Overview
XPath Injection, akin to other common injection attacks, specifically targets vulnerabilities within an application’s user input processing system. But what sets XPath Injection apart is its exploitation of XPath queries. The fallout? Unauthorized access to data, sidestepping of authentication measures, and in some instances, remote code execution.
What is XPath?
Essentially, XPath stands for XML Path Language. It’s a language that facilitates the selection of nodes from an XML document. When we dive into the realm of web applications, we often encounter XPath being used to query XML databases or XML files. The peril arises when an attacker manages to control any part of the XPath query that’s transmitted to the server. This control, in essence, is what we term as XPath Injection.
To draw a parallel, SQL Injection specifically targets SQL queries. Similarly, XPath Injection attempts to skew the query to the attacker’s advantage.
A Step-by-Step Example
Let’s consider a web application that uses XPath to authenticate users based on an XML user database.
XML User Database
<users>
<user>
<username>jeremy</username>
<password>cheesecake</password>
</user>
<user>
<username>jessamy</username>
<password>tiramisu</password>
</user>
</users>Vulnerable XPath Query
The application might use an XPath expression like the following to authenticate users:
/users/user[username/text()='<INPUT_USERNAME>' and password/text()='<INPUT_PASSWORD>']Attack Scenario
- Normal Login: A user inputs their username and password, and the application forms the query. E.g., with `jeremy` and `cheesecake`, the query becomes:
/users/user[username/text()='jeremy' and password/text()='cheesecake']- Injection Attack: An attacker inputs `jeremy’ or ‘1’=’1` as the username and leaves the password field blank. The query becomes:
/users/user[username/text()='jeremy' or '1'='1' and password/text()='']This altered query returns true for any user, allowing the attacker to bypass authentication.
Ways to Detect XPath Injection
Manual Testing
- Deciphering Error Messages: By strategically placing special characters like single quotes and subsequently analyzing the resultant error messages, one can pinpoint application vulnerabilities.
- Behavior Analysis: Deploying a range of payloads and observing the application’s response can be illuminating.
Automated Testing
- Fuzzing: Tools such as FFUF, when combined with prevalent payloads, can aid in detecting XPath injection.
- Scanners: A plethora of security tools have the capability to auto-detect XPath Injection loopholes.
- Code Review: Both manual and automated static code analysis can spotlight insecure code patterns, flagging potential XPath Injection risks.
Common Payloads
Here’s a list of payloads you may use to test for XPath Injection vulnerabilities:
`' or '1'='1`
`' or ''='`
`'] | //user/*`
`'] | //user/*[contains(text(),'admin')]`Mitigating XPath Injection
Guarding an application against XPath Injection attacks is paramount. Embracing robust input validation, opting for parameterized XPath queries, enforcing least privilege access, and timely software updates stand out as some of the quintessential best practices.
Conclusion
XPath Injection poses a formidable threat to web applications, particularly when developers overlook the significance of input validation and secure coding. By understanding its mechanics, recognizing its signs, and implementing safeguards, developers can create a robust defense line against potential attackers, ensuring data integrity and user trust.
About the Author: Alex Olsen
Alex is a Web Application Security specialist with experience working across multiple sectors, from single-developer applications all the way up to enterprise web apps with tens of millions of users. He enjoys building applications almost as much as breaking them and has spent many years supporting the shift-left movement by teaching developers, infrastructure engineers, architects, and anyone who would listen about cybersecurity. He created many of the web hacking courses in TCM Security Academy, as well as the PWPA and PWPP certifications.
Alex holds a Master’s Degree in Computing, as well as the PNPT, CEH, and OSCP certifications.
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