CVE-2025-27220 Overview
CVE-2025-27220 is a Regular Expression Denial of Service (ReDoS) vulnerability discovered in the Ruby CGI gem's Util#escapeElement method. This vulnerability affects versions of the CGI gem prior to 0.4.2 and can be exploited remotely without authentication to cause application denial of service through crafted input that triggers catastrophic backtracking in the regular expression engine.
Critical Impact
Attackers can exploit this vulnerability to cause significant CPU exhaustion and application unavailability by sending specially crafted input to Ruby applications using the vulnerable CGI gem's Util#escapeElement method.
Affected Products
- Ruby CGI gem versions before 0.4.2
- Ruby 3.1.0
- Ruby 3.2.0
Discovery Timeline
- 2025-03-04 - CVE-2025-27220 published to NVD
- 2025-11-03 - Last updated in NVD database
Technical Details for CVE-2025-27220
Vulnerability Analysis
This vulnerability is classified under CWE-1333 (Inefficient Regular Expression Complexity). The Util#escapeElement method in the Ruby CGI gem contains a poorly constructed regular expression that is susceptible to catastrophic backtracking when processing maliciously crafted input strings.
ReDoS vulnerabilities occur when regular expressions contain patterns that can be exploited to cause exponential time complexity during matching operations. In this case, the vulnerable method accepts user-controlled input that gets processed by the regex engine, allowing attackers to submit strings specifically designed to maximize processing time.
The network-accessible nature of this vulnerability means that remote attackers can trigger the denial of service condition without requiring any privileges or user interaction, making it particularly dangerous for internet-facing Ruby web applications.
Root Cause
The root cause of CVE-2025-27220 lies in the implementation of the Util#escapeElement method within the CGI gem. The method utilizes a regular expression pattern that exhibits inefficient complexity characteristics. When provided with adversarial input containing specific character sequences and repetition patterns, the regex engine enters a state of catastrophic backtracking, consuming excessive CPU resources and causing the application to become unresponsive.
Attack Vector
The attack vector for this vulnerability is network-based. An attacker can remotely send specially crafted HTTP requests or input data to any Ruby application that processes user input through the Util#escapeElement method. The attack requires no authentication or special privileges, and no user interaction is necessary for successful exploitation.
The vulnerability allows attackers to craft input strings that exploit the regex backtracking behavior. When such input is processed, the regular expression engine attempts numerous matching combinations, leading to exponential time complexity and effective denial of service. For detailed technical analysis, see the HackerOne Report #2890322.
Detection Methods for CVE-2025-27220
Indicators of Compromise
- Unusually high CPU utilization on application servers running Ruby CGI-based applications
- Application response time degradation or complete unresponsiveness during attack attempts
- Increased memory consumption in Ruby processes handling web requests
- Log entries showing timeout errors or request processing delays in CGI-related code paths
Detection Strategies
- Monitor Ruby application performance metrics for sudden spikes in CPU usage without corresponding increases in legitimate traffic
- Implement request timeout monitoring to detect requests that take abnormally long to process
- Analyze application logs for patterns indicating repeated requests with similar malformed input structures
- Deploy runtime application self-protection (RASP) solutions to detect regex-based DoS attempts
Monitoring Recommendations
- Configure alerting thresholds for CPU utilization anomalies in Ruby application environments
- Implement request duration tracking and alert on requests exceeding normal processing times
- Monitor the CGI gem usage patterns across your application portfolio to identify affected deployments
- Establish baseline metrics for normal application behavior to quickly identify deviation during attacks
How to Mitigate CVE-2025-27220
Immediate Actions Required
- Update the Ruby CGI gem to version 0.4.2 or later immediately across all affected systems
- Audit your Ruby applications to identify all instances where the Util#escapeElement method is used
- Implement input length restrictions on data processed by CGI methods as a temporary protective measure
- Consider implementing request rate limiting to reduce the impact of potential exploitation attempts
Patch Information
The vulnerability has been addressed in CGI gem version 0.4.2. Organizations should update to this version or later to remediate the vulnerability. For Ruby 3.1.0 and 3.2.0 users, ensure that the bundled CGI gem is updated to the patched version. Patch details and upgrade instructions are available in the GitHub Ruby Advisory. Debian users should consult the Debian LTS Advisory for distribution-specific guidance.
Workarounds
- Implement request timeouts at the web server or application level to terminate long-running requests
- Deploy a Web Application Firewall (WAF) with rules to detect and block potential ReDoS payloads
- Add input validation to restrict the length and character composition of data passed to Util#escapeElement
- Consider using alternative HTML escaping methods that do not rely on vulnerable regex patterns
# Update CGI gem to patched version
gem update cgi
# Verify installed version is 0.4.2 or later
gem list cgi
# For Bundler-managed applications, update Gemfile
bundle update cgi
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
