CVE-2021-26813
📋 TL;DR
CVE-2021-26813 is a regular expression denial of service (ReDoS) vulnerability in markdown2, a Python Markdown processor. Attackers can cause extended processing delays by providing specially crafted malicious strings. Any application using vulnerable versions of markdown2 to process untrusted Markdown input is affected.
💻 Affected Systems
- python-markdown2
📦 What is this software?
Fedora by Fedoraproject
Fedora by Fedoraproject
Fedora by Fedoraproject
Markdown2 by Markdown2 Project
⚠️ Risk & Real-World Impact
Worst Case
Complete service unavailability due to CPU exhaustion, potentially causing application timeouts, degraded performance for all users, and cascading failures in dependent systems.
Likely Case
Degraded performance and increased response times for users processing malicious Markdown content, potentially leading to partial service disruption.
If Mitigated
Minimal impact with proper input validation, rate limiting, and monitoring in place to detect and block malicious patterns.
🎯 Exploit Status
Exploitation requires only providing malicious Markdown input. The vulnerability is in the regular expression engine and can be triggered without authentication if the application processes untrusted input.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.4.0
Vendor Advisory: https://github.com/trentm/python-markdown2/pull/387
Restart Required: No
Instructions:
1. Update markdown2 to version 2.4.0 or later using pip: 'pip install --upgrade markdown2>=2.4.0' 2. Verify the update with 'pip show markdown2' 3. Test application functionality with updated version.
🔧 Temporary Workarounds
Input validation and sanitization
allImplement strict input validation to reject or sanitize Markdown input containing suspicious patterns before processing.
Rate limiting and timeouts
allImplement processing timeouts and rate limiting for Markdown processing operations to prevent resource exhaustion.
🧯 If You Can't Patch
- Implement strict input validation to reject Markdown input containing complex nested patterns or excessive backticks
- Deploy Web Application Firewall (WAF) rules to detect and block ReDoS patterns in incoming requests
🔍 How to Verify
Check if Vulnerable:
Check installed markdown2 version: 'pip show markdown2 | grep Version' and verify if version is >=1.0.1.18 and <2.4.0Check Version:
pip show markdown2 | grep VersionVerify Fix Applied:
Verify markdown2 version is 2.4.0 or later: 'pip show markdown2 | grep Version' should show Version: 2.4.0 or higher📡 Detection & Monitoring
Log Indicators:
- Unusually long processing times for Markdown conversion
- High CPU usage spikes during Markdown processing
- Request timeouts on Markdown processing endpoints
Network Indicators:
- Increased response times for Markdown-related API endpoints
- Pattern of repeated Markdown processing requests from single sources
SIEM Query:
source=application_logs "markdown2" AND ("timeout" OR "slow" OR "CPU" > 90%)🔗 References
- https://github.com/trentm/python-markdown2/pull/387
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BRP5RN35JZTSJ3JT4722F447ZDK7LZS5/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/J752422YELXLMLZJPVJVKD2KKHHQRVEH/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JTIX5UXRDJZJ57DO4V33ZNJTNKWGBQLY/
- https://github.com/trentm/python-markdown2/pull/387
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BRP5RN35JZTSJ3JT4722F447ZDK7LZS5/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/J752422YELXLMLZJPVJVKD2KKHHQRVEH/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JTIX5UXRDJZJ57DO4V33ZNJTNKWGBQLY/
