CVE-2025-27219

Executive Summary

CVE-2025-27219 is a Denial of Service (DoS) vulnerability in the Ruby CGI gem, specifically within the CGI::Cookie.parse method. The flaw allows an unauthenticated attacker to cause excessive resource consumption by sending an extremely large cookie value, leading to potential service disruption. The vulnerability affects versions prior to 0.4.2. The CVSS score is 5.8 (Medium), and the EPSS score is 0.0%, indicating low exploitation likelihood currently. No public proof-of-concept or exploit code is available.

Supply Chain & Package Security

This vulnerability affects the Ruby CGI gem, a core library for handling CGI requests. Applications using this gem to parse cookies are at risk. The supply chain risk is moderate, as the gem is widely used in Ruby web applications. Organizations should verify their dependency versions and update to the patched version (0.4.2) to mitigate the risk.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• Base Score: 5.8 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: Low (A:L)

The threat level is considered Medium due to the low complexity and network-based attack vector, but the impact is limited to availability (DoS) with no data breach risk.

Deep Dive & Mechanics

The vulnerability resides in the CGI::Cookie.parse method of the Ruby CGI gem. This method is responsible for parsing raw cookie strings from HTTP requests. The flaw is that the method does not impose any limit on the length of the raw cookie value it processes. When an attacker sends a crafted HTTP request with an extremely large cookie value (e.g., millions of characters), the parsing process consumes excessive CPU and memory resources, potentially causing the server to become unresponsive or crash.

The root cause is the lack of input validation or length checking on the cookie value before parsing. The method iterates over the entire string without any bounds, leading to resource exhaustion.

Vulnerable vs. Patched Code

Vulnerable Code (CGI gem < 0.4.2):

# In CGI::Cookie.parse
def self.parse(raw_cookie)
  cookies = {}
  raw_cookie.split(';').each do |pair|
    # No length check on raw_cookie
    key, value = pair.split('=', 2)
    cookies[key.strip] = value.strip if key && value
  end
  cookies
end

Patched Code (CGI gem >= 0.4.2):

# In CGI::Cookie.parse
MAX_COOKIE_LENGTH = 4096  # Example limit

def self.parse(raw_cookie)
  raise ArgumentError, "Cookie too long" if raw_cookie.length > MAX_COOKIE_LENGTH
  cookies = {}
  raw_cookie.split(';').each do |pair|
    key, value = pair.split('=', 2)
    cookies[key.strip] = value.strip if key && value
  end
  cookies
end

The patched version introduces a maximum length check (e.g., 4096 characters) on the raw cookie string, preventing excessively large values from being processed.

Exploitation & Lab Setup

Red Team

Exploitation Steps:

1. Identify a target Ruby web application using the CGI gem (version < 0.4.2).
2. Craft an HTTP request with an extremely large cookie value (e.g., 10 million characters).
3. Send the request to the target server.
4. Monitor server resource consumption (CPU/memory) to confirm DoS.

Example Request:

GET / HTTP/1.1
Host: target.com
Cookie: session=AAAA... (10 million 'A' characters)

Note: No public exploit code is available. This is a conceptual demonstration.

Blue Team

Detection and Mitigation:

1. Monitor for HTTP requests with abnormally large cookie values (e.g., > 4096 bytes).
2. Implement web application firewall (WAF) rules to block requests with oversized cookies.
3. Upgrade the CGI gem to version 0.4.2 or later.
4. Use rate limiting to mitigate potential DoS attacks.

Lab Setup:

• Use a vulnerable Ruby application (e.g., Sinatra with CGI gem 0.4.1).
• Simulate attack using curl or custom script.
• Monitor with tools like top, htop, or netstat.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests with Cookie header values exceeding 4096 bytes.
  • Repeated requests from the same IP with large cookie values.
• System IOCs:
  • High CPU usage by Ruby processes.
  • Memory exhaustion leading to OOM (Out of Memory) killer events.
  • Application logs showing ArgumentError: Cookie too long (if patched) or timeout errors.

Detection Engineering

YARA Rule

rule CVE_2025_27219_Cookie_DoS {
    meta:
        description = "Detects attempts to exploit CVE-2025-27219 via oversized cookie values"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-27219"
    strings:
        $cookie_header = /Cookie:\s*.{4097,}/ nocase
    condition:
        $cookie_header
}

Sigma Rule

title: CVE-2025-27219 - Oversized Cookie Value DoS Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests with cookie values exceeding 4096 bytes, indicating potential exploitation of CVE-2025-27219.
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: generic
detection:
    selection:
        cs-method: 'GET'
        cs-uri-stem: '*'
        cs(Cookie)|length: > 4096
    condition: selection
falsepositives:
    - Legitimate applications with large cookies (rare)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-27219 - Oversized Cookie Value DoS Attempt"; flow:to_server; http.cookie; content:"!"; pcre:"/^.{4097,}/R"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Upgrade the CGI gem to version 0.4.2 or later.

   gem update cgi
   

2. Workaround: If immediate upgrade is not possible, implement input validation in application code to reject cookies longer than a safe limit (e.g., 4096 bytes).
3. Network Level: Deploy WAF rules to block requests with oversized cookie headers.
4. Monitoring: Enable logging and alerting for large cookie values.
5. Long-term: Regularly update dependencies and perform security audits.

Valters IT