CVE-2021-41816: Ruby CGI Buffer Overflow Vulnerability

CVE-2021-41816 Overview

CVE-2021-41816 is an integer overflow vulnerability in the CGI.escape_html function found in Ruby versions before 2.7.5 and 3.x before 3.0.3. The vulnerability occurs when processing a long string on platforms where size_t and long have different numbers of bytes, such as Windows systems. This integer overflow leads to a resultant buffer overflow, potentially enabling attackers to execute arbitrary code or cause denial of service conditions. The vulnerability also affects the CGI gem before version 0.3.1 for Ruby.

Critical Impact

This vulnerability allows remote attackers to exploit the integer overflow via network-accessible Ruby applications, potentially achieving arbitrary code execution with no authentication required and no user interaction necessary.

Affected Products

• Ruby-lang CGI gem (versions before 0.3.1)
• Ruby-lang Ruby (versions before 2.7.5 and 3.x before 3.0.3)
• Fedoraproject Fedora (versions 34 and 35)

Discovery Timeline

• 2021-11-24 - Ruby-lang releases security advisory and patch
• 2022-02-06 - CVE-2021-41816 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-41816

Vulnerability Analysis

The vulnerability resides in the CGI.escape_html function, which is commonly used in Ruby web applications to sanitize HTML output and prevent cross-site scripting attacks. On platforms where size_t and long data types have different byte sizes (notably Windows systems using 64-bit architecture), an attacker can trigger an integer overflow by supplying an exceptionally long input string.

When the CGI.escape_html function calculates the required buffer size for the escaped output, it performs arithmetic operations that can overflow when the input string is sufficiently large. This integer overflow causes the function to allocate a buffer that is significantly smaller than required. Subsequent write operations to this undersized buffer result in a heap-based buffer overflow, potentially corrupting adjacent memory regions.

The vulnerability is particularly dangerous because CGI.escape_html is a fundamental security function used throughout Ruby web applications. Any application that processes user-controlled input through this function on affected platforms becomes vulnerable.

Root Cause

The root cause is an integer overflow vulnerability (CWE-190) in the buffer size calculation within CGI.escape_html. The function fails to properly validate or handle the arithmetic overflow that occurs when computing the output buffer size for very long input strings on platforms with mismatched size_t and long data type sizes. This improper integer handling leads to an undersized buffer allocation, which is subsequently overwritten during the HTML escaping process.

Attack Vector

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

1. Identifying a Ruby web application running on a vulnerable platform (Windows or other platforms where size_t and long differ in size)
2. Crafting a malicious HTTP request containing an extremely long string in a parameter that gets processed by CGI.escape_html
3. Submitting the request to trigger the integer overflow and subsequent buffer overflow
4. Potentially achieving arbitrary code execution or causing application crash

The exploitation is facilitated by the fact that CGI.escape_html is often called automatically on user input in many Ruby web frameworks, making vulnerable applications widespread.

Detection Methods for CVE-2021-41816

Indicators of Compromise

• Abnormally large HTTP request payloads targeting Ruby web applications
• Application crashes or unexpected termination of Ruby processes after receiving large string inputs
• Memory corruption errors in Ruby application logs
• Unusual memory allocation patterns in Ruby web server processes

Detection Strategies

• Monitor Ruby application logs for buffer overflow or memory corruption errors related to CGI processing
• Implement web application firewall rules to detect and block abnormally large HTTP parameters
• Use runtime application self-protection (RASP) tools to detect exploitation attempts
• Deploy SentinelOne Singularity Platform for real-time memory corruption detection and prevention

Monitoring Recommendations

• Set up alerts for Ruby process crashes on Windows servers running affected versions
• Monitor network traffic for unusually large HTTP requests to Ruby applications
• Implement logging for all CGI.escape_html calls processing external input
• Review application logs for signs of integer overflow or buffer boundary violations

How to Mitigate CVE-2021-41816

Immediate Actions Required

• Upgrade Ruby to version 2.7.5, 3.0.3, or later immediately
• Update the CGI gem to version 0.3.1 or later for Ruby applications using it as a standalone gem
• Conduct an inventory of all Ruby applications running on Windows or other affected platforms
• Apply input length validation as a defense-in-depth measure while patching

Patch Information

Ruby-lang has released patched versions addressing this vulnerability. The official security advisory is available at the Ruby-lang Security Advisory. Additional advisories have been published by Fedora, Debian, Gentoo, and NetApp. Organizations should apply vendor-specific patches according to their distribution.

Workarounds

• Implement strict input length validation before processing user input through CGI.escape_html
• Consider using alternative HTML escaping libraries that are not affected by this vulnerability as a temporary measure
• Deploy web application firewalls to reject requests with abnormally large parameters
• Restrict access to affected Ruby applications from untrusted networks until patching is complete

# Ruby version upgrade example
# Check current Ruby version
ruby --version

# Update Ruby using rbenv
rbenv install 2.7.5
rbenv global 2.7.5

# Or update Ruby using rvm
rvm install 2.7.5
rvm use 2.7.5 --default

# Update CGI gem for standalone installations
gem update cgi