CVE-2020-6817
📋 TL;DR
This CVE describes a regular expression denial of service (ReDoS) vulnerability in Mozilla's bleach library when parsing style attributes. Attackers can cause excessive CPU consumption and service degradation by providing malicious style attribute values. Any application using bleach with allowed style attributes is affected.
💻 Affected Systems
- Mozilla bleach
📦 What is this software?
Bleach by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
Complete service unavailability due to CPU exhaustion, affecting all users of the vulnerable application.
Likely Case
Degraded performance and intermittent service disruptions for users accessing affected endpoints.
If Mitigated
Minimal impact with proper input validation and resource limits in place.
🎯 Exploit Status
Exploitation requires sending specially crafted style attribute values to vulnerable endpoints.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.1.4
Vendor Advisory: https://github.com/mozilla/bleach/security/advisories/GHSA-vqhp-cxgc-6wmm
Restart Required: No
Instructions:
1. Update bleach to version 3.1.4 or later using pip: pip install --upgrade bleach>=3.1.4
2. Verify the update was successful
3. No application restart required for Python library updates
🔧 Temporary Workarounds
Disable style attribute parsing
allRemove style from allowed attributes in bleach.clean calls
Modify bleach.clean calls to exclude 'style' from allowed attributes
Input validation and sanitization
allImplement additional validation before passing content to bleach.clean
Implement custom validation to reject or sanitize style attribute values before bleach processing
🧯 If You Can't Patch
- Implement rate limiting and request throttling to mitigate DoS impact
- Deploy web application firewalls (WAF) with ReDoS protection rules
🔍 How to Verify
Check if Vulnerable:
Check if bleach version is below 3.1.4: python -c "import bleach; print(bleach.__version__)"Check Version:
python -c "import bleach; print('bleach version:', bleach.__version__)"Verify Fix Applied:
Verify bleach version is 3.1.4 or higher: python -c "import bleach; print(bleach.__version__ >= '3.1.4')"📡 Detection & Monitoring
Log Indicators:
- Unusually high CPU usage on web servers
- Increased request processing times
- Multiple requests with similar style attribute patterns
Network Indicators:
- Repeated requests containing complex style attribute values
- Traffic spikes to endpoints using bleach.clean
SIEM Query:
source="web_logs" AND (uri="*bleach*" OR message="*style*attribute*") AND duration>5000ms