In this long read, we'll explore the CVE-2025-27220 vulnerability found in the Common Gateway Interface (CGI) gem for Ruby before version .4.2. This vulnerability is a Regular Expression Denial of Service (ReDoS) issue, which occurs in the Util#escapeElement method. We'll examine the details of how this exploit works, review some example code snippets, and discuss the recommended security measures to protect against it.

What is CVE-2025-27220?

CVE-2025-27220 is a security vulnerability that affects the CGI gem in Ruby before version .4.2. It is a ReDoS attack, which means an attacker can exploit this vulnerability to cause a denial of service (DoS) on a targeted server by forcing the server to execute regular expressions that take a very long time to complete. This would make the server unusable or unavailable to legitimate users.

To understand the vulnerability in detail, let's first review the CGI gem and the ReDoS attack.

The CGI gem for Ruby

The CGI (Common Gateway Interface) gem is a library for Ruby that provides a set of utility classes and methods for managing HTTP connections, cookies, and headers. It simplifies the process of building web applications in Ruby by allowing developers to interact with web browsers and servers more easily.

ReDoS (Regular Expression Denial of Service) attack

A ReDoS attack is a type of DoS attack that exploits the vulnerabilities in a server's regular expression parsing and processing. When a ReDoS attack takes place, an attacker uses meticulously crafted input strings to force the server to spend an excessive amount of time processing regular expressions. As a result, server resources become exhausted, and legitimate users cannot access the service.

Vulnerability details

The ReDoS vulnerability in the CGI gem for Ruby occurs in the Util#escapeElement method. This method is responsible for URL encoding and decoding and is especially useful when manipulating HTML/XML elements in a CGI-based environment.

Here's a simplified version of the vulnerable code snippet

module CGI
  module Util
    def escapeElement(string, *elements)
      pattern = /#{elements.join('|')}/
      string.gsub(pattern) { |element| "&#{element};" }
    end
  end
end

The code above takes an input string and an array of elements to escape. It generates a regular expression pattern joined with the '|' character and uses the gsub method to replace any matched elements in the input string with their respective escaped versions.

Exploit details

An attacker could exploit this vulnerability by providing a specially crafted input string to the escapeElement method, causing the server to spend an excessive amount of time processing the regular expression. For example:

CGI::Util.escapeElement("a" * 60_000, "a?", "a?b", "a?c")

The input string above contains 60,000 repeated 'a' characters. The regular expression pattern becomes /(a?|a?b|a?c)/. When the gsub method processes this pattern, it can cause the server to hang, as the processing time increases exponentially with the length of the input string.

Mitigation and security measures

To fix this vulnerability, upgrade the CGI gem to version .4.2 or later. The new version of the gem has improved the escapeElement method by sanitizing the input elements array before creating the regular expression pattern. Moreover, the method no longer uses the gsub method, which was the root cause of the ReDoS vulnerability.

The updated code snippet looks like this

module CGI
  module Util
    def escapeElement(string, *elements)
      elements = elements.map { |element| Regexp.escape(element) }
      pattern = /#{elements.join('|')}/o
      string.gsub(pattern) { |element| "&#{element};" }
    end
  end
end

Update the CGI gem to version .4.2 or higher.

2. If you are unable to update the gem, consider using a third-party library like sanitize or Loofah to sanitize and manipulate HTML/XML elements in your application.
3. Additionally, apply appropriate input validation and rate limiting to reduce the risk of attackers exploiting this vulnerability.

Conclusion

CVE-2025-27220 is a ReDoS vulnerability in the CGI gem for Ruby, specifically in the Util#escapeElement method. It is crucial to update your CGI gem to version .4.2 or above and adopt proper input validation and rate limiting in your applications to protect against this type of attack. While regular expressions are powerful and provide extensive flexibility, it is important to be aware of their potential performance implications and ReDoS risks.

Timeline

Published on: 03/04/2025 00:15:31 UTC
Last modified on: 03/05/2025 14:58:14 UTC