CVE-2021-41819
📋 TL;DR
This vulnerability in Ruby's CGI::Cookie.parse function mishandles security prefixes in cookie names, allowing attackers to bypass cookie security mechanisms. It affects Ruby versions through 2.6.8 and the CGI gem through 0.3.0, potentially enabling session hijacking or privilege escalation in web applications using CGI cookie parsing.
💻 Affected Systems
- Ruby
- CGI gem
📦 What is this software?
Cgi by Ruby Lang
Cgi by Ruby Lang
Cgi by Ruby Lang
Factory by Opensuse
Fedora by Fedoraproject
Fedora by Fedoraproject
Leap by Opensuse
Ruby by Ruby Lang
Ruby by Ruby Lang
Ruby by Ruby Lang
⚠️ Risk & Real-World Impact
Worst Case
Attackers could bypass cookie security controls to hijack user sessions, escalate privileges, or perform actions as authenticated users without proper authorization.
Likely Case
Session hijacking where attackers can impersonate legitimate users and access their accounts or sensitive data.
If Mitigated
Limited impact if applications implement additional authentication layers, rate limiting, and proper session management outside of cookie parsing.
🎯 Exploit Status
Exploitation requires the attacker to craft malicious cookie names with security prefixes. The HackerOne report provides technical details.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Ruby 2.6.9+, CGI gem 0.3.1+
Vendor Advisory: https://www.ruby-lang.org/en/news/2021/10/15/cookie-security-bypass-cve-2021-41819/
Restart Required: Yes
Instructions:
1. Update Ruby to version 2.6.9 or later. 2. Update CGI gem to version 0.3.1 or later. 3. Restart all Ruby applications and services.
🔧 Temporary Workarounds
Implement custom cookie validation
allAdd server-side validation to reject cookies with suspicious names or prefixes before parsing.
# Example Ruby code to validate cookie names
# Add this before CGI::Cookie.parse calls
cookies.each do |name, value|
if name.start_with?('__Secure-', '__Host-')
# Reject or handle specially
end
end
🧯 If You Can't Patch
- Implement Web Application Firewall (WAF) rules to block requests with malicious cookie patterns.
- Use alternative session management mechanisms that don't rely on CGI cookie parsing.
🔍 How to Verify
Check if Vulnerable:
Check Ruby version with 'ruby -v' and CGI gem version with 'gem list cgi'. If Ruby <= 2.6.8 or CGI gem <= 0.3.0, you are vulnerable if using CGI::Cookie.parse.Check Version:
ruby -v && gem list cgiVerify Fix Applied:
After updating, verify with 'ruby -v' shows >= 2.6.9 and 'gem list cgi' shows >= 0.3.1. Test cookie parsing functionality in your application.📡 Detection & Monitoring
Log Indicators:
- Unusual cookie names with security prefixes in web server logs
- Multiple failed authentication attempts followed by successful logins from different IPs
Network Indicators:
- HTTP requests containing cookies with manipulated __Secure- or __Host- prefixes
- Abnormal session cookie patterns
SIEM Query:
source="web_server" AND (cookie_name="__Secure-*" OR cookie_name="__Host-*") AND status=200🔗 References
- https://hackerone.com/reports/910552
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/IUXQCH6FRKANCVZO2Q7D2SQX33FP3KWN/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/UTOJGS5IEFDK3UOO7IY4OTTFGHGLSWZF/
- https://security.gentoo.org/glsa/202401-27
- https://security.netapp.com/advisory/ntap-20220121-0003/
- https://www.ruby-lang.org/en/news/2021/11/24/cookie-prefix-spoofing-in-cgi-cookie-parse-cve-2021-41819/
- https://hackerone.com/reports/910552
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/IUXQCH6FRKANCVZO2Q7D2SQX33FP3KWN/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/UTOJGS5IEFDK3UOO7IY4OTTFGHGLSWZF/
- https://security.gentoo.org/glsa/202401-27
- https://security.netapp.com/advisory/ntap-20220121-0003/
- https://www.ruby-lang.org/en/news/2021/11/24/cookie-prefix-spoofing-in-cgi-cookie-parse-cve-2021-41819/
