CVE-2021-39594

What it actually does

A NULL pointer dereference in the updateusage() function in swftext.c causes the application to crash when processing a specially crafted SWF file.

Why it matters to the business

Automated or manual workflows that convert, inspect, or manipulate SWF files can be interrupted by crashes, leading to failed jobs, delayed processing pipelines, and the need for manual intervention or restarts.

How an attack unfolds

An attacker supplies a malformed SWF file to any swftools utility that invokes updateusage(); when the file is parsed the NULL dereference is triggered and the process terminates.

How to fix it

Upgrade swftools to a version containing a fix for the NULL dereference if one becomes available; otherwise replace the tool with a maintained alternative or wrap invocations in a sandbox that limits crash impact and restarts.

How to confirm the fix worked

Replay known crashing SWF samples against the updated binary and confirm the process no longer terminates with a segmentation fault or abort; inspect the updateusage() function for a NULL check before use.

Typical attack path

1. Locate a system or service that invokes swftools on untrusted SWF input
2. Craft a minimal SWF file that reaches the vulnerable code path in swftext.c
3. Deliver the file through normal channels (upload, email attachment, build artifact, etc.)
4. Trigger processing so that updateusage() dereferences NULL and crashes the tool

How to detect it

Watch system and application logs for repeated segmentation faults, aborts, or sudden exits of swftools processes immediately after SWF file handling; correlate with file-creation or upload events involving .swf extensions.

What your security team should do

• Restrict swftools execution to trusted input only
• Run the utilities inside a container or sandbox with resource limits and crash monitoring
• Replace swftools with actively maintained SWF tooling where possible
• Add file-type validation and size limits before any swftools invocation