How to remediate – XPath Injection

1. Introduction

XPath Injection is a web security vulnerability that allows attackers to interfere with XML Path Language queries used by applications. This can lead to unauthorised access to data, including user information and potentially privileged credentials. Web applications using XML to store or process data are usually affected. A successful attack could compromise confidentiality of sensitive data, integrity of application logic, and availability if critical resources are targeted.

2. Technical Explanation

XPath Injection occurs when untrusted data is incorporated into XPath queries without proper sanitisation. This allows attackers to modify the query’s behaviour, potentially retrieving unintended information or performing actions they shouldn’t be able to. The scanner detected this by injecting special characters and observing server responses. There is no specific CVE associated with this generic vulnerability class.

• Root cause: Lack of input validation when constructing XPath queries from user-supplied data.
• Exploit mechanism: An attacker could submit a malicious string as part of an authentication request, altering the XPath query to bypass login checks or access other users’ data. For example, submitting ‘//user[username=’admin’ and password=’password’]’ might allow access without correct credentials if not properly handled.
• Scope: Web applications using XML for data storage or processing are affected. Specific versions depend on the application framework used.

3. Detection and Assessment

Confirming a vulnerability requires checking how user input is processed when constructing XPath queries. A quick check involves examining the application’s source code, if accessible. Thorough assessment uses automated scanners.

• Quick checks: Review application code for direct use of user-supplied data in XPath query construction without sanitisation or parameterisation.
• Scanning: Burp Suite and OWASP ZAP can identify potential XPath injection points using active scanning techniques. These are examples only, as accuracy varies.
• Logs and evidence: Examine web server logs for unusual XPath queries containing special characters like single quotes (‘), double slashes (‘//’), or other XML syntax elements.

4. Solution / Remediation Steps

The best way to prevent XPath Injection is to use parameterized queries, which treat user input as data rather than part of the query itself. Alternatively, precompiled queries can be used.

4.1 Preparation

• Ensure you have a rollback plan in place – revert to the previous code version if issues arise.
• Changes should be deployed during a maintenance window with appropriate approval from security or development teams.

4.2 Implementation

1. Step 1: Identify all locations in your application where XPath queries are constructed using user-supplied data.
2. Step 2: Replace direct string concatenation with parameterized (prepared) XPath queries, if the framework supports them.
3. Step 3: If parameterisation isn’t possible, use precompiled XPath queries that do not process user input directly.

4.3 Config or Code Example

Before

String query = "SELECT * FROM users WHERE username = '" + userInput + "'";

After

PreparedStatement statement = connection.prepareStatement("SELECT * FROM users WHERE username = ?");
statement.setString(1, userInput);
ResultSet result = statement.executeQuery();

4.4 Security Practices Relevant to This Vulnerability

• Practice 2: Least privilege – limit the permissions of database accounts used by the application to only what is necessary.

4.5 Automation (Optional)

If suitable, provide a small script or infrastructure code that applies the fix at scale. Only include if safe and directly relevant.

5. Verification / Validation

Confirming the fix involves re-running the tests used during detection to ensure they no longer identify the vulnerability. A smoke test verifies core functionality remains operational.

• Post-fix check: Re-run the scanner and verify that it does not report any XPath injection vulnerabilities in the affected areas.
• Re-test: Attempt to inject malicious payloads through the application’s interface and confirm they are no longer successful.
• Monitoring: Monitor web server logs for any unexpected errors or unusual XPath queries.

6. Preventive Measures and Monitoring

Preventing this vulnerability requires a layered approach, including secure coding practices and regular security assessments.

• Baselines: Update your application’s security baseline to include requirements for input validation and parameterized queries.
• Pipelines: Integrate Static Application Security Testing (SAST) tools into your CI/CD pipeline to identify potential vulnerabilities during development.
• Asset and patch process: Implement a regular code review process that specifically checks for XPath injection vulnerabilities.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Incorrectly implemented parameterisation could still leave the application vulnerable. Thorough testing is essential.
• Roll back: Revert to the previous code version and restore any modified configuration files.

8. References and Resources

• Vendor advisory or bulletin: N/A – this is a generic vulnerability class.
• NVD or CVE entry: N/A – no specific CVE for XPath Injection in general.
• Product or platform documentation relevant to the fix: Refer to your application framework’s documentation on parameterized queries and input validation.