CVE-2021-21289

Command Injection Vulnerability in Mechanize

This security advisory has been created for public disclosure of a Command Injection vulnerability that was responsibly reported by @kyoshidajp (Katsuhiko YOSHIDA).

Workarounds

No workarounds are available. We recommend upgrading to v2.7.7 or later.

References

See https://docs.rubocop.org/rubocop/cops_security.html#securityopen for background on why Kernel.open should not be used with untrusted input.

For more information

If you have any questions or comments about this advisory, please open an issue in sparklemotion/mechanize.

Mechanize >= v2.0, < v2.7.7 allows for OS commands to be injected using several classes' methods which implicitly use Ruby's Kernel.open method. Exploitation is possible only if untrusted input is used as a local filename and passed to any of these calls:

• Mechanize::CookieJar#load: since v2.0 (see 208e3ed)
• Mechanize::CookieJar#save_as: since v2.0 (see 5b776a4)
• Mechanize#download: since v2.2 (see dc91667)
• Mechanize::Download#save and #save! since v2.1 (see 98b2f51, bd62ff0)
• Mechanize::File#save and #save_as: since v2.1 (see 2bf7519)
• Mechanize::FileResponse#read_body: since v2.0 (see 01039f5)

Untrusted input reaches a shell command, allowing arbitrary commands to run on the host. Typical impact: code execution in the application's environment.

CVE-2021-21289 has a CVSS score of 7.4 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.7.7); upgrading removes the vulnerable code path.

Kodem intelligence

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These vulnerabilities are patched in Mechanize v2.7.7.

1. What is CVE-2021-21289? CVE-2021-21289 is a high-severity OS command injection vulnerability in mechanize (rubygems), affecting versions >= 2.0.0, < 2.7.7. It is fixed in 2.7.7. Untrusted input reaches a shell command, allowing arbitrary commands to run on the host.
2. How severe is CVE-2021-21289? CVE-2021-21289 has a CVSS score of 7.4 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of mechanize are affected by CVE-2021-21289? mechanize (rubygems) versions >= 2.0.0, < 2.7.7 is affected.
4. Is there a fix for CVE-2021-21289? Yes. CVE-2021-21289 is fixed in 2.7.7. Upgrade to this version or later.
5. Is CVE-2021-21289 exploitable, and should I be worried? Whether CVE-2021-21289 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-21289 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-21289? Upgrade mechanize to 2.7.7 or later.

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