Web Application Defender's Cookbook

Recipe 9-9: Preventing XML Attacks

This recipe shows how you can tell when attackers are attempting to send injection attacks within XML payloads.

Ingredients

• OWASP ModSecurity Core Rule Set (CRS)
• ModSecurity
  • XML variable
  • @validateSchema operator
  • @rx operator

CAPEC-83: XPath Injection

An attacker can craft special user-controllable input consisting of XPath expressions to inject the XML database and bypass authentication or glean information that he normally would not be able to. XPath Injection enables an attacker to talk directly to the XML database, thus bypassing the application completely. XPath Injection results from the failure of an application to properly sanitize input used as part of dynamic XPath expressions used to query an XML database.

http://urecapec.mitre.org/data/definitions/83.html

Sample Attacks

Let’s look at a web service that sends XML data in POST requests:

POST /axis/getBalance.jws HTTP/1.0 Content-Type: text/xml; charset=utf-8 SOAPAction: "" Content-Length: 576 Expect: 100-continue Host: www.bluebank.example.com <?xml version="1.0" encoding="utf-8"?> <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/ " xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/" xmlns:tns="http://www.bluebank.example.com/axis/getBalance.jws" xmlns:types=" http://www.bluebank.example.com/axis/getBalance.jws/encodedTypes" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <soap:Body soap:encodingStyle="http://schemas.xmlsoap.org/ ...