UBUNTU-CVE-2026-125270: Medium severity ubuntu/ffmpeg vulnerability

FFmpeg through 9.0.2 contains a heap out-of-bounds read vulnerability in v4l2bufferswframetobuf() in libavcodec/v4l2buffers.c that mishandles frames with negative linesize. Attackers can supply bottom-up AVI or PNM/PFM files that are transcoded to a V4L2 M2M encoder, crashing the application or leaking heap memory into encoded output.