CVE-2021-39595

What it actually does

Stack-based buffer overflow in rfx_alloc() within mem.c allows an attacker to overwrite adjacent stack memory and redirect execution flow to attacker-controlled code.

Why it matters to the business

Processing a malicious SWF file with the affected tools can result in full system compromise, data theft, or ransomware deployment on developer or analysis workstations.

How an attack unfolds

An attacker creates a specially crafted SWF file that triggers the overflow in rfx_alloc when parsed; the victim opens or processes the file with vulnerable swftools utilities, allowing the attacker to execute arbitrary code with the privileges of the tool.

How to fix it

Update swftools to a version newer than 20200710 if a fix becomes available; otherwise replace the tool with a maintained alternative and avoid processing untrusted SWF content.

How to confirm the fix worked

Confirm the installed swftools version exceeds 20200710; recompile from source with stack-protection flags and test against known proof-of-concept inputs to verify the overflow is no longer reachable.

Typical attack path

1. Craft malicious SWF file containing oversized input to trigger rfx_alloc overflow
2. Deliver the file via email, web download, or shared storage
3. Victim processes the file using any swftools command-line utility
4. Overflow overwrites return address and executes attacker-supplied shellcode
5. Attacker establishes persistence or exfiltrates data

How to detect it

Search process-creation and file-access logs for swftools utilities handling files from untrusted sources; monitor for segmentation faults or unexpected child processes spawned by swftools binaries.

What your security team should do

• Immediately restrict swftools usage to trusted input only
• Add swftools binaries to application allow-lists with strict sandboxing
• Monitor upstream project or packaging channels for any security updates
• Replace swftools workflows with modern, maintained equivalents