How to remediate – Web Cache Poisoning Denial of Service

1. Introduction

Web Cache Poisoning Denial of Service is a vulnerability where an attacker manipulates a web cache, causing it to store and serve malicious content. This can lead to service disruption as legitimate users receive incorrect information or the application becomes unreachable. It typically affects applications using caching systems like proxy servers or CDNs, impacting confidentiality, integrity, and availability.

2. Technical Explanation

The root cause is unkeyed inputs in a caching system – headers or cookies not included when generating the cache key. An attacker can craft requests with specific input values that force the cache to store a response containing user-controlled data. Subsequent users requesting the same resource will receive this poisoned content. The scanner performs a safe check, affecting only itself and not live traffic.

• Root cause: Unkeyed inputs in the caching system allow for manipulation of cached responses.
• Exploit mechanism: An attacker sends requests with crafted headers or cookies to poison the cache. For example, an attacker could inject a malicious script into a header that is not part of the cache key, causing it to be stored and served to other users.
• Scope: Applications using caching systems (proxy servers, CDNs) are affected.

3. Detection and Assessment

Confirming vulnerability involves checking for unkeyed inputs and testing if they affect the cache. A quick check is reviewing application configuration for caching settings. Thorough assessment requires analysing request/response headers during a controlled test.

• Quick checks: Review web server or CDN configuration files for caching rules and identify any unkeyed input parameters.
• Scanning: Nessus plugin 16895 can detect some instances of cache poisoning vulnerabilities, but results should be verified manually.
• Logs and evidence: Examine web server logs for unusual request patterns with manipulated headers or cookies. Look for responses with unexpected content.

curl -I https://example.com/resource -H "X-Custom-Header: malicious_value"

4. Solution / Remediation Steps

Fixing this issue requires either disabling caching for affected inputs or including those inputs in the cache key. Prioritise disabling caching if possible, otherwise configure the cache correctly.

4.1 Preparation

• Ensure you have access to revert configuration changes if needed. A roll back plan is to restore the previous configuration from the snapshot.
• Change windows may be required depending on service impact, and approval should be sought from the application owner.

4.2 Implementation

1. Step 1: Identify affected input parameters (headers or cookies).
2. Step 2: Disable caching for those specific inputs in your web server configuration (e.g., Nginx, Apache) or CDN settings.
3. Step 3: If both the input and caching are required, configure the cache to include the input parameter in the cache key.
4. Step 4: Restart the web server or apply the CDN configuration changes.

4.3 Config or Code Example

Before

# Nginx example - caching without considering X-Custom-Header
proxy_cache_key "$scheme$request_method$host$request_uri";

After

# Nginx example - caching including X-Custom-Header
proxy_cache_key "$scheme$request_method$host$request_uri$http_x_custom_header";

4.4 Security Practices Relevant to This Vulnerability

Several security practices help prevent this issue. Input validation is crucial for blocking malicious data. Safe defaults ensure caching systems are configured securely. Secure headers can also mitigate risks.

• Practice 1: Input validation prevents attackers from injecting harmful content into cache keys or responses.
• Practice 2: Least privilege limits the impact of a successful attack by restricting access to sensitive resources.

4.5 Automation (Optional)

Configuration management tools can automate changes across multiple servers. Use caution when modifying caching configurations, as incorrect settings can cause service outages.

# Ansible example - updating Nginx configuration
- name: Update Nginx cache key
  lineinfile:
    path: /etc/nginx/nginx.conf
    regexp: 'proxy_cache_key "$scheme$request_method$host$request_uri";'
    line: 'proxy_cache_key "$scheme$request_method$host$request_uri$http_x_custom_header";'
  notify: Restart Nginx

5. Verification / Validation

Confirm the fix by re-running the initial test and verifying that the cache is no longer poisoned. A service smoke test should confirm normal application functionality.

• Post-fix check: Run `curl -I https://example.com/resource -H “X-Custom-Header: malicious_value”` again and verify that the response does not contain the injected value.
• Re-test: Repeat the initial scanning process to confirm the vulnerability is no longer detected.
• Smoke test: Access key application features (e.g., login, search) to ensure they function as expected.
• Monitoring: Monitor web server logs for any unusual request patterns or errors related to caching.

curl -I https://example.com/resource -H "X-Custom-Header: malicious_value" # Should not return the injected value

6. Preventive Measures and Monitoring

Update security baselines with secure caching configurations. Implement checks in CI/CD pipelines to prevent unkeyed inputs from reaching production. Regular patch reviews help identify and address vulnerabilities promptly.

• Baselines: Update your web server or CDN baseline configuration to include best practices for cache key generation.
• Pipelines: Integrate SAST tools into your CI/CD pipeline to scan code for potential caching vulnerabilities.
• Asset and patch process: Review security advisories and apply patches promptly to address known vulnerabilities in caching systems.

7. Risks, Side Effects, and Roll Back

Disabling caching can impact application performance. Incorrect cache key configuration can lead to unexpected behaviour or increased resource usage. Roll back by restoring the previous configuration from the snapshot.

• Risk or side effect 1: Disabling caching may increase server load and response times.
• Risk or side effect 2: Incorrect cache key configuration could result in cached data becoming stale or invalid.
• Roll back: Restore the web server or CDN configuration from the pre-change snapshot. Restart related services.

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://cpdos.org/