CVE-2022-28738: Ruby-lang Ruby Use After Free Vulnerability

CVE-2022-28738 Overview

A double free vulnerability was discovered in the Regexp compiler in Ruby 3.x before 3.0.4 and 3.1.x before 3.1.2. If a victim attempts to create a Regexp from untrusted user input, an attacker may be able to write to unexpected memory locations, potentially leading to remote code execution or denial of service.

Critical Impact

This double free vulnerability in Ruby's Regexp compiler allows attackers to potentially achieve remote code execution by supplying crafted regular expression patterns, affecting all Ruby 3.x versions prior to the patched releases.

Affected Products

• Ruby 3.x versions before 3.0.4
• Ruby 3.1.x versions before 3.1.2
• Ruby-lang Ruby (various distributions including Debian and Gentoo)

Discovery Timeline

• 2022-04-12 - Ruby releases security advisory for CVE-2022-28738
• 2022-05-09 - CVE-2022-28738 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-28738

Vulnerability Analysis

This vulnerability is classified as CWE-415 (Double Free), a memory corruption issue that occurs when memory is freed more than once. In the context of Ruby's Regexp compiler, the flaw exists in how the compiler handles memory allocation and deallocation during regular expression processing.

When processing certain regular expression patterns, the Ruby Regexp compiler incorrectly frees the same memory region twice. This double free condition can corrupt the memory allocator's internal data structures, leading to undefined behavior. An attacker who can influence the regular expression patterns being compiled can potentially exploit this condition to achieve arbitrary memory writes.

The vulnerability is particularly dangerous because regular expressions are commonly used to validate user input in web applications, meaning the attack surface is substantial in applications that construct Regexp objects from user-supplied data.

Root Cause

The root cause is a memory management error in Ruby's Regexp compilation process where the same memory block is deallocated twice. This occurs due to improper tracking of memory ownership during the compilation of complex regular expression patterns. When memory is freed more than once, the heap allocator's metadata becomes corrupted, creating conditions for exploitation.

Attack Vector

The attack vector is network-based and requires no privileges or user interaction. An attacker can exploit this vulnerability by:

1. Identifying a Ruby application that creates Regexp objects from user-controlled input
2. Crafting a malicious regular expression pattern that triggers the double free condition
3. Submitting this pattern to the vulnerable application
4. The double free corrupts heap metadata, potentially allowing the attacker to write to arbitrary memory locations

The vulnerability manifests during Regexp compilation when processing untrusted input. Attackers can craft specific regular expression patterns that trigger the memory management flaw. For detailed technical analysis, refer to the HackerOne Report #1220911 and the official Ruby security advisory.

Detection Methods for CVE-2022-28738

Indicators of Compromise

• Unexpected application crashes or segmentation faults in Ruby processes, particularly during regex operations
• Memory corruption errors in application logs related to Regexp compilation
• Unusual memory consumption patterns in Ruby applications processing regular expressions
• Application behavior anomalies when handling user-supplied pattern matching input

Detection Strategies

• Monitor Ruby application logs for memory allocation errors or double-free warnings
• Implement runtime memory sanitizers (such as AddressSanitizer) in development and testing environments to detect double-free conditions
• Use SentinelOne's behavioral AI to detect anomalous memory access patterns indicative of heap corruption exploits
• Audit codepaths where user input flows into Regexp.new() or similar regex construction methods

Monitoring Recommendations

• Enable verbose logging for Ruby applications to capture memory-related errors
• Deploy application performance monitoring (APM) solutions to track unusual memory behavior in production
• Implement alerting for application crashes in Ruby services, especially those handling user input for pattern matching
• Review web application firewall (WAF) logs for suspicious regex patterns in request parameters

How to Mitigate CVE-2022-28738

Immediate Actions Required

• Upgrade Ruby to version 3.0.4 or later for Ruby 3.0.x installations
• Upgrade Ruby to version 3.1.2 or later for Ruby 3.1.x installations
• Audit applications to identify code paths where user input is used to construct Regexp objects
• Implement input validation and sanitization for any user-controlled data used in regular expressions

Patch Information

Ruby has released patched versions that address this vulnerability:

• Ruby 3.0.4 - Contains the fix for Ruby 3.0.x branch
• Ruby 3.1.2 - Contains the fix for Ruby 3.1.x branch

Organizations should apply these updates immediately. For detailed patch information, consult the Ruby security advisory. Distribution-specific advisories are available from Debian, Gentoo, and NetApp.

Workarounds

• Avoid constructing Regexp objects directly from untrusted user input until patches can be applied
• Implement strict allowlists for acceptable regex patterns if user input must be used
• Use precompiled, static regular expressions instead of dynamically generated patterns where possible
• Consider using alternative string matching methods that don't require Regexp compilation for untrusted input

# Verify Ruby version and upgrade if vulnerable
ruby --version

# For Ruby version managers (rbenv), upgrade to patched version
rbenv install 3.0.4
rbenv global 3.0.4

# For RVM users
rvm install 3.0.4
rvm use 3.0.4 --default

# Verify the upgrade
ruby --version