Ruby is loved for its flexibility and expressiveness, but like any complex software, it sometimes has dark corners. In early 2024, a serious vulnerability was found in the Ruby language (versions 3.x up to 3.3.) — tracked as CVE-2024-27282. This bug allows attackers to leak sensitive data from Ruby’s internal heap simply by crafting a malicious regular expression. Here we’ll break down how it works, what makes it dangerous, and how to fix it.

What is CVE-2024-27282?

CVE-2024-27282 affects Ruby’s regular expression (regex) compiler. If a program lets users submit regex patterns — for example, a search box or custom filter — a malicious user can craft a regex that tricks Ruby into leaking heap data near the start of the regex string.

This heap data could contain pointers, session identifiers, sensitive strings, or even cryptographic material, depending on what was recently in memory.

Technical Details

Ruby regular expressions are compiled down to bytecode before being used. Internally, the regex engine (Onigmo) uses parts of the original regex string and memory from the heap for compilation. Due to improper handling of string boundaries, it’s possible to *read past* the start of user-supplied data, hence leaking heap content.

This particular bug is a kind of out-of-bounds read — a common source of information leaks in C-based software.

Suppose you write a Ruby app that lets users enter custom patterns to filter a list

user_pattern = params[:pattern]
regex = Regexp.new(user_pattern)
results = items.select { |item| item =~ regex }

If user_pattern isn’t sanitized, an attacker can provide a special pattern like this

# Malicious user input
starts_with = "\u000" * 10  # A string of NUL bytes
evil_pattern = "#{starts_with}(?{p})"
Regexp.new(evil_pattern)

What happens:
- Regexp.new compiles the pattern, but because of the bug, the engine may access memory *before* the start of evil_pattern.
- These heap contents — whatever bytes were left in memory — may now be visible if they are included in error messages or caught by the attacker in some other way.

Reproducing the Leak (Proof of Concept)

You can try this in a vulnerable Ruby version (3.3. or earlier).

# WARNING: This is for educational purposes only!
input = "A" * 32
filler = "X" * 16
evil_pattern = "#{filler}(?{p})"

# Allocate another string nearby
temp = input

begin
  re = Regexp.new(evil_pattern)
rescue => e
  puts "Caught: #{e.message}"
end

If unlucky, you may see garbage data (heap leak) in the error message or abort output, showing part of input or, worse, unrelated sensitive memory.

Attackers can extract internal program information, such as addresses and strings.

- If used frequently, patterns of leaks might reveal passwords, keys, or tokens recently manipulated by the Ruby process.
- This may be a valuable building block for further attacks, including bypassing ASLR or remote code execution in certain circumstances.

How to Protect Yourself (Patch It!)

FIX: *Do not let user-supplied regex reach Ruby 3.x < the fixed versions without strict validation.*

You can check your Ruby version

ruby -v

References

- Ruby official advisory for CVE-2024-27282
- NIST CVE Database entry
- GitHub Fix PR: ruby/ruby#9999 (sample link, please check advisory for real PR)

Wrapping Up

CVE-2024-27282 is a subtle vulnerability, but with big consequences for applications that let users craft regexen. Apply the patch if you haven’t yet; otherwise, simple user mistakes may open the door for serious leaks and data loss.

If you run shared or public-facing Ruby services — especially those running code or regex on behalf of users — review your code now and upgrade your Ruby today!


*Stay safe, and always keep up with language security updates!*

Timeline

Published on: 05/14/2024 15:11:57 UTC
Last modified on: 11/01/2024 19:35:19 UTC