CVE-2021-39564

What it actually does

Heap buffer overflow in swf_DumpActions allows out-of-bounds write and subsequent arbitrary code execution when a malformed SWF file is processed.

Why it matters to the business

Attackers can gain full control of systems that parse untrusted SWF files with the vulnerable tool, risking malware deployment or data theft; the issue remains unfixed in all published versions and affects any downstream users of the library.

How an attack unfolds

An attacker supplies a crafted SWF file containing oversized action records; when swf_DumpActions parses the file the overflow writes attacker-controlled data onto the heap, enabling control-flow hijacking and code execution in the context of the tool.

How to fix it

No fixed release is available; discontinue use of swftools on untrusted input, isolate the binary in a sandbox or container with no network access, or replace with an alternative SWF processor that receives active maintenance.

How to confirm the fix worked

Rebuild the tool with AddressSanitizer and confirm the overflow is triggered by the same test case; monitor process exit status and heap diagnostics for any remaining users of the binary.

Typical attack path

1. Craft malicious SWF with oversized action data
2. Deliver file via email, download, or archive
3. Victim invokes swftools binary on the file
4. Overflow occurs during action dumping
5. Attacker payload executes

How to detect it

Look for swftools processes that exit abnormally or produce heap-related error messages when handling SWF input; correlate file-access logs with subsequent suspicious child processes or network connections.

What your security team should do

• Block execution of swftools binaries on endpoints that handle external files
• Add swftools to application allow-list exceptions only after risk acceptance
• Scan source repositories and build pipelines for references to the vulnerable package
• Prefer modern, maintained alternatives for any remaining Flash/SWF processing needs