How to remediate – Web Site Cross-Domain Policy File Detection

1. Introduction

The Web Site Cross-Domain Policy File Detection vulnerability means a ‘crossdomain.xml’ file exists on a web server. This XML file controls whether Adobe Flash Player can access data from different domains. It matters because unrestricted policies can allow attackers to bypass security measures and potentially steal information or take control of the server. Confidentiality, integrity, and availability could be impacted if exploited.

2. Technical Explanation

The vulnerability occurs when a web server hosts a crossdomain.xml file that allows broad access to data from any domain. An attacker can exploit this by creating a malicious Flash application hosted on their own domain, which then attempts to access sensitive data or perform actions on the vulnerable server as if it were originating from a trusted source. The main precondition is the presence of an overly permissive crossdomain.xml file.

• Root cause: An improperly configured ‘crossdomain.xml’ file allowing unrestricted access using wildcard characters (*).
• Exploit mechanism: An attacker hosts a Flash application on their server and uses it to make requests to the vulnerable web server, bypassing same-origin policy restrictions due to the permissive crossdomain.xml file. For example, an attacker could read files from the server or execute commands if the server allows it.
• Scope: Web servers hosting Adobe Flash content are affected. Older versions of Flash Player are particularly susceptible.

3. Detection and Assessment

You can confirm a system is vulnerable by checking for the presence of the crossdomain.xml file and reviewing its contents. A quick check involves browsing to the root directory of the web server in a browser. A thorough method requires examining the file’s XML content directly.

• Quick checks: Use your browser to navigate to http://yourserver.com/crossdomain.xml. If the file loads, it exists.
• Scanning: Nessus vulnerability ID 8a58aa76 can detect this issue. This is an example only and may require updated plugins.
• Logs and evidence: Web server access logs will show requests for ‘crossdomain.xml’. Look for any unusual activity around this file.

curl -I http://yourserver.com/crossdomain.xml

4. Solution / Remediation Steps

Fix the issue by carefully reviewing and restricting access in the crossdomain.xml file or removing it if Flash content is no longer used. Only allow specific domains that legitimately need access to data on your server.

4.1 Preparation

• Change window needs: Changes should be made during a scheduled maintenance period with appropriate approval from IT security and application owners.

4.2 Implementation

1. Step 1: Locate the ‘crossdomain.xml’ file on the web server.
2. Step 2: Open the ‘crossdomain.xml’ file in a text editor.
3. Step 3: Modify the policy to allow only specific domains that require access, replacing ‘*’ with explicit domain names.
4. Step 4: Save the changes to the ‘crossdomain.xml’ file.
5. Step 5: Restart the web server or related services if necessary.

4.3 Config or Code Example

Before

<cross-domain-policy>
  <allow-access-from domain="*" />
</cross-domain-policy>

After

<cross-domain-policy>
  <allow-access-from domain="https://allowed.example.com" />
  <allow-access-from domain="https://anotherallowed.example.com" />
</cross-domain-policy>

4.4 Security Practices Relevant to This Vulnerability

List only practices that directly address this vulnerability type. Use neutral wording and examples instead of fixed advice. For example: least privilege, input validation, safe defaults, secure headers, patch cadence. If a practice does not apply, do not include it.

• Practice 1: Least privilege – restrict access to only the domains that absolutely need it.
• Practice 2: Secure Defaults – avoid using wildcard characters (*) in security configurations unless absolutely necessary.

4.5 Automation (Optional)

# Example PowerShell script to check crossdomain.xml content
$filePath = "C:inetpubwwwrootcrossdomain.xml"
if (Test-Path $filePath) {
  $content = Get-Content $filePath -Raw
  if ($content -match 'domain="*"' ) {
    Write-Host "Warning: crossdomain.xml contains wildcard access."
  }
} else {
  Write-Host "crossdomain.xml not found."
}

5. Verification / Validation

Confirm the fix by checking that the ‘crossdomain.xml’ file no longer allows unrestricted access. Use your browser to view the file and verify the allowed domains are correct. Test with a Flash application from an untrusted domain to ensure it cannot access data on the server.

• Post-fix check: Browse to http://yourserver.com/crossdomain.xml. The output should list only approved domains, not ‘*’.
• Re-test: Re-run the curl command from step 3. It should no longer show wildcard access.
• Monitoring: Monitor web server logs for requests to ‘crossdomain.xml’ and alert on any unexpected activity.

curl -I http://yourserver.com/crossdomain.xml

6. Preventive Measures and Monitoring

Suggest only measures that are relevant to the vulnerability type. Use “for example” to keep advice conditional, not prescriptive.

• Baselines: Update your security baseline to include a policy requiring specific domain restrictions in crossdomain.xml files.
• Asset and patch process: Regularly review server configurations for unnecessary or insecure files like ‘crossdomain.xml’.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 2: Removing ‘crossdomain.xml’ entirely will prevent all Flash content from accessing server data. Mitigation: Ensure you no longer require Flash functionality.
• Roll back: Restore the original ‘crossdomain.xml’ file from backup. Restart web services if necessary.

8. References and Resources

• Vendor advisory or bulletin: