CVE-2021-41816
📋 TL;DR
This vulnerability in Ruby's CGI.escape_html function allows integer overflow and buffer overflow when processing very long strings on platforms where size_t and long have different byte sizes (like Windows). Attackers can exploit this to execute arbitrary code or cause denial of service. Affected are Ruby applications using CGI.escape_html on vulnerable Ruby versions or the CGI gem.
💻 Affected Systems
- Ruby
- Ruby CGI gem
📦 What is this software?
Cgi by Ruby Lang
Cgi by Ruby Lang
Cgi by Ruby Lang
Fedora by Fedoraproject
Fedora by Fedoraproject
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data theft, or ransomware deployment.
Likely Case
Denial of service through application crashes or limited remote code execution in constrained environments.
If Mitigated
Application instability or crashes without code execution if memory protections are enabled.
🎯 Exploit Status
Exploitation requires sending specially crafted long strings to applications using CGI.escape_html. Public HackerOne report demonstrates the issue.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Ruby 2.7.5, Ruby 3.0.3, CGI gem 0.3.1
Vendor Advisory: https://www.ruby-lang.org/en/news/2021/11/24/buffer-overrun-in-cgi-escape_html-cve-2021-41816/
Restart Required: Yes
Instructions:
1. Update Ruby to 2.7.5+ or 3.0.3+ using your package manager. 2. Update CGI gem to 0.3.1+ via 'gem update cgi'. 3. Restart all Ruby applications and services.
🔧 Temporary Workarounds
Input validation and length limiting
allImplement input validation to reject or truncate excessively long strings before they reach CGI.escape_html.
Use alternative HTML escaping
allReplace CGI.escape_html with alternative HTML escaping libraries like Rack::Utils.escape_html or ERB::Util.html_escape.
🧯 If You Can't Patch
- Implement WAF rules to block requests with extremely long parameter values
- Isolate vulnerable applications in network segments with strict egress filtering
🔍 How to Verify
Check if Vulnerable:
Check Ruby version with 'ruby -v' and CGI gem version with 'gem list cgi'. If Ruby <2.7.5 or 3.x <3.0.3, or CGI gem <0.3.1, you are vulnerable.Check Version:
ruby -v && gem list cgiVerify Fix Applied:
After update, verify Ruby version is 2.7.5+ or 3.0.3+ and CGI gem is 0.3.1+. Test application functionality with normal inputs.📡 Detection & Monitoring
Log Indicators:
- Application crashes or segmentation faults in Ruby processes
- Unusually long parameter values in web logs (10,000+ characters)
- Memory allocation errors in application logs
Network Indicators:
- HTTP requests with extremely long parameter values
- Repeated requests causing application instability
SIEM Query:
source="web_logs" AND (param_length>10000 OR status=500) AND process="ruby"🔗 References
- https://hackerone.com/reports/1328463
- 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-tracker.debian.org/tracker/CVE-2021-41816
- https://security.gentoo.org/glsa/202401-27
- https://security.netapp.com/advisory/ntap-20220303-0006/
- https://www.ruby-lang.org/en/news/2021/11/24/buffer-overrun-in-cgi-escape_html-cve-2021-41816/
- https://hackerone.com/reports/1328463
- 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-tracker.debian.org/tracker/CVE-2021-41816
- https://security.gentoo.org/glsa/202401-27
- https://security.netapp.com/advisory/ntap-20220303-0006/
- https://www.ruby-lang.org/en/news/2021/11/24/buffer-overrun-in-cgi-escape_html-cve-2021-41816/
