Internet Bug Bounty: Security Unfavorable Specifications and Implementations in the CGI::Cookie Class

CVE-2021-33621: HTTP response splitting in CGI
Posted by mame on 22 Nov 2022

We have released the cgi gem version 0.3.5, 0.2.2, and 0.1.0.2 that has a security fix for a HTTP response splitting vulnerability. This vulnerability has been assigned the CVE identifier CVE-2021-33621.

Details
If an application that generates HTTP responses using the cgi gem with untrusted user input, an attacker can exploit it to inject a malicious HTTP response header and/or body.

Also, the contents for a CGI::Cookie object were not checked properly. If an application creates a CGI::Cookie object based on user input, an attacker may exploit it to inject invalid attributes in Set-Cookie header. We think such applications are unlikely, but we have included a change to check arguments for CGI::Cookie#initialize preventatively.

Please update the cgi gem to version 0.3.5, 0.2.2, and 0.1.0.2, or later. You can use gem update cgi to update it. If you are using bundler, please add gem “cgi”, “>= 0.3.5” to your Gemfile.

Credits
Thanks to Hiroshi Tokumaru for discovering this issue.

Full Security Advisory: https://www.ruby-lang.org/en/news/2022/11/22/http-response-splitting-in-cgi-cve-2021-33621/