CVE-2021-39577
📋 TL;DR
CVE-2021-39577 is a heap buffer overflow vulnerability in swftools' swfdump utility that allows attackers to execute arbitrary code by providing a malicious SWF file. This affects users who process untrusted SWF files with swftools versions through 2020-07-10. Attackers can achieve remote code execution if they can supply malicious input to the vulnerable component.
💻 Affected Systems
- swftools
📦 What is this software?
Swftools by Swftools
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining the same privileges as the swfdump process, potentially leading to lateral movement, data exfiltration, or persistent backdoors.
Likely Case
Local privilege escalation or remote code execution when processing attacker-controlled SWF files, potentially leading to application compromise and data leakage.
If Mitigated
Denial of service or application crash if exploit fails, with limited impact due to proper sandboxing and least privilege configurations.
🎯 Exploit Status
Proof of concept available in GitHub issue #121. Exploitation requires providing a malicious SWF file to swfdump, which can be done remotely if swfdump processes external input.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Versions after 2020-07-10
Vendor Advisory: https://github.com/matthiaskramm/swftools/issues/121
Restart Required: No
Instructions:
1. Update swftools to version after 2020-07-10. 2. For Linux: Use package manager or compile from updated source. 3. For Windows: Download updated binary from official repository. 4. Verify installation with 'swfdump --version'.
🔧 Temporary Workarounds
Disable swfdump usage
linuxRemove or restrict execution permissions for swfdump binary to prevent exploitation.
sudo chmod 000 /usr/bin/swfdump
sudo mv /usr/bin/swfdump /usr/bin/swfdump.disabled
Input validation and sandboxing
allImplement strict input validation for SWF files and run swfdump in isolated containers with minimal privileges.
docker run --read-only --cap-drop=ALL -v /safe/input:/input:ro swftools swfdump /input/file.swf
🧯 If You Can't Patch
- Implement strict access controls to prevent untrusted users from executing swfdump
- Deploy application allowlisting to block execution of swfdump except from trusted processes
🔍 How to Verify
Check if Vulnerable:
Run 'swfdump --version' and check if version is 2020-07-10 or earlier. Test with known malicious SWF file if available.Check Version:
swfdump --versionVerify Fix Applied:
Confirm swftools version is after 2020-07-10 with 'swfdump --version'. Test with the same malicious SWF file that previously caused crashes.📡 Detection & Monitoring
Log Indicators:
- Segmentation fault or abnormal termination of swfdump process
- Unexpected memory access errors in system logs
- Execution of swfdump with unusual file paths or network locations
Network Indicators:
- Network transfers of SWF files to systems running swftools
- Outbound connections from swfdump process to unexpected destinations
SIEM Query:
process_name:"swfdump" AND (event_id:"1000" OR event_id:"1001") OR process_name:"swfdump" AND cmdline:"*.swf"