CVE-2021-39577

What it actually does

Heap buffer overflow in the main function of swfdump.c allows out-of-bounds memory writes when processing a crafted SWF file, enabling arbitrary code execution in the context of the swfdump process.

Why it matters to the business

Processing untrusted SWF files with the affected tool can lead to full system compromise on developer or analysis workstations, increasing risk of malware deployment or data theft in environments that still rely on legacy Flash tooling.

How an attack unfolds

An attacker supplies a malicious SWF file that triggers the heap overflow during parsing in swfdump; when a user or automated process invokes swfdump on the file, the overflow is used to overwrite memory and redirect execution to attacker-controlled code.

How to fix it

Remove or isolate swftools installations; if continued use is required, rebuild from source after applying any upstream memory-safety patches and restrict execution to trusted inputs only. No official fixed versions have been published.

How to confirm the fix worked

Confirm swftools version is newer than 20200710 or that the binary has been rebuilt with the overflow fixed; test against publicly available proof-of-concept SWF files that previously triggered the crash.

Typical attack path

1. Create a specially crafted SWF file that triggers the heap overflow in swfdump.c
2. Deliver the file via email, web download, or shared storage to a target system
3. Convince the victim or an automated workflow to run swfdump against the file
4. Achieve arbitrary code execution within the swfdump process

How to detect it

Search process execution logs and file-access telemetry for swfdump invocations against files originating outside trusted sources; monitor for segmentation faults or abnormal memory usage during SWF processing.

• swfdump
• swfdump.c
• heap-buffer-overflow

What your security team should do

• Immediately restrict or block execution of swfdump on any untrusted input
• Inventory all systems still shipping or using swftools and schedule removal
• Replace legacy SWF analysis workflows with maintained, memory-safe alternatives
• Add swfdump to application allow-listing or sandbox policies where removal is not feasible