CVE-2025-27219: Ruby-lang CGI Gem DoS Vulnerability

CVE-2025-27219 Overview

CVE-2025-27219 is a Denial of Service (DoS) vulnerability in the Ruby CGI gem affecting versions before 0.4.2. The vulnerability exists in the CGI::Cookie.parse method, which fails to impose any limit on the length of raw cookie values it processes. This oversight allows attackers to cause excessive resource consumption by sending HTTP requests with extremely large cookies, potentially rendering affected web applications unresponsive.

Critical Impact

Attackers can exhaust server resources by sending maliciously crafted HTTP requests containing oversized cookies, leading to service disruption for Ruby web applications utilizing the CGI gem.

Affected Products

• Ruby CGI gem versions prior to 0.4.2
• Ruby CGI gem version 0.3.6
• Applications using CGI::Cookie.parse method for cookie handling

Discovery Timeline

• 2025-03-04 - CVE-2025-27219 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-27219

Vulnerability Analysis

This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling). The CGI::Cookie.parse method in the Ruby CGI library is responsible for parsing raw cookie strings from HTTP requests into structured Cookie objects. The flaw stems from the method's acceptance of arbitrarily large cookie values without implementing any bounds checking or size limitations.

When a web application processes incoming HTTP requests containing cookies, the CGI library parses these values. An attacker can exploit this behavior by crafting requests with exceptionally large cookie values, forcing the parser to allocate significant memory resources. This can lead to memory exhaustion, CPU starvation during parsing operations, and ultimately service degradation or complete denial of service.

Root Cause

The root cause is the absence of input validation on cookie value length within the CGI::Cookie.parse method. The method processes the entire raw cookie string regardless of size, allocating memory proportional to the input without any defensive limits. This design oversight violates the principle of resource allocation constraints, allowing unbounded consumption of server resources.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can craft malicious HTTP requests containing oversized Cookie headers and send them to any Ruby web application that utilizes the vulnerable CGI gem for cookie parsing.

The exploitation is straightforward: by including extremely large cookie values (potentially megabytes or more) in HTTP requests, an attacker forces the target application to allocate excessive memory and CPU cycles during the parsing operation. Repeated requests can amplify the effect, exhausting server resources and causing service unavailability for legitimate users.

Since this is a resource exhaustion attack, the vulnerability can be exploited remotely without requiring any special privileges or complex attack chains. The attacker simply needs network access to the target web application's HTTP endpoint.

Detection Methods for CVE-2025-27219

Indicators of Compromise

• Unusually large Cookie headers in HTTP access logs (exceeding typical sizes of a few kilobytes)
• Sudden spikes in memory usage correlated with incoming web requests
• Application server slowdowns or crashes without other apparent causes
• Repeated requests from the same source with abnormally large payloads

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block HTTP requests with Cookie headers exceeding reasonable size thresholds
• Monitor application logs for parsing errors or timeouts related to cookie processing
• Deploy network intrusion detection systems (NIDS) with signatures for oversized HTTP headers
• Use application performance monitoring (APM) tools to correlate resource spikes with specific request patterns

Monitoring Recommendations

• Set up alerts for memory usage anomalies on servers running Ruby web applications
• Configure logging to capture full Cookie header sizes for forensic analysis
• Monitor request latency metrics to identify degradation patterns indicative of DoS attacks
• Implement rate limiting at the load balancer level to mitigate volumetric attacks

How to Mitigate CVE-2025-27219

Immediate Actions Required

• Upgrade the Ruby CGI gem to version 0.4.2 or later immediately
• Audit applications to identify all instances using CGI::Cookie.parse method
• Implement request size limits at the web server or reverse proxy level
• Review and apply security patches for the Ruby environment

Patch Information

The vulnerability has been addressed in CGI gem version 0.4.2, which introduces proper bounds checking on cookie value lengths. Organizations should update their gem dependencies by running bundle update cgi or specifying the patched version in their Gemfile. Additional information is available through the GitHub Ruby Advisory and the HackerOne Report #2936778. Debian users should refer to the Debian LTS Security Announcement for distribution-specific patch information.

Workarounds

• Configure web servers (Nginx, Apache) to limit maximum Cookie header size to a reasonable threshold (e.g., 8KB)
• Implement middleware or request filters to validate and reject oversized cookies before they reach the CGI parsing layer
• Use a reverse proxy with request size limiting capabilities to provide defense-in-depth
• Consider temporarily disabling cookie-dependent functionality if immediate patching is not feasible

# Nginx configuration to limit cookie header size
# Add to server or location block
large_client_header_buffers 4 8k;
client_header_buffer_size 1k;

# Update Ruby CGI gem to patched version
gem update cgi --version '>= 0.4.2'

# Or update via Bundler
bundle update cgi