CVE-2021-39595
📋 TL;DR
This vulnerability in swftools allows attackers to execute arbitrary code through a stack buffer overflow in the rfx_alloc() function. It affects all users of swftools up to version 20200710 who process untrusted SWF files. Successful exploitation could lead to complete system compromise.
💻 Affected Systems
- swftools
📦 What is this software?
Swftools by Swftools
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution with the privileges of the swftools process, potentially leading to full system takeover, data theft, or ransomware deployment.
Likely Case
Local privilege escalation or denial of service when processing malicious SWF files, especially in automated workflows.
If Mitigated
Limited impact if swftools runs in sandboxed environments with minimal privileges and processes only trusted files.
🎯 Exploit Status
Proof-of-concept exists in the GitHub issue. Exploitation requires the attacker to provide a malicious SWF file that gets processed by swftools.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Version after 20200710 (check latest release)
Vendor Advisory: https://github.com/matthiaskramm/swftools/issues/141
Restart Required: No
Instructions:
1. Check current version with 'swfrender --version'. 2. Update to latest version from official repository. 3. Recompile from source if using custom build. 4. Test with known safe SWF files to verify functionality.
🔧 Temporary Workarounds
Disable swftools processing
linuxTemporarily disable swftools usage until patched
sudo systemctl stop swftools-service
sudo chmod 000 /usr/bin/swf*
Restrict file processing
linuxOnly allow swftools to process files from trusted sources
sudo chown root:root /usr/bin/swf*
sudo chmod 4755 /usr/bin/swf*
🧯 If You Can't Patch
- Run swftools in a container or sandbox with minimal privileges
- Implement strict input validation to reject untrusted SWF files
🔍 How to Verify
Check if Vulnerable:
Check if swftools version is 20200710 or earlier: 'swfrender --version' or 'dpkg -l | grep swftools'Check Version:
swfrender --version 2>&1 | head -1Verify Fix Applied:
Verify version is newer than 20200710 and test processing of safe SWF files📡 Detection & Monitoring
Log Indicators:
- Segmentation fault or crash logs from swftools processes
- Unusual process spawning from swftools
Network Indicators:
- Unexpected network connections originating from swftools processes
SIEM Query:
process_name:swf* AND (event_id:1000 OR signal:SIGSEGV)