CVE-2021-21289 – CVE-2021-21289 is a command injection vulnerabi... (How to Fix)

Q: What is the severity of CVE-2021-21289?

CVE-2021-21289 has a CVSS score of 7.4 (HIGH). CVE-2021-21289 is a command injection vulnerability in the Mechanize Ruby library that allows attackers to execute arbitrary operating system commands. It affects Mechanize versions 2.0.0 through 2.7.6 when untrusted input is passed to specific file handling methods. Applications using Mechanize for web automation with user-controlled filenames are vulnerable.

📋 TL;DR

CVE-2021-21289 is a command injection vulnerability in the Mechanize Ruby library that allows attackers to execute arbitrary operating system commands. It affects Mechanize versions 2.0.0 through 2.7.6 when untrusted input is passed to specific file handling methods. Applications using Mechanize for web automation with user-controlled filenames are vulnerable.

💻 Affected Systems

Products:

Mechanize Ruby library

Versions: 2.0.0 through 2.7.6

Operating Systems: All operating systems where Ruby and Mechanize are installed

Default Config Vulnerable: ⚠️ Yes

Notes: Only vulnerable when untrusted input is passed to Mechanize::CookieJar#load, Mechanize::CookieJar#save_as, Mechanize#download, Mechanize::Download#save, Mechanize::File#save, or Mechanize::FileResponse#read_body methods.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Full system compromise with arbitrary command execution as the application user, potentially leading to data theft, lateral movement, or ransomware deployment.

🟠

Likely Case

Limited command execution within the application's context, potentially accessing sensitive files or establishing persistence.

🟢

If Mitigated

No impact if input validation prevents untrusted filenames or if the vulnerability is patched.

🌐 Internet-Facing: MEDIUM - Exploitation requires user input to specific Mechanize methods, which may be exposed in web applications.

🏢 Internal Only: LOW - Typically requires internal user interaction or specific automation workflows.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ✅ No

Complexity: LOW

Exploitation requires attacker-controlled input to specific Mechanize methods. Proof-of-concept code is available in the advisory.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 2.7.7

Vendor Advisory: https://github.com/sparklemotion/mechanize/security/advisories/GHSA-qrqm-fpv6-6r8g

Restart Required: Yes

Instructions:

1. Update Gemfile to specify 'mechanize', '>= 2.7.7'. 2. Run 'bundle update mechanize'. 3. Restart all Ruby processes using Mechanize. 4. Test application functionality.

🔧 Temporary Workarounds

Input validation and sanitization

all

Validate and sanitize all user input passed to Mechanize file methods to prevent command injection.

Gem version pinning

all

Temporarily pin to a safe version if immediate upgrade isn't possible.

gem 'mechanize', '2.7.7'

🧯 If You Can't Patch

Implement strict input validation for all parameters passed to vulnerable Mechanize methods
Use application-level firewalls or WAF rules to block suspicious file operations

🔍 How to Verify

Check if Vulnerable:

Check Gemfile.lock or run 'gem list mechanize' to see if version is between 2.0.0 and 2.7.6 inclusive.

Check Version:

gem list mechanize | grep mechanize

Verify Fix Applied:

Confirm mechanize version is 2.7.7 or higher using 'gem list mechanize'.

📡 Detection & Monitoring

Log Indicators:

Unusual file operations with shell metacharacters in Mechanize logs
Unexpected process execution from Ruby applications

Network Indicators:

Outbound connections from Ruby processes to unexpected destinations

SIEM Query:

process.name:ruby AND cmdline:*Kernel.open* OR cmdline:*|* OR cmdline:*`*

📊 Metadata

CVE ID: CVE-2021-21289

CVSS v3 Score: 7.4 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N

CWE: CWE-78

Published: February 2, 2021

Last Updated: November 21, 2024

🔗 References

https://github.com/sparklemotion/mechanize/commit/66a6a1bfa653a5f13274a396a5e5441238656aa0 Patch,Third Party Advisory
https://github.com/sparklemotion/mechanize/releases/tag/v2.7.7 Release Notes,Third Party Advisory
https://github.com/sparklemotion/mechanize/security/advisories/GHSA-qrqm-fpv6-6r8g Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/02/msg00021.html Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/LBVVJUL4P4KCJH4IQTHFZ4ATXY7XXZPV/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/YNFZ7ROYS6V4J5L5PRAJUG2AWC7VXR2V/
https://rubygems.org/gems/mechanize/ Product,Third Party Advisory
https://security.gentoo.org/glsa/202107-17 Third Party Advisory
https://github.com/sparklemotion/mechanize/commit/66a6a1bfa653a5f13274a396a5e5441238656aa0 Patch,Third Party Advisory
https://github.com/sparklemotion/mechanize/releases/tag/v2.7.7 Release Notes,Third Party Advisory
https://github.com/sparklemotion/mechanize/security/advisories/GHSA-qrqm-fpv6-6r8g Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/02/msg00021.html Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/LBVVJUL4P4KCJH4IQTHFZ4ATXY7XXZPV/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/YNFZ7ROYS6V4J5L5PRAJUG2AWC7VXR2V/
https://rubygems.org/gems/mechanize/ Product,Third Party Advisory
https://security.gentoo.org/glsa/202107-17 Third Party Advisory

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