Web Cache Deception-Exploiting Misconfigured Caches

Read more articles on my Medium

Web caching is a technique used to store copies of web pages, images, and other resources to improve performance and reduce server load. When a user requests a cached resource, the cache serves the stored version instead of fetching it from the server. Caching is implemented at various levels, including browsers, Content Delivery Networks (CDNs), and proxy servers.

A well-configured cache enhances website speed, reduces bandwidth consumption, and minimizes latency. However, misconfigurations can introduce security vulnerabilities, such as Web Cache Deception (WCD) and Cache Poisoning.

Web Cache Deception: A Dangerous Attack

Web Cache Deception is a technique that attackers use to manipulate caching systems and mislead users or web applications into serving unintended content. This attack involves crafting malicious requests to exploit vulnerabilities in caching mechanisms, leading to the caching of sensitive data associated with other users through legitimate URLs.

Subsequent users who unknowingly request malicious URLs may be unknowingly served highly confidential and personal sensitive information, putting their privacy and security at significant risk.

How Web Cache Deception Works

In this attack, a victim user visits a sensitive URL, such as:

https://example.com/profile/settings

The attacker then modifies the URL to append a static file extension, such as:

https://example.com/profile/settings.css

If the server does not validate the URL correctly and serves the dynamic content for the modified URL, the cache may store the response as settings.css. Now, anyone accessing the modified URL may view cached sensitive user data without authentication.

Explanation of the Attack

A successful Web Cache Deception attack relies on three key conditions:

1. Misconfigured Cache: The web cache should be configured to cache static files based on extensions without considering any caching headers provided by the server.

2. Unvalidated Requests: When accessing a page like http://www.example.com/home.php/non-existent.css, the server must serve the content of home.php, despite the requested CSS file non-existent.css not existing.

3. No Authentication for Static Files: The server must not require authentication for accessing public static files. As a result, the cached files become publicly accessible to all users, including unauthenticated ones.

Exploitation

In this scenario, attackers can append non-existent static file extensions, such as .css, .js, or .jpg, to URLs that point to sensitive pages. This allows the cache to store dynamic content as if it were static content. When attackers share these crafted URLs with victims, they may end up revealing sensitive data, such as user profiles or account settings, through cached versions of the page.

This type of attack is a significant concern because cached static files are often served without any authentication, exposing sensitive data to unauthorized users.

Cache Poisoning vs. Web Cache Deception

Cache Poisoning is another attack method that targets caching systems. It involves injecting malicious content into cached responses. While Web Cache Deception tricks the cache into storing sensitive information under publicly accessible URLs, Cache Poisoning manipulates the cached content itself to serve malicious payloads, which can lead to phishing, defacement, or code execution.

Both techniques exploit misconfigurations in caching mechanisms and can have serious security implications when left unchecked.

How to Exploit Web Cache Deception

To exploit a Web Cache Deception vulnerability, attackers follow these steps:

1. Identify Sensitive Pages: Look for pages containing sensitive data, such as user profiles, payment details, or account dashboards.

2. Modify the URL: Append static file extensions like .css, .js, or .jpg to the URL to trick the cache into storing the page content.

   Example: https://example.com/user/account.css

3. Test the Response: If the web server returns sensitive data for the modified URL, it means the cache has stored dynamic content, which can now be accessed by anyone.

4. Verify the Attack: Check the modified URL from another session or IP address to see if the same sensitive data is returned, confirming the attack’s success.

5. Automate Exploitation: Attackers might automate the process to identify vulnerable endpoints across the site.

A Diagram of the Web Cache Deception Attack

In this attack, malicious requests are crafted to exploit caching misconfigurations, which store dynamic content as static resources. A diagram can help visualize how this attack unfolds, showing how attackers trick the caching system into storing sensitive data and allowing unauthorized access to cached pages.

Mitigating Web Cache Deception and Cache Poisoning

Mitigating Web Cache Deception

To protect against Web Cache Deception:

1. Avoid Caching Sensitive Pages: Ensure that sensitive pages, like user profile settings or payment history, are not cached.

2. Use Cache-Control Headers: Implement headers like Cache-Control: no-store, Cache-Control: private for sensitive data to prevent caching by intermediate proxies.

3. Validate URL Requests: Ensure that requests with non-existent static file extensions return a 404 or 302 error, rather than serving dynamic content.

4. Implement Authentication: Use authentication checks before serving cached responses to ensure only authorized users can access sensitive content.

5. Vary Header Implementation: Use the Vary header to ensure responses are cached based on specific factors, such as cookies or authorization tokens:

   Vary: Cookie, Authorization

Mitigating Cache Poisoning

To mitigate Cache Poisoning:

1. Sanitize User Input: Always sanitize user input to prevent malicious content from being injected into cached responses.

2. Enforce Strict Cache Policies: Ensure only static content is cached and apply strong validation for HTTP headers and query parameters.

3. Use Cache-Control for Static Files: Use Cache-Control: immutable for static files to avoid accidental poisoning.

4. Content Security Policy (CSP): Implement a CSP to restrict the execution of untrusted JavaScript and prevent malicious scripts from being executed on cached pages.

Conclusion

Web caching is essential for optimizing web performance, but when misconfigured, it can introduce significant security vulnerabilities like Web Cache Deception and Cache Poisoning. These attacks can expose sensitive data or inject malicious content into cached responses, leading to unauthorized access and security breaches.

By implementing strict cache-control policies, securing routing mechanisms, and ensuring proper validation of cached content, organizations can protect against these attacks. Regular cache monitoring and audits are also crucial to detect and mitigate risks promptly.

Organizations must prioritize cybersecurity assessments and penetration testing to safeguard their web caching systems and prevent Web Cache Deception and Cache Poisoning from compromising sensitive data.

Clear Gate, a trusted cybersecurity provider, offers in-depth manual penetration tests and security assessments to help businesses secure their web caching systems and protect valuable data from potential threats.

References:

1. [Web cache deception

   Web Security Academy](https://portswigger.net/web-security/web-cache-deception)

2. Web cache deception — PortSwigger