As of 2026-08-22 the latest npm release is 0.2.1, which is the affected version, and the sink is still present on the repository's default branch. There is nothing to upgrade to. See Mitigation for what users can do in the meantime.

Threat model

MCP tool arguments are attacker-influenced. They are produced by an LLM from the content the agent processes — documents, web pages, tool output — so indirect prompt injection puts an attacker in control of the values a tool handler receives. A handler that feeds any of those values into a shell turns that influence into code execution on the host running the MCP server. This is the same model as the precedent case aws-mcp-server / CVE-2026-5058.

A media-conversion server is a particularly natural target for this: the files it is asked to process are exactly the untrusted content an agent picks up from elsewhere.

Execution

src/utils/ffmpeg.ts — the command is assembled as a string and the shell is enabled explicitly:

spawn("ffmpeg " + command, [], { shell: true }) runs the whole string through /bin/sh -c. Everything the caller contributed to command is shell syntax.

src/tools/handlers.ts reaches that helper from three tools, interpolating parameters that are never validated for content:

Only inputPath and outputPath are checked at all, and the Zod schemas validate type (z.string()), never content — so metacharacters pass straight through.

A convert_video call with options set to ; touch /tmp/marker ; produces:

Reproduction

poc/driver.mjs speaks MCP over stdio: it initializes the server, issues one tools/call for convert_video with an injected options value, and checks for a marker file. The payload is a benign touch.

Expected on an affected version:

ffmpeg does not need to be installed — the injected command runs in the same /bin/sh -c string regardless of whether the leading binary resolves. Validated 2026-06-01 in an isolated container, re-validated 2026-06-13, and the sink re-confirmed present on the default branch on 2026-08-22. Run it only against infrastructure you are authorized to test.

Detection

• An ffmpeg command line containing ;, &&, ||, |, a backtick or $(. Legitimate ffmpeg invocations from this server do not.
• A sh -c process whose command line starts with ffmpeg and contains a second command after a separator — a process-tree signal (auditd, eBPF, Falco, EDR) that does not depend on application logging.
• Unexpected children of the MCP server process. ffmpeg and ffprobe are expected; a shell, an interpreter or a network client is not.
• The server logs every invocation to stderr as Running FFmpeg command: ffmpeg <command>, which is the cheapest place to spot an injected value if the logs are collected at all.
• MCP tool-call logs where options, format, codec, pixelFormat or extraOptions contain shell metacharacters.

MITRE ATT&CK T1059.004 Command and Scripting Interpreter: Unix Shell.

Mitigation

No patched release exists, so the options are containment and removal:

• Stop using the package if the agent it serves processes any untrusted content. This is the only complete mitigation available today.
• If it must stay, run it in a container or VM as an unprivileged user, with a read-only filesystem apart from the media working directory, no credentials in its environment, and no network egress. Code execution then buys the attacker a sandbox rather than the host.
• Do not expose it over a network transport. The stdio-only deployment is what keeps this at 8.4 rather than 9.8.
• Wrap or fork the server to reject shell metacharacters in the five parameters listed above before they reach runFFmpegCommand.

The upstream fix is to drop shell: true and pass ffmpeg an argument vector — spawn("ffmpeg", ["-i", inputPath, …]) — building that array from allowlisted format, codec and pixelFormat values rather than from free-form strings. Content validation, not just Zod type validation, is the underlying requirement.

NIST SP 800-53r5 SI-10; OWASP ASVS v4 §5.3.8; CWE-78 mitigations M1 and M2.

Timeline

References

• npm package — https://www.npmjs.com/package/@sworddut/mcp-ffmpeg-helper
• Repository — https://github.com/sworddut/mcp-ffmpeg-helper
• Precedent for the MCP threat model — CVE-2026-5058 (aws-mcp-server)

Companion advisories from the same research pass: CVE-2026-52616, CVE-2026-52618.

License

MIT — see LICENSE.