with all the credit card users. The keyword ALL is necessary to get around the query that make use of 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 query must have an equal number of parameters, in order to avoid a syntax error.

Blind Sql Injection Testing
We have pointed out that exists another category of sql injection, called Blind Sql Injection, in which nothing is known on the outcome of an operation. This behavior happens in cases where the programmer has created a customed error page that does not reveal anything on the structure of the query or on the database. (Does not return a SQL error, it may just return a HTTP 500).  Thanks to the inference methods it is possible to avoid this obstacle and thus to succeed to recover the values of some desired fields. The method consists in carrying out a series of booloean 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 vulnerable to sql injection of name id. 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 is to obtain 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 practically in every database. For our examples we will use the following pseudo-functions:

SUBSTRING (text, start, length): it 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 length in characters of the input text.

Through such functions we will execute our tests on the first character and, when we will 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 time (selecting a single character means to impose the length parameter to 1) and 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 ASCII table, until finding the desired value. As an example we will insert 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 returns a result if and only if the first character of field username is equal to the ASCII value 97. If we get a false value then we increase the index of 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 table and we pass to analyze the next character, modifying the parameters of SUBSTRING function. The problem is to understand in that way we distinguish the test that has carried a true value, from the one that has carried a false value. In order to make this we create a query that we are sure returns a false value. This is possible by the following value as field Id:

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

by which will create the following query:

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

The answer of the server obtained (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 exposed before. Sometimes this method does not work. In the case the server returns two defferent 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 template in order to decide the result of the test. In the previous tests, we are supposed to know in what way it is possible to understand when we have ended the inference beacause we have obtained the value. In order to understand when we have ended, we will use one characteristic of the SUBSTRING function and the LENGTH function. When our test will return a true value and we would have used an ASCII code equals to 0 (that is the value null), then that mean that we have ended to make inference, or that the value we have analyzed effectively contains the value null.

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 with now (excluded the null value). The query will be:

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

that gives back a true or false value. If we have a true value, then we have ended to make inference and therefore we have gained the value of the parameter. If we obtain a false value, this means that the null character is present on the value of the parameter, then we must continue to analyze the next parameter until we will 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. A simple tool which performs this task, via GET requests on MySql DB is SqlDumper, is shown below.