What is cache poisoning? How hackers manipulate web caches and how to avoid It

Cache poisoning is a cyberattack that targets web caches, manipulating stored data to deliver malicious content to users. To understand this, it’s important to first grasp how web caches work and how attackers exploit them.

What is a web cache?

A web cache stores copies of web content, such as pages and images, to speed up the browsing experience. Caching reduces load times and decreases the data that needs to be fetched from the server. Web caches can exist in different forms:

• Browser cache: Stores recently visited web pages and elements locally.
• CDN cache: Stores web resources across servers worldwide to reduce load times for users.
• Reverse proxy cache: Sits between users and web servers to cache and speed up requests.

Caches improve performance but also come with vulnerabilities that hackers can exploit through cache poisoning.

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How cache poisoning works

In a cache poisoning attack, hackers manipulate a web cache by injecting malicious content. When users request cached data, the altered version is served instead of the original, potentially running harmful scripts on their browsers.

Here’s how the attack typically unfolds:

• Identifying targets: The attacker identifies which parts of the website are cached.
• Crafting a malicious request: They send a crafted request that looks legitimate but includes harmful content.
• Poisoning the cache: If the cache server doesn’t properly validate the request, it stores the malicious content.
• Delivering the poisoned data: When future users request that resource, they receive the manipulated content.

The attack may lead to phishing, malware installation, or data theft, all without the user's awareness.

Common cache poisoning techniques

• Host header attacks: Attackers manipulate the "Host" header in HTTP requests to trick the cache into storing malicious data.
• HTTP parameter pollution: Hackers add unexpected parameters to URLs, which the cache might store and serve as part of a poisoned page.
• Vary header manipulation: Attackers manipulate the “Vary” header, which dictates how different versions of web resources are cached based on request headers.

How to protect against cache poisoning

1. Input validation: Always sanitise user inputs, especially request headers and URL parameters. Proper validation prevents hackers from injecting harmful data into the cache.

2. Secure caching headers: Use headers like "Cache-Control" and "Expires" wisely. Sensitive data should not be cached, and for dynamic content, headers like "no-cache" or "no-store" should be employed.

3. Control cache keys: Set cache keys to avoid caching responses that are specific to user data or parameters. This prevents attackers from manipulating query parameters.

4. Implement HTTPS: HTTPS secures communication between the user and server, reducing the risk of cache poisoning by preventing interception and modification of requests.

Summing up, cache poisoning is a serious risk that can turn a website’s speed advantage into a security flaw. By understanding how cache poisoning works and taking the right steps—such as validating inputs, securing headers, and using HTTPS—you can significantly reduce the threat and protect both your users and your web applications. Remember that using a hide VPN service adds an extra layer of security to your online activities.