Web Cache Poisoning Attack : A Helpful Guide

11/24/2022

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Web cache poisoning attacks put your website/application in danger. 

An online attack, known as “web cache poisoning,” involves an attacker using a web server and it is associated caching server’s weaknesses to cache and serve malicious HTTP responses to other web server users. It’s a mouthful, but let’s break it down. 

This article will define web cache poisoning, describe how it functions, and discuss how to stop them.

Web Cache: What is it?

In a particular time frame, most web pages are frequently browsed many times. The web server briefly retains a duplicate of the webpage in its memory whenever a client requests to access a certain web page. The duplicate stored in memory is used to fulfill subsequent incoming requests for the same resource, keeping the distant server from becoming overburdened.

Expedited delivery of web objects to the client is made possible by a web cache, which is a copy of the server response. Physically storing data closer to the user reduces network traffic and improves the functionality of a website.

The Function of Web Cache

The distant server fulfills its request when a user accesses a web page for the first time. Following receipt of the client request and retrieval of the necessary data from a remote server, the web server transforms the requested web page into an HTML file that is then loaded onto the client browser.

The cache server keeps this response for a specific amount of time. The next time a user requests to see this page, the cache server transmits the ready static files to the browser rather than processing the request from the remote server because it is located between the user and the remote server.

When determining whether a response is in the cache memory or if the request needs to be processed by a remote server, the cache server uses a “cache key.” The host header and request line are often included in the cache key, a preset subset of the request’s elements.

The request is processed by the cache server, which acts as a copy of the cached answer if the cache key of an incoming request matches that of a prior request.

Caching has a wide range of advantages for contemporary websites, including:

  • less work for the webserver to do
  • increased reaction rate
  • a reduction in network traffic
  • website performance that can be predicted 

Even requirements for emptying cache memory when certain circumstances are met are part of caching techniques. For instance, website developers can set the cached versions of the homepage and blog pages to be removed when a post is published while maintaining the original versions of all other sites.

Web Cache Poisoning Attack: What Is It?

In a web cache poisoning attack, a perpetrator takes advantage of flaws in the caching process to store a modified and negative answer in the cache entry, forcing the website to display harmful information to its users. A hacker often tries the following actions to build a successful web cache poisoning attack:

Identification and evaluation of unkeyed inputs: The cache ignores some sections of a request while deciding whether to deliver the requested resource from the cache. Malicious headers are injected into the HTTP response header field by a hacker who finds invalid code.

A poisoned response is given to all users whose request for the resource matches the cache key by taking advantage of this vulnerability to modify the unkeyed inputs.

To invoke the negative response correctly, the malicious actor must comprehend how the web server interprets the unkeyed input. Web cache poisoning attacks are only feasible when these inputs may be utilized to build other replies dynamically or if the server reflects this input in its response without sufficient validation or input sanitation.

Caching the malicious response – The effective storing of the destructive answer in the cache memory is essential to the success of a web caching attack. Attackers must use a variety of observations and trial-and-error techniques to study the behavior of the cache.

Responses may or may not be cached depending on the status code, response headers, route, file extension, content type, etc. Potential targets can receive the exploit once the response has been successfully stored.

A web cache poisoning attack is fairly difficult to perform due to the multiple requirements that must be met. The effectiveness of caching responses and traffic pertaining to the targeted cache often determine an attack’s effects.

Cross-site scripting payloads are frequently used in conjunction with web cache poisoning to increase the effects of the exploit. The impact on most users, should an attacker successfully poison a browser cache, is minimal. In contrast, server-side cache poisoning can have a large-scale effect on subscribers without the hacker being constantly involved.

The Best Way To Stop Web Cache Poisoning Attacks

Disabling caching entirely is the only proven approach to stop web cache poisoning attacks. Larger websites are unlikely to do this, although some smaller websites might be able to. For instance, you might want to consider if caching is truly necessary if it is just enabled since it is the CDN you are using’s default setting. If you don’t, turn it off.

Here are some recommendations you should go by if caching is actually necessary for your website or application: 

1. Cache only static files

Only static files that never change and don’t require user input to produce a cached response should have caching enabled. This will prevent an attacker from tricking your caching server into retrieving a malicious version of a file instead of the intended safe file.

2. Use third-party software with caution.

Today, many, if not most, websites and applications are created using third-party software. Your internal development team can have a strict code of conduct regarding security procedures. However, as soon as you include third-party software, you implicitly depend on the strength of the security procedures used by that third-party development team.

If their code is poorer than yours, it becomes your weakest link, making your website or application just as vulnerable.

3. Never trust or rely on information obtained in HTTP headers.

HTTP headers can be used to exploit a variety of client-side flaws, which can result in reflected cross-site scripting attacks.

An application or website that accepts user input and displays the results to the user without validating the input is vulnerable to reflected XSS attacks. An example of this would be a search field. It only reproduces what the user entered. Therefore, don’t rely on HTTP header values that aren’t included in your cache key. And the web cache should never return HTTP headers.

4. Never believe GET requests.

GET request bodies should not be trusted, and you should ensure they cannot alter the contents of a response. Consider utilizing a POST request or avoiding the cache entirely if the body of a GET request can change the content of a response.

5. Regularly test for vulnerabilities.

Run a vulnerability study on your website or application regularly to ensure that it is not particularly vulnerable to cross-site scripting attacks. Regular vulnerability scanning will shield you from further online dangers.

What distinguishes web cache poisoning from DNS cache poisoning?

The Domain Name Server is affected by DNS cache poisoning. In a DNS cache poisoning attack, the attacker inserts erroneous data into a DNS server’s cache.

Users are redirected to a different DNS entry with each incorrect response to a DNS query. By forcing the DNS caching server log to forbid IP addresses from being stored in the memory cache, a DNS poisoning attack forces the DNS request to recall the incorrect website.

On the other hand, Web cache poisoning entails changing request bodies to trigger a malicious response kept in the cache memory of the server or browser.

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Conclusion

The only way to ensure that a web cache poisoning attack won’t affect you is to disable caching. However, taking the above-mentioned precautions will increase your chances of avoiding them. Any website or business that operates online must contend with unfriendly conditions. And it won’t survive very long without adequate defenses. In this way, any amount is helpful.