CVE-2021-39593

What it actually does

NULL pointer dereference in swf_FontExtract_DefineFontInfo() within swftext.c causes the process to crash when processing a malformed SWF file containing DefineFontInfo data.

Why it matters to the business

Processing pipelines or forensic workflows that rely on swftools to handle Flash files can be halted by a single crafted file, leading to service disruption and manual intervention; the issue has been public for years with public exploits available but no vendor fixes listed.

How an attack unfolds

An attacker crafts a minimal SWF file that triggers the DefineFontInfo path with invalid data, causing the NULL dereference when the file is opened locally via any swftools command that invokes swf_FontExtract_DefineFontInfo().

How to fix it

Remove or isolate swftools from any automated or user-facing workflows until a patched build is obtained; monitor upstream repository for updates that address the dereference in swftext.c; consider alternative SWF parsing libraries that receive active maintenance.

How to confirm the fix worked

Run the affected swftools commands against a known-good test corpus and a minimal crashing SWF sample; confirm process exit status and absence of NULL dereference via debugger or address sanitizer; verify binary version strings against any future fixed releases.

Typical attack path

1. Generate malformed SWF containing crafted DefineFontInfo structure
2. Deliver file via email, download link, or shared storage
3. Victim or automated process invokes swftools binary on the file
4. Tool crashes with segmentation fault, producing denial of service

How to detect it

Monitor process logs and core dumps for segmentation faults originating from swftext.c or swf_FontExtract_DefineFontInfo; inspect command-line history and file-access logs for invocations of swftools utilities on external or untrusted SWF files.

• swf_FontExtract_DefineFontInfo
• swftext.c
• segmentation fault.*swftools

What your security team should do

• Inventory all systems and scripts that invoke swftools binaries and remove from production paths where possible
• Block untrusted SWF files from reaching any remaining swftools instances
• Subscribe to swftools repository notifications and apply updates immediately upon release
• Add crash-monitoring rules for swftools processes to detect attempted exploitation