CVE-2021-42202

What it actually does

NULL pointer dereference in swf_DeleteFilter() within swffilter.c triggers an immediate crash when processing certain malformed SWF inputs.

Why it matters to the business

Local users or automated workflows that invoke swftools on untrusted files experience abrupt process termination, leading to interrupted media processing pipelines and potential loss of productivity until the tool is avoided or replaced.

How an attack unfolds

An attacker crafts a minimal SWF file containing structures that reach swf_DeleteFilter with a NULL filter pointer. When the victim opens or converts the file with any swftools utility, the dereference occurs and the process terminates.

How to fix it

Remove or isolate swftools from environments that process untrusted files; replace with maintained alternatives. Monitor the upstream repository for any future patches addressing the swffilter.c NULL dereference.

How to confirm the fix worked

Attempt to process known crashing SWF test cases with updated builds; confirm the process no longer terminates with a segmentation fault or equivalent signal. Verify that no swftools binaries remain in default PATHs on production systems.

Typical attack path

1. Attacker generates a malformed SWF file designed to hit the NULL filter path.
2. Victim receives the file via email, download, or shared storage.
3. Victim or an automated script invokes an swftools binary on the file.
4. swf_DeleteFilter dereferences the NULL pointer and the process crashes.
5. Attacker observes denial of service or uses the crash as a stepping stone for further attacks.

How to detect it

Review system and application logs for segmentation faults or abrupt terminations of swftools binaries (swfcombine, swfdump, swfrender, etc.) immediately after SWF file operations. Correlate timestamps with file access events involving .swf extensions.

• swf_DeleteFilter
• swffilter.c
• segmentation fault.*swf
• null pointer.*swftools

What your security team should do

• Inventory all systems and build pipelines that depend on swftools and mark them for decommissioning.
• Block execution of swftools binaries via application allow-listing where possible.
• Add SWF file type restrictions or sandboxing to any remaining media conversion workflows.
• Subscribe to swftools repository notifications for any future security commits.