CVE-2025-27220
📋 TL;DR
A Regular Expression Denial of Service (ReDoS) vulnerability exists in the CGI gem for Ruby versions before 0.4.2. This vulnerability allows attackers to cause excessive CPU consumption by sending specially crafted input to the Util#escapeElement method, potentially leading to service degradation or unavailability. Affected systems include any Ruby applications using vulnerable versions of the CGI gem.
💻 Affected Systems
- Ruby CGI gem
📦 What is this software?
Cgi by Ruby Lang
Cgi by Ruby Lang
Cgi by Ruby Lang
⚠️ Risk & Real-World Impact
Worst Case
Complete service unavailability due to CPU exhaustion, leading to denial of service for legitimate users and potential cascading failures in dependent systems.
Likely Case
Performance degradation and intermittent service disruptions when attackers send malicious payloads to vulnerable endpoints.
If Mitigated
Minimal impact with proper input validation, rate limiting, and monitoring in place to detect and block malicious patterns.
🎯 Exploit Status
Exploitation requires sending specially crafted input to endpoints using the vulnerable method, which is straightforward for attackers.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 0.4.2
Vendor Advisory: https://github.com/rubysec/ruby-advisory-db/blob/master/gems/cgi/CVE-2025-27220.yml
Restart Required: Yes
Instructions:
1. Update the CGI gem to version 0.4.2 or later using: gem update cgi
2. Restart all Ruby applications using the CGI gem
3. Verify the update with: gem list cgi
🔧 Temporary Workarounds
Input validation and sanitization
allImplement strict input validation to reject or sanitize potentially malicious patterns before they reach the vulnerable method.
Rate limiting
allImplement rate limiting on endpoints using the CGI gem to prevent attackers from sending multiple malicious requests.
🧯 If You Can't Patch
- Implement Web Application Firewall (WAF) rules to block patterns known to trigger ReDoS vulnerabilities
- Monitor CPU usage and implement alerting for abnormal spikes that could indicate exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check the CGI gem version with: gem list | grep cgi
If version is below 0.4.2, the system is vulnerable.Check Version:
gem list | grep cgiVerify Fix Applied:
After updating, verify with: gem list | grep cgi
Should show version 0.4.2 or higher.📡 Detection & Monitoring
Log Indicators:
- Unusually long processing times for CGI requests
- High CPU usage spikes correlated with specific request patterns
- Repeated failed requests with similar payloads
Network Indicators:
- Multiple requests with similar patterns targeting CGI endpoints
- Unusual request sizes or patterns in HTTP traffic
SIEM Query:
source="web_logs" AND (uri="*cgi*" OR process="ruby") AND (duration>5000 OR cpu_usage>90)