# CVE-2026-125257: FFmpeg off-by-one heap overflow in Nellymoser encoder

Source: https://junglewise.ai/threats/cve-2026-125257-ffmpeg-through-9-0-2-contains-an-off-by-one-heap-overflow  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-09. Updated: 2026-11-09  
License: CC BY 4.0 with a link back

CVE-2026-125257 · Severity: high · CVSS 7 · Published 2026-11-09

FFmpeg is a multimedia framework used to encode and decode audio and video. When encoding Nellymoser audio with trellis optimization enabled, a crafted audio file can trigger a heap memory overflow that overwrites adjacent data. An attacker can exploit this to crash applications or potentially execute arbitrary code by providing specially crafted audio to an application using vulnerable FFmpeg versions.

Technologies: [Ffmpeg](https://junglewise.ai/threats/technologies/ffmpeg). Vendors: [Ffmpeg](https://junglewise.ai/threats/vendors/ffmpeg).

## Quick answers

**Is CVE-2026-125257 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-125257 being exploited in the wild.

**How severe is CVE-2026-125257?** High severity, with a CVSS score of 7, published 9 November 2026.

**Which products and versions are affected?** Ffmpeg through 9.0.2.

**Is there a fix for CVE-2026-125257?** Yes: fixed in FFmpeg 9.1. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-125257 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-125257?** Check whether you run Ffmpeg at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in FFmpeg 9.1

## Technical details

An off-by-one error in the get_exponent_dynamic() function in libavcodec/nellymoserenc.c allows the trellis index (idx) to equal OPT_SIZE instead of remaining below it, causing a write one element past the opt and path heap allocations. The vulnerability is triggered when encoding Nellymoser audio with trellis enabled and requires attacker-supplied crafted audio input. The overflow corrupts adjacent heap memory and was fixed in FFmpeg 9.1.

## Affected products

- FFmpeg FFmpeg through 9.0.2

## Timeline

- 2026-11-09: disclosed
- 2026-09-19: patched

## References

- [https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24471](https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24471)
- [https://github.com/FFmpeg/FFmpeg](https://github.com/FFmpeg/FFmpeg)

## Related threats

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- [FFmpeg double free in H.264 SEI message handling](https://junglewise.ai/threats/cve-2026-125269-ffmpeg-through-9-0-2-contains-a-double-free-vulnerability-in)
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- [FFmpeg use-after-free in NVIDIA codec decoding](https://junglewise.ai/threats/cve-2026-125265-ffmpeg-through-9-0-2-contains-a-use-after-free-vulnerability-in)
- [FFmpeg JPEG XL parser uncontrolled resource consumption](https://junglewise.ai/threats/cve-2026-125264-ffmpeg-through-9-0-2-contains-an-uncontrolled-resource)