Web Cache Poisoning Affecting rack package, versions <3.0.0.beta1



    Attack Complexity High
    User Interaction Required
    Availability High

    Threat Intelligence

    Exploit Maturity Proof of concept

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  • Snyk ID SNYK-RUBY-RACK-1061917
  • published 19 Apr 2021
  • disclosed 13 Oct 2020
  • credit James Kettle

Introduced: 13 Oct 2020

CVE NOT AVAILABLE CWE-444 Open this link in a new tab
First added by Snyk

How to fix?

Upgrade rack to version 3.0.0.beta1 or higher.


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.


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.