CVE-2021-42203
📋 TL;DR
This vulnerability is a heap-use-after-free flaw in swftools that allows attackers to execute arbitrary code by exploiting font extraction in SWF files. It affects all users processing untrusted SWF files with vulnerable swftools versions. The vulnerability can lead to complete system compromise.
💻 Affected Systems
- swftools
📦 What is this software?
Swftools by Swftools
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to full system compromise, data theft, or ransomware deployment.
Likely Case
Local privilege escalation or denial of service when processing malicious SWF files.
If Mitigated
Limited impact with proper sandboxing and input validation in place.
🎯 Exploit Status
Exploitation requires crafting a malicious SWF file that triggers the heap-use-after-free during font extraction. Public proof-of-concept exists in GitHub issues.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Version after 20201222 (check latest release)
Vendor Advisory: https://github.com/matthiaskramm/swftools/issues/176
Restart Required: No
Instructions:
1. Check current swftools version. 2. Update to latest version from official repository. 3. Recompile if using source. 4. Test with known vulnerable SWF files.
🔧 Temporary Workarounds
Disable swftools processing
linuxTemporarily disable swftools usage for SWF file processing
sudo systemctl stop swftools
sudo chmod 000 /usr/bin/swftools
Input validation
allReject SWF files from untrusted sources before processing
🧯 If You Can't Patch
- Sandbox swftools execution in isolated containers with minimal privileges
- Implement strict file upload validation and only process SWF files from trusted sources
🔍 How to Verify
Check if Vulnerable:
Check swftools version: swftools --version. If version is 20201222 or earlier, system is vulnerable.Check Version:
swftools --versionVerify Fix Applied:
Update swftools and verify version is newer than 20201222. Test with sample SWF files to ensure no crashes.📡 Detection & Monitoring
Log Indicators:
- swftools process crashes
- segmentation faults in system logs
- unexpected process termination
Network Indicators:
- Unusual outbound connections from swftools process
- File uploads containing SWF files to processing systems
SIEM Query:
process_name:swftools AND (event_type:crash OR exit_code:139)