How to remediate – Web Cache Deception

1. Introduction

Web Cache Deception is a vulnerability where a caching system incorrectly caches content based on URL manipulation, allowing access to authenticated pages without proper login. This can lead to sensitive data exposure and unauthorised actions. Systems with any form of web caching enabled are typically affected. Impact is high: confidentiality, integrity, and availability may be compromised.

2. Technical Explanation

The root cause is insufficient validation when determining what content should be cached. The application caches pages based on the URL alone, without considering whether the user has permission to view them. An attacker can craft a request for an authenticated page and force the cache to store it. Subsequent requests from any user will then serve the cached version. CWE-444 describes this issue.

• Root cause: Caching logic only checks URL and extension, not Content-Type or authentication status.
• Exploit mechanism: An attacker accesses a restricted page while logged in, forcing the cache to store it. Then, they access the same URL without logging in, retrieving the cached version.
• Scope: Web applications using any caching system (e.g., Varnish, Akamai, Cloudflare) are potentially affected.

3. Detection and Assessment

Confirming vulnerability requires checking cache behaviour with authenticated and unauthenticated requests. A quick check is to examine the application’s configuration for caching settings. Thorough assessment involves attempting to access cached pages without authentication.

• Quick checks: Check web server configuration files (e.g., Nginx, Apache) or application code for cache-related directives.
• Scanning: Burp Suite’s spider can identify potential caching endpoints. Nessus and OpenVAS may have relevant plugins but results should be verified.

curl -v "https://example.com/restricted-page" #Check response headers for caching directives

4. Solution / Remediation Steps

Fixing this requires modifying the caching logic to consider Content-Type and authentication status when storing content. This ensures only publicly accessible pages are cached.

4.1 Preparation

• Ensure you have access to the application’s source code or configuration settings. Roll back by restoring the original configuration files.
• Change windows should be scheduled during off-peak hours with approval from the IT security team.

4.2 Implementation

1. Step 1: Modify the caching logic to include a check for Content-Type. Only cache content if it is publicly accessible (e.g., HTML, CSS, JavaScript).
2. Step 3: Restart the web server or application to apply the changes.

4.3 Config or Code Example

Before

# Simple caching based on URL
cache_path /tmp/cache levels 1:2 keys_zone my_cache:10m max_size 10g;
server {
  location / {
    proxy_cache my_cache;
  }
}

After

# Caching with Content-Type check and authentication
cache_path /tmp/cache levels 1:2 keys_zone my_cache:10m max_size 10g;
server {
  location / {
    proxy_cache my_cache;
    proxy_cache_valid 200 301 302 60m;
    proxy_cache_bypass $http_authorization; # Don't cache authenticated requests
    proxy_cache_revalidate on;
  }
}

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent Web Cache Deception. Least privilege limits the impact of a successful attack. Input validation prevents malicious data from being processed. Secure headers protect against cache poisoning. A regular patch cadence ensures systems are up-to-date with the latest security fixes.

• Practice 1: Least privilege reduces the scope of damage if an attacker bypasses caching controls.
• Practice 2: Input validation prevents attackers from manipulating URLs to exploit caching vulnerabilities.

4.5 Automation (Optional)

# Example Ansible task to update Nginx config
- name: Update Nginx caching configuration
  lineinfile:
    path: /etc/nginx/nginx.conf
    regexp: 'proxy_cache my_cache;'
    line: 'proxy_cache my_cache; proxy_cache_bypass $http_authorization;'
  notify: Restart Nginx

5. Verification / Validation

• Post-fix check: `curl -v “https://example.com/restricted-page”` – Verify no cache headers are present when unauthenticated.
• Re-test: Repeat the initial detection steps to ensure the vulnerability is resolved.
• Smoke test: Confirm users can still log in and access their authenticated pages as expected.
• Monitoring: Monitor web server logs for unexpected caching behaviour or repeated requests from unauthenticated users.

curl -v "https://example.com/restricted-page" #Check response headers, should not show cache directives when logged out

6. Preventive Measures and Monitoring

Update security baselines to include caching best practices. Integrate SAST tools into CI pipelines to identify potential vulnerabilities in application code. Implement a regular patch management process to address known issues promptly.

• Baselines: Update CIS benchmarks or internal security policies to reflect secure caching configurations.
• Pipelines: Add Static Application Security Testing (SAST) tools to CI/CD pipelines to detect insecure caching logic during development.

7. Risks, Side Effects, and Roll Back

Modifying caching configuration could impact performance or cause unexpected behaviour. Thorough testing is crucial. If issues arise, roll back by restoring the original configuration files.

• Risk or side effect 2: Changes could break existing functionality if not tested properly. Have a roll back plan in place.
• Roll back: Restore the original web server configuration files from backup. Restart the web server or application to apply the changes.

8. References and Resources

• Vendor advisory or bulletin: N/A – This is a general web application issue, not tied to one vendor.
• NVD or CVE entry: CWE-444
• Product or platform documentation relevant to the fix: Consult your specific web server (e.g., Nginx, Apache) documentation for caching configuration options.