Ruby’s CGI gem is a widely used library that helps web applications handle HTTP requests and escape HTML. In early 2025, a critical bug was discovered in this gem, classified as CVE-2025-27220. The bug is a "Regular Expression Denial of Service" (ReDoS) issue found specifically in the Util#escapeElement method. In this long read, we’ll explain what this vulnerability is, show a practical exploit, and share how to keep your apps safe.

What Is ReDoS (Regular Expression Denial of Service)?

A Regular Expression Denial of Service (ReDoS) means the attacker feeds crafted input that gets stuck in a time-consuming regex operation. This can freeze up the server, making it unable to handle real users—effectively a denial of service, triggered by just one malicious request.

Where Does the Bug Happen?

In versions of the CGI gem before .4.2, the vulnerability sits in the escapeElement utility, which is used to escape user input for safe output in HTML. This makes it very tempting for attackers to target.

Here’s the bug’s original announcement

- Original advisory on GitHub
- NVD CVE-2025-27220

The Problem Code

Under the hood, Ruby’s CGI gem used a vulnerable regular expression. Here’s what a simplified version looks like:

def escapeElement(string)
  # Vulnerable regex for example
  string.gsub(/(<([a-zA-Z][a-zA-Z-9]*)\b[^>]*>)/, '') 
end

This regex tries to match HTML tags, but if given a massive crafted input — say, thousands of <a characters with no closing >, it gets stuck for a long time trying to decide how to parse it.

Build Malicious Input

Construct a string that matches the start of the vulnerable regex, but doesn’t allow it to finish parsing quickly.

Pass this string into the method.

Here’s a Ruby script to demonstrate

require 'cgi'

# Only vulnerable with CGI gem < .4.2!
evil = "<a" + " " * 50_000 + ">"
puts "Calling escapeElement – this may hang!"
CGI::escapeElement(evil, 'a')
puts "Done"

Running this with a vulnerable CGI gem will make the program hang for a long time.

Real-World Impact

* Any Ruby web app using CGI to sanitize user input can be targeted.
* Attackers don’t need authentication.
* One POST or GET of a crafted string may be enough to block the app (resource exhaustion).
* If your server allows parallel requests, an attacker can use multiple connections to leverage the bug.

If you’re using the CGI gem below .4.2, upgrade now!

gem update cgi

Or:

gem "cgi", ">= .4.2"

Why does this work?

From .4.2 and later, the CGI maintainers fixed the regex to avoid catastrophic backtracking

# Simplified fix – non-greedy, safer regex
string.gsub(/(<[a-z][\s\S]*?>)/im, '')

This safer regex outsmarts the kind of input that causes ReDoS.

References

- GitHub Security Advisory for cgi
- NVD entry for CVE-2025-27220
- How ReDoS Works

Summary

CVE-2025-27220 is a clear reminder that even seemingly safe code like user-input sanitization can be dangerous if built on unsafe regexes.
If you use Ruby’s CGI gem, check your version and upgrade ASAP.
Simple crafted input can put your uptime—and your reputation—at risk.

Stay safe: keep libraries up to date, and always test user-facing code for edge cases and performance traps!

Timeline

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