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XML Injection

Description:

XML Injection is an attack technique used to manipulate or compromise the logic of an XML application or service. The injection of unintended XML content and/or structures into an XML message can alter the intend logic of the application. Further, XML injection can cause the insertion of malicious content into the resulting message/document.


How to Exploit:

Inserting hacker@evil.com</Email><UniqueID>0</UniqueID><Email>hacker@evil.com in Email field will yield the following result:

<UserRecord>
<UniqueID>123</UniqueID>
<Name>Henry Ackerman</Name><Email>hacker@evil.com</Email><UniqueID>0</UniqueID><Email>hacker@evil.com&lt;/Email>
<Address>123 Disk Drive</Address>
<ZipCode>98103</ZipCode>
<PhoneNumber>206-123-4567</PhoneNumber>
</UserRecord>

This will add more one record in XML database with UniqueID=0.


One Live Example:

In order of doing a Web Application Server Assessment we came across something which was vulnerable to XML Injection. We found that the input field was not properly validating the input given by the user. That means there was no XML validation on part of contents and length of the input supplied by the user. The application didn’t have XML Schema (xsd file) against which the XML was being validated.

The application had one admin module where User management function was implemented. The admin can add user, delete user, manage its profile and assign roles and privileges to them.


We entered valid user information in all the respective filed in order to add a user:



We captured the request in proxy. All the data in respective fields were going into XML format as you can see in the here:


We inserted a valid XML payload confirming to the all the required fields required to add a valid user and that adheres to the valid XML structure. You can see the highlighted payload which is being inserted into ‘CustomerView’ filed of the XML:



Upon forwarding the result the application successfully accepts the request and instead of creating a user in the name of ‘validuser’ the application creates another user in the name of ‘hackman’:



Thus an attacker acting as Man in the Middle attack will successfully create a user of his choice and login into the application in unauthorized way.


Countermeasures:


• Do not trust client input.

• Validate input: length, range, format, and type.

• Validate XML streams.

• Constrain, reject, and sanitize input.

• Encode output.

• Restrict the size, length, and depth of parsed XML messages.

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