CVE-2021-39563

What it actually does

NULL pointer dereference in swf_DumpActions within swfaction.c triggers an application crash when processing a specially crafted SWF file.

Why it matters to the business

Processing untrusted SWF files with the affected tools can cause unexpected crashes and service interruptions for any workflow that relies on swftools.

How an attack unfolds

An attacker crafts a malformed SWF file that reaches the vulnerable swf_DumpActions function, causing the tool to dereference a NULL pointer and terminate when the file is opened or dumped.

How to fix it

Upgrade to a patched release of swftools if one becomes available; otherwise replace swftools usage with maintained alternatives and avoid processing untrusted SWF files with the vulnerable binaries.

How to confirm the fix worked

Test that the updated or replacement tools no longer crash on the same malformed SWF samples used for validation; scan source builds for the presence of the vulnerable swf_DumpActions implementation.

Typical attack path

1. Create or obtain a malformed SWF file engineered to trigger the NULL dereference
2. Deliver the file to a target system running swftools
3. Invoke a swftools utility (such as swfdump) on the malicious file
4. Observe application crash and denial of service

How to detect it

Review process crash logs and core dumps from swftools binaries; correlate unexpected terminations of swfdump or similar utilities with recent receipt of SWF files.

What your security team should do

• Remove or isolate swftools from any automated or user-facing pipelines that handle external files
• Monitor for new upstream releases or community patches addressing the NULL dereference
• Implement file-type sandboxing or conversion steps before any remaining swftools invocations