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How to remediate – Insecure Cross-Domain Policy (allow-http-request-headers-from)

Contents

1. Introduction

Insecure Cross-Domain Policy (allow-http-request-headers-from) occurs when a website’s `crossdomain.xml` file is configured too permissively, allowing any domain to make cross-origin requests. This can allow malicious websites to access sensitive data from the server. Systems running Silverlight are typically affected. A successful exploit could lead to information disclosure, impacting confidentiality.

2. Technical Explanation

The browser security model prevents web content from one domain accessing data from another by default (same-origin policy). URL policy files like `crossdomain.xml` grant exceptions. When a wildcard “*” is used in the `crossdomain.xml` file, it allows any origin to access resources on the server. An attacker could host a malicious webpage that attempts to read data from the vulnerable server, bypassing the same-origin policy.

  • Root cause: The `crossdomain.xml` file is configured with overly broad permissions (using “*”).
  • Exploit mechanism: A malicious website makes cross-origin requests to the vulnerable server, reading data that would normally be inaccessible.
  • Scope: Silverlight applications are affected when deployed on servers with permissive `crossdomain.xml` files.

3. Detection and Assessment

To confirm vulnerability, check for the presence of a `crossdomain.xml` file in the root directory of your web server and examine its contents. A thorough method involves reviewing the policy file to identify any wildcard entries.

  • Quick checks: Use a web browser or command-line tool (like `curl`) to check for the existence of `/crossdomain.xml`.
  • Scanning: Nessus plugin ID 30819 can detect permissive crossdomain.xml files. This is an example only, and results should be verified manually.
  • Logs and evidence: Web server logs may show cross-origin requests being made to the vulnerable server. Look for requests originating from unexpected domains.
curl -I https://www.example.com/crossdomain.xml

4. Solution / Remediation Steps

Carefully evaluate which sites need access and restrict permissions in the `crossdomain.xml` file accordingly. Only allow trusted domains to make cross-origin calls.

4.1 Preparation

  • Ensure you understand the network topology and authentication mechanisms in place. Change windows should be planned during off-peak hours, with approval from security or IT operations teams.

4.2 Implementation

  1. Step 1: Edit the `crossdomain.xml` file on the web server.
  2. Step 2: Remove the wildcard “*” entry.
  3. Step 3: Add specific domain names that are allowed to make cross-origin requests, using the `` tag. For example, ``.
  4. Step 4: Save the changes to the `crossdomain.xml` file.

4.3 Config or Code Example

Before

<?xml version="1.0"?>
<cross-domain-policy>
  <allow-access-from domain="*" />
</cross-domain-policy>

After

<?xml version="1.0"?>
<cross-domain-policy>
  <allow-access-from domain="https://trusteddomain.com" />
</cross-domain-policy>

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege reduces the impact if exploited, and secure configuration management ensures that default settings are not overly permissive.

  • Practice 1: Implement least privilege by only granting access to necessary domains in the `crossdomain.xml` file.
  • Practice 2: Regularly review and update security configurations to ensure they align with current best practices.

4.5 Automation (Optional)

Automation is not typically suitable for this vulnerability due to the need for careful domain evaluation.

5. Verification / Validation

Confirm the fix by checking that only allowed domains are present in the `crossdomain.xml` file and attempting a cross-origin request from an untrusted domain. A service smoke test should verify core functionality remains operational.

  • Post-fix check: Use `curl -I https://www.example.com/crossdomain.xml` to confirm the wildcard entry has been removed and only trusted domains are listed.
  • Re-test: Attempt a cross-origin request from a non-allowed domain; it should be blocked by the browser.
  • Smoke test: Verify that Silverlight applications continue to function as expected with the updated `crossdomain.xml` configuration.
  • Monitoring: Monitor web server logs for unexpected cross-origin requests originating from unknown domains.
curl -I https://www.example.com/crossdomain.xml

6. Preventive Measures and Monitoring

Update security baselines to include restrictions on `crossdomain.xml` file permissions. Implement CI/CD pipeline checks to prevent overly permissive configurations from being deployed. A regular patch review cycle helps identify and address vulnerabilities promptly.

  • Baselines: Update your security baseline or policy to require specific domain names in the `crossdomain.xml` file, prohibiting wildcards.
  • Pipelines: Add checks in CI/CD pipelines to scan for overly permissive configurations in deployment packages.
  • Asset and patch process: Establish a regular review cycle (e.g., quarterly) to assess and update security configurations on all servers.

7. Risks, Side Effects, and Roll Back

Changing the `crossdomain.xml` file may break existing Silverlight applications that rely on cross-origin access from untrusted domains. The roll back steps involve restoring the original `crossdomain.xml` file.

  • Risk or side effect 1: Existing Silverlight applications may stop functioning if their origin is not allowed in the updated policy.
  • Risk or side effect 2: Incorrectly configured `crossdomain.xml` files can lead to unexpected errors and service disruptions.
  • Roll back: 1) Stop any affected services. 2) Restore the original `crossdomain.xml` file from backup. 3) Restart the services.

8. References and Resources

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