CVE-2025-27220: Ruby-lang CGI ReDoS Vulnerability

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

bash

# 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