FFmpeg through 9.0.2 contains an uninitialized memory disclosure vulnerability in av_dynamic_hdr_plus_to_t35() that leaves up to three payload bytes uninitialized when tone_mapping_flag is 0. Attackers can supply crafted Matroska T.35 BlockAdditional or HEVC/AV1 SEI metadata so that remuxing or transcoding writes leaked process memory into output files.