Difference between revisions of "Blind XPath Injection"

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{{Template:Attack}}
 
{{Template:Attack}}
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[[Category:OWASP ASDR Project]]
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Last revision (mm/dd/yy): '''{{REVISIONMONTH}}/{{REVISIONDAY}}/{{REVISIONYEAR}}'''
  
 
==Description==
 
==Description==
  
XPath is a sort of query language that describes how to locate specific elements (including attributes, processing instructions,
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XPath is a type of query language that describes how to locate specific elements (including attributes, processing instructions,
etc.) in an XML document. Since it is a query language, XPath is somewhat similar to Structured Query Language (SQL). However, XPath
+
etc.) in an XML document. Since it is a query language, XPath is somewhat similar to Structured Query Language (SQL), however, XPath is different in that it
can be used to reference almost any part of any XML document without access control restrictions, whereas with SQL, a "user" (which is a term undefined in the XPath/XML context) may be restricted to certain tables, columns or queries.
+
can be used to reference almost any part of an XML document without access control restrictions. In SQL, a "user" (which is a term undefined in the XPath/XML context) may be restricted to certain databases, tables, columns, or queries. Using an XPATH Injection attack, an attacker is able to modify the XPATH query to perform an action of his choosing.
  
 +
Blind XPath Injection attacks can be used to extract data from an application that embeds user supplied data in an unsafe way. When input is not properly sanitized, an attacker can supply valid XPath code that is executed. This type of attack is used in situations where the attacker has no knowledge about the structure of the XML document, or perhaps error message are suppressed, and is only able to pull once piece of information at a time by asking true/false questions(booleanized queries), much like [[Blind SQL Injection]].
  
More information may be found in the article dedicated to [[XPATH Injection]]. Conducting Blind XPath Injection attack the attacker has no knowledge about the structure of the XML document. However his situation is better comparing to [[Blind_SQL_Injection]], because there are functions, which allows for performing tests (XML Crawling) and in the end getting know the document structure.
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For more information, please see the article on regular [[XPATH Injection]].
  
==Examples ==
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==Risk Factors==
  
Using [[XPATH_Injection]] attack the attacker is able to e.g. log in to the system without entering valid login and password. If he wants to know information about other users he must take one step further. The attacker may be successfull using two methods: Boolenization and XML Crawling. By adding to the XPath syntax, which allows to log in without entering a login and password, additional expressions (replacing what the attacker entered in the place of login to the specially crafted expression).
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TBD
  
'''Boolenization'''
 
  
Using so called "Boolenization" method the attacker may find out if the given XPath expression is True or False. Let's assume that the
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==Examples==
aim of the attacker is to log in to the account. Successfull log in would be equal "True" and failed log in attempt would equals "False". Only a smart portion of the information is analyzed "character" or the number. When the attacker focuses on the string he may reveal it in its entirety by checking every single character within the class/range of characters this string belongs to.
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The attacker may mount a successful attack using two methods: Boolenization and XML Crawling. By adding to the XPath syntax, the attacker uses additional expressions (replacing what the attacker entered in the place of the injection).
  
 +
===Boolenization===
 +
Using the "Boolenization" method the attacker may find out if the given XPath expression is True or False. Let's assume that the aim of the attacker is to log in to an account in a web application. A Successful log in would return "True" and failed log in attempt would return "False". Only a small portion of the information is targeted via the analyzed character or number. When the attacker focuses on a string he may reveal it in its entirety by checking every single character within the class/range of characters this string belongs to.
  
Using ''string-length(S)'' function, where S is a string, the attacker may find out the length of this string. With the appropriate number of ''substring(S,N,1)'' function iterations, where S is a previously mentioned string, N is a start character, and "1" is a next character counting from N character, the attacker is able to find out the whole string.
 
  
  
Example:
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Using a ''string-length(S)'' function, where S is a string, the attacker may find out the length of this string. With the appropriate number of ''substring(S,N,1)'' function iterations, where S is a previously mentioned string, N is a start character, and "1" is a next character counting from N character, the attacker is able to enumerate the whole string.
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Code:
 
<pre>
 
<pre>
 
<?xml version="1.0" encoding="UTF-8"?>
 
<?xml version="1.0" encoding="UTF-8"?>
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* ''substring((//user[position()=1]/child::node()[position()=2),1,1)'' returns the first character of this user ('r').
 
* ''substring((//user[position()=1]/child::node()[position()=2),1,1)'' returns the first character of this user ('r').
  
 
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===XML Crawling===
'''XML Crawling'''
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To get to know the XML document structure the attacker may use:
 
To get to know the XML document structure the attacker may use:
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count(//user/child::node()
 
count(//user/child::node()
 
</pre>
 
</pre>
Will return the number of nodes (in this case 2).
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This will return the number of nodes (in this case 2).
  
 
* stringlength(string)
 
* stringlength(string)
Line 68: Line 76:
  
  
If the log in form would look like that:
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If the log in form would look like this:
  
 
C#:
 
C#:
Line 81: Line 89:
 
</pre>
 
</pre>
  
The XPath syntax may remind common [[SQL_Injection]] attacks but the attacker must consider, that this language disallows commenting
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The XPath syntax may remind you of common [[SQL Injection]] attacks but the attacker must consider that this language disallows commenting
out the rest of expresssion. To ommit this limitation the attacker should use OR expressions to void all expressions, which may disrupt the attack.
+
out the rest of expresssion. To omit this limitation the attacker should use OR expressions to void all expressions, which may disrupt the attack.
  
Because of ''Boolenization'' the number of queries, even within a small XML document, may be very high (thousands, houndred of thousands and more). That is why this attack is not conducted manually. Knowing few basic XPath functions the attacker is able to write an application in a short time, which will rebuild the structure of the document and will fill it with a data by itself.
+
Because of ''Boolenization'' the number of queries, even within a small XML document, may be very high (thousands, houndred of thousands and more). That is why this attack is not conducted manually. Knowing a few basic XPath functions, the attacker is able to write an application in a short time which will rebuild the structure of the document and will fill it with data by itself.
  
References:
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==Related [[Threat Agents]]==
 
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TBD
* http://www.modsecurity.org/archive/amit/blind-xpath-injection.pdf - by Amit Klein (much more detailes, in my opinion the best source about Blind XPath Injection).
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* http://www.ibm.com/developerworks/xml/library/x-xpathinjection.html
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==Related Threats==
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* [[:Category:Command_Execution]]
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==Related Attacks==
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 +
==Related [[Attacks]]==
 
* [[Blind_SQL_Injection]]
 
* [[Blind_SQL_Injection]]
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* [[XPATH_Injection]]
  
==Related Vulnerabilities==
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==Related [[Vulnerabilities]]==
 
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* [[Injection_problem]]
 
* [[Injection_problem]]
  
==Related Countermeasures==
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==Related [[Controls]]==
 
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* [[:Category:Input Validation]]
 
* [[:Category:Input Validation]]
  
More in the article [[XPATH_Injection]]
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==References==
 
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* http://dl.packetstormsecurity.net/papers/bypass/Blind_XPath_Injection_20040518.pdf - by Amit Klein (much more detailes, in my opinion the best source about Blind XPath Injection).
==Categories==
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* http://www.ibm.com/developerworks/xml/library/x-xpathinjection/index.html
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* http://projects.webappsec.org/w/page/13247005/XPath%20Injection
  
 
[[Category:Injection]]
 
[[Category:Injection]]
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[[Category:Attack]]

Latest revision as of 21:17, 30 May 2013

This is an Attack. To view all attacks, please see the Attack Category page.




Last revision (mm/dd/yy): 05/30/2013

Description

XPath is a type of query language that describes how to locate specific elements (including attributes, processing instructions, etc.) in an XML document. Since it is a query language, XPath is somewhat similar to Structured Query Language (SQL), however, XPath is different in that it can be used to reference almost any part of an XML document without access control restrictions. In SQL, a "user" (which is a term undefined in the XPath/XML context) may be restricted to certain databases, tables, columns, or queries. Using an XPATH Injection attack, an attacker is able to modify the XPATH query to perform an action of his choosing.

Blind XPath Injection attacks can be used to extract data from an application that embeds user supplied data in an unsafe way. When input is not properly sanitized, an attacker can supply valid XPath code that is executed. This type of attack is used in situations where the attacker has no knowledge about the structure of the XML document, or perhaps error message are suppressed, and is only able to pull once piece of information at a time by asking true/false questions(booleanized queries), much like Blind SQL Injection.

For more information, please see the article on regular XPATH Injection.

Risk Factors

TBD


Examples

The attacker may mount a successful attack using two methods: Boolenization and XML Crawling. By adding to the XPath syntax, the attacker uses additional expressions (replacing what the attacker entered in the place of the injection).

Boolenization

Using the "Boolenization" method the attacker may find out if the given XPath expression is True or False. Let's assume that the aim of the attacker is to log in to an account in a web application. A Successful log in would return "True" and failed log in attempt would return "False". Only a small portion of the information is targeted via the analyzed character or number. When the attacker focuses on a string he may reveal it in its entirety by checking every single character within the class/range of characters this string belongs to.


Using a string-length(S) function, where S is a string, the attacker may find out the length of this string. With the appropriate number of substring(S,N,1) function iterations, where S is a previously mentioned string, N is a start character, and "1" is a next character counting from N character, the attacker is able to enumerate the whole string.


Code:

<?xml version="1.0" encoding="UTF-8"?>
<data>
   <user>
   <login>admin</login>
   <password>test</password>
   <realname>SuperUser</realname>
   </user>
   <user>
   <login>rezos</login>
   <password>rezos123</password>
   <realname>Simple User</realname>
   </user>
</data>

Function:

  • string.stringlength(//user[position()=1]/child::node()[position()=2]) returns the length of the second string of the first user (8),
  • substring((//user[position()=1]/child::node()[position()=2),1,1) returns the first character of this user ('r').

XML Crawling

To get to know the XML document structure the attacker may use:

  • count(expression)
count(//user/child::node()

This will return the number of nodes (in this case 2).

  • stringlength(string)
string-length(//user[position()=1]/child::node()[position()=2])=6

Using this query the attacker will find out if the second string (password) of the first node (user 'admin') consists of 6 characters.

  • substring(string, number, number)
substring((//user[position()=1]/child::node()[position()=2]),1,1)="a"

This query will confirm (True) or deny (False) that the first character of the user ('admin') password is an "a" character.


If the log in form would look like this:

C#:

String FindUser;
FindUser = "//user[login/text()='" + Request("Username") + "' And
      password/text()='" + Request("Password") + "']";

then the attacker should inject the following code:

Username: ' or substring((//user[position()=1]/child::node()[position()=2]),1,1)="a" or ''='

The XPath syntax may remind you of common SQL Injection attacks but the attacker must consider that this language disallows commenting out the rest of expresssion. To omit this limitation the attacker should use OR expressions to void all expressions, which may disrupt the attack.

Because of Boolenization the number of queries, even within a small XML document, may be very high (thousands, houndred of thousands and more). That is why this attack is not conducted manually. Knowing a few basic XPath functions, the attacker is able to write an application in a short time which will rebuild the structure of the document and will fill it with data by itself.

Related Threat Agents

TBD

Related Attacks

Related Vulnerabilities

Related Controls

References