Web Cache Poisoning Affecting rack package, versions <3.0.0.beta1
Snyk CVSS
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Test your applications- Snyk ID SNYK-RUBY-RACK-1061917
- published 19 Apr 2021
- disclosed 13 Oct 2020
- credit James Kettle
How to fix?
Upgrade rack to version 3.0.0.beta1 or higher.
Overview
rack is a minimal, modular and adaptable interface for developing web applications in Ruby. By wrapping HTTP requests and responses in the simplest way possible, it unifies and distills the API for web servers, web frameworks, and software in between (the so-called middleware) into a single method call.
Affected versions of this package are vulnerable to Web Cache Poisoning by using a vector called parameter cloaking. When the attacker can separate query parameters using a semicolon (;), they can cause a difference in the interpretation of the request between the proxy (running with default configuration) and the server. This can result in malicious requests being cached as completely safe ones, as the proxy would usually not see the semicolon as a separator, and therefore would not include it in a cache key of an unkeyed parameter.
PoC
GET /?q=legitimate&utm_content=1;q=malicious HTTP/1.1
Host: somesite.com
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.83 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,imag e/webp,image/apng,/;q=0.8,application/signed-exchange;v=b3;q=0.9 Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9 Connection: close
The server sees 3 parameters here: q, utm_content and then q again. On the other hand, the proxy considers this full string: 1;q=malicious as the value of utm_content, which is why the cache key would only contain somesite.com/?q=legitimate.