CVE-2021-39598

What it actually does

A NULL pointer dereference in the callcode() function in code.c allows crafted input to trigger an immediate crash of the affected process.

Why it matters to the business

Processing of untrusted SWF files can be interrupted at will, causing downtime for any workflow or service that depends on swftools for conversion or analysis.

How an attack unfolds

An attacker supplies a specially crafted SWF file to any swftools binary that invokes callcode(); the function dereferences a NULL pointer and the process terminates, producing a denial-of-service condition.

How to fix it

No fixed versions are published. Remove or isolate swftools from any workflow that accepts untrusted input; replace with maintained alternatives and enforce strict file-type allow-listing.

How to confirm the fix worked

Run the affected binaries against known crashing SWF samples in a controlled environment and confirm that the process terminates with a segmentation fault or equivalent NULL-dereference signal; verify that no newer upstream release resolves the issue.

Typical attack path

1. Obtain or generate a malicious SWF file that reaches the vulnerable code path
2. Invoke a swftools utility (e.g., swfextract, swfrender) on the file
3. callcode() executes and dereferences a NULL pointer
4. Process crashes, resulting in denial of service

How to detect it

Inspect process crash dumps, syslog entries, and any custom logging that records the call stack when swftools utilities terminate abnormally; look for references to code.c or the callcode symbol.

What your security team should do

• Inventory all systems and containers that ship or execute swftools
• Block or sandbox any use of the tools on untrusted files
• Monitor crash-reporting pipelines for repeated callcode NULL dereference signatures
• Plan migration to supported SWF or vector-graphics tooling