CVE-2021-39558
📋 TL;DR
This vulnerability in swftools allows attackers to execute arbitrary code through a stack buffer overflow when processing malicious SWF files. It affects all users of swftools up to version 20200710 who process untrusted SWF files. The vulnerability resides in the VectorGraphicOutputDev::drawGeneralImage() function.
💻 Affected Systems
- swftools
📦 What is this software?
Swftools by Swftools
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with remote code execution leading to complete control of the affected system, data theft, and lateral movement within the network.
Likely Case
Local privilege escalation or remote code execution when processing malicious SWF files, potentially leading to malware installation or data exfiltration.
If Mitigated
Denial of service or application crash if exploit fails, with limited impact due to proper sandboxing and privilege separation.
🎯 Exploit Status
Proof of concept exists in the GitHub issue. Exploitation requires the victim to process a malicious SWF file, which could be delivered via email, web uploads, or other file transfer methods.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Version after 20200710
Vendor Advisory: https://github.com/matthiaskramm/swftools/issues/106
Restart Required: No
Instructions:
1. Update swftools to the latest version from the official repository. 2. Recompile from source if using self-compiled versions. 3. Verify the fix by checking the version number.
🔧 Temporary Workarounds
Disable SWF file processing
allPrevent swftools from processing SWF files by removing or restricting access to the tool.
sudo apt remove swftools
sudo yum remove swftools
brew uninstall swftools
Sandbox swftools execution
linuxRun swftools in a container or sandboxed environment to limit potential damage from exploitation.
docker run --rm -v $(pwd):/data alpine/swftools
firejail --net=none swftools
🧯 If You Can't Patch
- Implement strict input validation for SWF files and only allow processing from trusted sources
- Monitor for abnormal swftools process behavior and implement application whitelisting
🔍 How to Verify
Check if Vulnerable:
Check swftools version: swfrender --version or swftools --version. If version is 20200710 or earlier, the system is vulnerable.Check Version:
swfrender --version 2>&1 | head -1Verify Fix Applied:
After updating, verify the version is newer than 20200710 and test with known safe SWF files to ensure functionality.📡 Detection & Monitoring
Log Indicators:
- Segmentation fault or crash logs from swftools processes
- Abnormal memory usage patterns in swftools
Network Indicators:
- Unexpected outbound connections from swftools processes
- File transfers following swftools execution
SIEM Query:
process_name:swf* AND (event_id:1000 OR exception_code:c0000005)