How to remediate – Cross-Site Script Inclusion (XSSI)

1. Introduction

Cross-Site Script Inclusion (XSSI) is a vulnerability where a malicious webpage includes scripts from another source, bypassing the Same-Origin Policy. This can allow attackers to steal cookies and sensitive information from authenticated users. Systems that include content from external sources without proper validation are usually affected. A successful exploit could compromise confidentiality, integrity, and availability of user data.

2. Technical Explanation

XSSI occurs when a web application dynamically includes content from other websites or resources without properly sanitizing the input. This allows an attacker to inject malicious scripts into the included content, which are then executed by the victim’s browser. The vulnerability relies on the browser interpreting remote content as valid JavaScript code.

• Root cause: Lack of strict validation and sanitization when including external resources or files in a web application.
• Exploit mechanism: An attacker crafts a malicious URL that, when included by the vulnerable application, executes arbitrary JavaScript code within the context of the victim’s browser session. For example, an attacker could create a webpage containing a script tag pointing to a resource controlled by them and then trick the target web application into including this page.
• Scope: Web applications using Javascript to include remote files or pages are affected.

3. Detection and Assessment

To confirm vulnerability, check for inclusion of external JavaScript sources in your code. Thoroughly review all areas where external content is included.

• Quick checks: Review application source code for instances of `include`, `require`, or similar functions that load external resources.
• Scanning: Static analysis tools can identify potential XSSI vulnerabilities by flagging insecure inclusion patterns. Examples include SonarQube and Veracode.
• Logs and evidence: Monitor web server logs for requests to external JavaScript sources, especially those from untrusted domains. Look for unusual or unexpected script inclusions.

grep -r "include" /path/to/web/application/sourcecode

4. Solution / Remediation Steps

To fix this issue, avoid including sensitive data in files that can be called with Javascript and enforce strict security measures for cookies and requests.

4.1 Preparation

• Ensure you have a rollback plan in case of unexpected issues. A simple revert to the previous code version should suffice.
• Changes require testing and approval from security team.

4.2 Implementation

1. Step 1: Enforce cookies with the `SameSite` flag set to `Strict` or `Lax`. This prevents cross-site cookie transmission.
2. Step 2: Only allow requests to pages using the `POST` method.
3. Step 3: Implement an anti-CSRF token on all POST requests to prevent unauthorized actions.

4.3 Config or Code Example

Before

// Insecure cookie setting
Set-Cookie: sessionid=12345; Path=/

After

// Secure cookie setting with SameSite flag
Set-Cookie: sessionid=12345; Path=/; SameSite=Strict; HttpOnly; Secure

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent XSSI vulnerabilities. Least privilege reduces the impact of exploitation, while input validation blocks unsafe data. Safe defaults and secure headers further enhance protection. A regular patch cadence ensures timely updates against known vulnerabilities.

• Practice 1: Implement least privilege principles to limit the access rights of web application users.
• Practice 2: Validate all user inputs to prevent malicious scripts from being injected into included content.

4.5 Automation (Optional)

Automated tools can help enforce secure cookie settings and validate input data across your applications.

# Example Bash script to check for SameSite flag in cookies
curl -s --cookie "sessionid=12345" http://example.com/ | grep 'SameSite='

5. Verification / Validation

Confirm the fix by verifying that cookies are properly protected and external resources are validated. Re-run earlier detection methods to ensure the issue is resolved. Perform a simple service smoke test to confirm functionality remains intact.

• Post-fix check: Use browser developer tools to inspect cookie settings and verify the `SameSite` flag is set correctly.
• Re-test: Re-run the source code review from step 3 to ensure no insecure inclusion patterns remain.
• Smoke test: Verify that users can still log in, access core features, and perform basic actions without issues.
• Monitoring: Monitor web server logs for any errors related to cookie handling or external resource loading.

curl -s --cookie "sessionid=12345" http://example.com/ | grep 'SameSite=Strict'

6. Preventive Measures and Monitoring

Update security baselines to include secure cookie settings and input validation rules. Integrate checks into CI/CD pipelines to prevent insecure code from being deployed. Establish a sensible patch or config review cycle based on the risk profile of your applications.

• Baselines: Update security baselines with CIS controls related to web application security, including cookie handling and input validation.
• Pipelines: Add SAST tools to CI/CD pipelines to identify potential XSSI vulnerabilities during development.
• Asset and patch process: Implement a regular patch review cycle for all web applications and dependencies.

7. Risks, Side Effects, and Roll Back

Enforcing strict cookie settings may cause compatibility issues with older browsers or systems. Incorrect input validation can lead to false positives or broken functionality. Roll back by reverting the code changes if any unexpected issues occur.

• Risk or side effect 1: Compatibility issues with older browsers due to `SameSite` flag. Mitigation: Test thoroughly across different browser versions.
• Risk or side effect 2: False positives from strict input validation. Mitigation: Carefully review and adjust validation rules as needed.
• Roll back: Revert the code changes made in steps 1-3. Restore the previous cookie settings and remove any new input validation logic.

8. References and Resources

• Vendor advisory or bulletin: N/A
• NVD or CVE entry: N/A
• Product or platform documentation relevant to the fix: https://owasp.org/www-project-web-security-testing-guide/v41/4-Web_Application_Security_Testing/11-Client_Side_Testing/13-Testing_for_Cross_Site_Script_Inclusion