CVE-2022-24724: GitHub Cmark-gfm RCE Vulnerability

CVE-2022-24724 Overview

CVE-2022-24724 is a critical integer overflow vulnerability in cmark-gfm, GitHub's extended version of the C reference implementation of CommonMark. The vulnerability exists in the table row parsing function table.c:row_from_string and can lead to heap memory corruption when parsing markdown tables whose marker rows contain more than UINT16_MAX (65,535) columns.

Critical Impact

This vulnerability enables Remote Code Execution (RCE) in applications that use cmark-gfm to render user-controlled markdown content, with impacts ranging from information disclosure to arbitrary code execution.

Affected Products

• GitHub cmark-gfm versions prior to 0.29.0.gfm.3
• GitHub cmark-gfm versions prior to 0.28.3.gfm.21
• Fedora 34, 35, and 36 (via bundled cmark-gfm packages)

Discovery Timeline

• 2022-03-03 - CVE-2022-24724 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-24724

Vulnerability Analysis

This vulnerability is classified as CWE-190 (Integer Overflow or Wraparound). The flaw resides in cmark-gfm's table extension, specifically within the row_from_string function in table.c. When parsing markdown tables, the function uses a 16-bit unsigned integer (UINT16) to track the number of columns in a table's marker row.

When a maliciously crafted markdown table contains more than 65,535 columns (UINT16_MAX), the column counter overflows, wrapping around to a small value. This integer overflow leads to incorrect memory allocation calculations and subsequent heap memory corruption. The severity of exploitation depends on the application's memory layout and how cmark-gfm is integrated.

Root Cause

The root cause is insufficient bounds checking on the column count variable during table parsing. The row_from_string function stores the column count in a 16-bit integer type, which cannot safely represent values exceeding 65,535. When an attacker supplies a table with an excessive number of columns, the counter overflows, causing the allocated buffer size to be significantly smaller than required. Subsequent write operations then corrupt adjacent heap memory.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by submitting specially crafted markdown content containing a table with more than UINT16_MAX columns to any application that uses vulnerable versions of cmark-gfm for markdown rendering.

The exploitation flow involves:

1. Crafting a markdown table with an excessive number of pipe characters (|) in the marker row
2. Submitting this malicious markdown to a vulnerable application
3. When cmark-gfm parses the table, the integer overflow occurs
4. The resulting heap corruption can be leveraged for information leakage or arbitrary code execution

For detailed technical analysis of this vulnerability, refer to the GitHub Security Advisory GHSA-mc3g-88wq-6f4x and the Packet Storm Exploit Report.

Detection Methods for CVE-2022-24724

Indicators of Compromise

• Unusual markdown content containing extremely long lines with thousands of pipe (|) characters
• Application crashes or unexpected behavior when processing markdown content
• Memory-related errors or segmentation faults in processes using cmark-gfm
• Log entries showing heap corruption or memory allocation failures in markdown parsing components

Detection Strategies

• Monitor for markdown input containing anomalously high numbers of table columns or pipe characters
• Implement runtime memory corruption detection tools (AddressSanitizer, Valgrind) in development and staging environments
• Deploy application-level input validation to reject markdown tables exceeding reasonable column limits
• Use software composition analysis (SCA) tools to identify vulnerable cmark-gfm versions in your codebase

Monitoring Recommendations

• Enable detailed logging for markdown processing components to capture parsing errors
• Monitor system resource usage for processes handling user-submitted markdown content
• Implement anomaly detection for unusually large markdown payloads targeting your applications
• Set up alerts for application crashes or restarts that may indicate exploitation attempts

How to Mitigate CVE-2022-24724

Immediate Actions Required

• Upgrade cmark-gfm to version 0.29.0.gfm.3 or 0.28.3.gfm.21 (or later) immediately
• If immediate patching is not possible, disable the table extension in cmark-gfm as a temporary workaround
• Audit your applications to identify all instances where cmark-gfm is used to render user-controlled markdown
• Implement input validation to limit the size and complexity of markdown tables before processing

Patch Information

GitHub has released patched versions of cmark-gfm that address this vulnerability:

• Version 0.29.0.gfm.3 - for the 0.29.x branch
• Version 0.28.3.gfm.21 - for the 0.28.x branch

Fedora users should apply the latest security updates for their respective Fedora versions (34, 35, 36) through the standard package management system. Multiple Fedora Package Announcements have been issued addressing this vulnerability.

Workarounds

• Disable the table markdown extension in cmark-gfm configuration if table rendering is not required
• Implement pre-processing validation to reject markdown content with excessive table columns
• Use input sanitization to limit the maximum number of columns in markdown tables before passing to cmark-gfm
• Consider running cmark-gfm in a sandboxed environment to limit potential RCE impact

# Example: Disable table extension when invoking cmark-gfm
# Check your application's cmark-gfm configuration and disable CMARK_OPT_TABLE_PREFER_STYLE_ATTRIBUTES
# or remove CMARK_EXT_TABLE from enabled extensions

# For applications using cmark-gfm CLI, avoid table extension:
cmark-gfm --unsafe input.md  # Review extensions enabled by default

# Update cmark-gfm via package manager (Fedora example)
sudo dnf update cmark-gfm