CVE-2021-39561
📋 TL;DR
CVE-2021-39561 is a stack buffer overflow vulnerability in swftools that allows remote code execution when processing malicious SWF files. Attackers can exploit this by tricking users into opening specially crafted SWF files, potentially taking full control of affected systems. Anyone using swftools versions through 20200710 is vulnerable.
💻 Affected Systems
- swftools
📦 What is this software?
Swftools by Swftools
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining complete control over the affected system, enabling data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Local privilege escalation or remote code execution when processing untrusted SWF files, leading to malware installation or data exfiltration.
If Mitigated
Denial of service or application crash if exploit fails, with proper sandboxing preventing system-wide compromise.
🎯 Exploit Status
Proof of concept available in GitHub issue; exploitation requires user to process malicious SWF file but no authentication needed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Versions after 20200710
Vendor Advisory: https://github.com/matthiaskramm/swftools/issues/102
Restart Required: No
Instructions:
1. Update swftools to latest version from official repository. 2. Recompile if using source code. 3. Replace existing swftools binaries with patched versions.
🔧 Temporary Workarounds
Disable SWF file processing
linuxPrevent swftools from processing untrusted SWF files by restricting file access or using alternative tools.
chmod 000 /usr/bin/swftools
mv /usr/bin/swftools /usr/bin/swftools.disabled
Sandbox execution
linuxRun swftools in isolated containers or virtual machines to limit exploit impact.
docker run --rm -v $(pwd):/data alpine/swftools
firejail --private swftools input.swf
🧯 If You Can't Patch
- Implement strict file upload validation to block SWF files from untrusted sources
- Deploy endpoint protection with memory corruption detection and exploit prevention
🔍 How to Verify
Check if Vulnerable:
Check swftools version with 'swftools --version' and verify if it's 20200710 or earlierCheck Version:
swftools --versionVerify Fix Applied:
Verify version is newer than 20200710 and test with known malicious SWF file from PoC📡 Detection & Monitoring
Log Indicators:
- Segmentation fault errors in system logs
- Unexpected process termination of swftools
- Large memory allocation failures
Network Indicators:
- Unusual outbound connections after SWF file processing
- File uploads to swftools endpoints
SIEM Query:
process_name:"swftools" AND (event_id:"1000" OR event_id:"1001")