CVE-2025-27788 - Out-of-Bounds Read in Ruby's JSON Gem

Ruby’s json gem is a staple for handling JSON data in many Ruby applications. But recently, a critical vulnerability—CVE-2025-27788—was found that could let an attacker crash your application with a specially crafted JSON document. If your project uses versions 2.10. or 2.10.1 of the json gem, you are at risk. The only fix is to upgrade, and there are no workarounds.

This post breaks down exactly what the issue is, shows you how it can be exploited, and offers clear guidance—step by step—on how to secure your Ruby application.

1. What is CVE-2025-27788?

CVE-2025-27788 is a vulnerability in the Ruby json gem. Starting with version 2.10. and before 2.10.2, it's possible to trigger an *out-of-bounds read*—typically causing a crash—by passing a maliciously crafted JSON document.

Patched: 2.10.2

There are no known workarounds other than upgrading the library.

Official advisory:
GitHub Security Advisory GHSA-xxxx-xxxx-xxxx
RubyGems Advisory

2. Why Does This Matter?

If your Ruby app or Rails project parses JSON from users, attackers can use this bug to crash your server. In the worst case, attackers might chain other vulnerabilities, or gain unintended access to memory, though right now only *crashing* is confirmed.

3. The Technical Root

The bug occurs due to bad checking of buffer boundaries while parsing certain JSON input. Basically, json gem’s parser could read outside allocated memory buffers—often resulting in a segmentation fault or abort.

Here’s a conceptual Ruby snippet that demonstrates parsing crashing input (This input is simplified; the actual malicious value can be slightly different):

require 'json'

# This input might trigger the vulnerable code
malicious_json = <<~JSON
  [
    "normal",
    "#{"\xFF" * 100}"
  ]
JSON

begin
  JSON.parse(malicious_json)
rescue => e
  puts "Exception: #{e.class} - #{e.message}"
end

On a vulnerable system (with version 2.10. or 2.10.1), this can crash or cause severe exceptions.

4. Proof-of-Concept Exploit

Here’s a minimal proof-of-concept (PoC) that may crash your Ruby process if the vulnerable gem is installed. Do not run this on production!

gem 'json', '2.10.1' # Explicitly use the vulnerable version
require 'json'

# Crafted invalid UTF-8 sequence can trigger the bug
crash_input = "[\"\\udc00\"]"

puts "Parsing malicious JSON..."
JSON.parse(crash_input)
puts "Done."

If your json gem is vulnerable, this can provoke an out-of-bounds read under the hood. You may see a segmentation fault (Segfault) or another error, instantly terminating the Ruby process.

5. How to Fix

Upgrade immediately to json gem version 2.10.2 or newer.

If your Gemfile looks like this

gem 'json', '2.10.'

Change it to

gem 'json', '>= 2.10.2'

Then run

bundle update json

You should see

Using json 2.10.2

After that, your application will be safe from CVE-2025-27788!

6. Frequently Asked Questions

Q: Am I affected if I use Rails?
A: Many Rails apps include json (directly or indirectly), so you may be at risk. Check with bundle list | grep json or bundle info json.

Q: Are earlier versions at risk?
A: No, versions before 2.10. do not have this problem.

Q: Is there a workaround?
A: None is known. The only fix is to upgrade.

7. More Details & References

- GitHub Advisory
- RubyGems Advisory
- Upstream Fix PR
- Release Notes

Add dependency scanning (like Dependabot or Brakeman) to catch such issues early.

With a little attention, your Ruby app will be safe from CVE-2025-27788. Don’t leave your system vulnerable—update now!

*Feel free to share this post or reference it in team security discussions. If you want more details or have questions, the official GitHub repository and advisories are the best places to look.*

Timeline

Published on: 03/12/2025 14:15:16 UTC
Last modified on: 04/02/2025 12:35:54 UTC