CVE-2021-39564
📋 TL;DR
CVE-2021-39564 is a heap buffer overflow vulnerability in swftools that allows attackers to execute arbitrary code by exploiting the swf_DumpActions() function. This affects all users of swftools up to version 20200710 who process malicious 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 full system compromise, allowing attacker to install malware, exfiltrate data, or pivot to other systems.
Likely Case
Local privilege escalation or denial of service when processing malicious SWF files, potentially leading to system instability.
If Mitigated
Limited impact with proper sandboxing and file validation, potentially only causing application crashes.
🎯 Exploit Status
Proof of concept available in GitHub issue, requires crafting malicious SWF file to trigger buffer overflow.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Versions after 20200710
Vendor Advisory: https://github.com/matthiaskramm/swftools/issues/116
Restart Required: No
Instructions:
1. Update swftools to latest version from official repository. 2. Recompile if using source. 3. Replace any vulnerable binaries with patched versions.
🔧 Temporary Workarounds
Disable SWF processing
linuxPrevent swftools from processing SWF files by removing or restricting access to vulnerable components.
sudo chmod 000 /usr/bin/swf*
sudo mv /usr/bin/swf* /root/disabled/
Sandbox execution
linuxRun swftools in isolated containers or virtual machines to limit potential damage.
docker run --read-only --cap-drop=ALL -v /safe_input:/input swftools
🧯 If You Can't Patch
- Implement strict input validation for SWF files before processing
- Deploy application allowlisting to prevent unauthorized swftools execution
🔍 How to Verify
Check if Vulnerable:
Check swftools version: swfrender --version or swfextract --version. If version is 20200710 or earlier, system is vulnerable.Check Version:
swfrender --version 2>&1 | head -1Verify Fix Applied:
Verify updated version reports a date after 20200710. Test with known safe SWF files to ensure functionality.📡 Detection & Monitoring
Log Indicators:
- Segmentation fault errors from swftools processes
- Unusual memory allocation patterns in system logs
Network Indicators:
- Unexpected outbound connections from swftools processes
- File uploads to SWF processing endpoints
SIEM Query:
process_name:swf* AND (event_id:1000 OR exception_code:c0000005)