^{CVE-2021-21289:}Command Injection Vulnerability in Mechanize Ruby Library

Overview

The vulnerability (CWE-78: OS Command Injection) affects several methods within the Mechanize library that handle file operations. When untrusted input is passed as a local filename to specific methods, an attacker can inject OS commands that will be executed with the privileges of the application running the Mechanize code. The vulnerable methods include:

• Mechanize::CookieJar#load
• Mechanize::CookieJar#save_as
• Mechanize#download
• Mechanize::Download#save
• Mechanize::File#save
• Mechanize::FileResponse#read_body

The root cause is the unsafe use of Ruby's Kernel.open method, which can execute commands if the input string begins with a pipe character (|).

Remediation

Users should upgrade to Mechanize version 2.7.7 or later, which fixes this vulnerability by implementing safer file handling methods. If upgrading is not immediately possible, implement the following mitigations:

1. Validate and sanitize all file path inputs before passing them to any of the vulnerable methods
2. Avoid using untrusted or user-supplied input as filenames in Mechanize operations
3. Consider implementing application-level controls to restrict the characters allowed in filenames

System administrators using package managers should apply the following updates:

• Debian: Apply the ruby-mechanize security update (DLA 2561-1)
• Fedora: Update to the patched versions referenced in advisories FEDORA-2021-24fdc228e4 and FEDORA-2021-db8ebc547e
• Gentoo: Apply the security update referenced in GLSA-202107-17

References

1. GitHub Security Advisory: https://github.com/sparklemotion/mechanize/security/advisories/GHSA-qrqm-fpv6-6r8g
2. Fix Commit: https://github.com/sparklemotion/mechanize/commit/66a6a1bfa653a5f13274a396a5e5441238656aa0
3. Release Notes: https://github.com/sparklemotion/mechanize/releases/tag/v2.7.7
4. Debian Security Advisory: https://lists.debian.org/debian-lts-announce/2021/02/msg00021.html
5. Fedora Security Advisory: https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/LBVVJUL4P4KCJH4IQTHFZ4ATXY7XXZPV/
6. Gentoo Security Advisory: https://security.gentoo.org/glsa/202107-17
7. RubyGems Package: https://rubygems.org/gems/mechanize/