CVE-2026-53701: Gstreamer1-plugins-bad-free: gstreamer: out-of-bounds write in h.266/vvc pps picture partition parser

An out-of-bounds write vulnerability was found in GStreamer's H.266/VVC PPS picture partition parser in gst-plugins-bad. In the multi-slice-in-tile processing of gsth266parserparsepicturepartition() (gsth266parser.c), the loop iterates without checking that the slice index stays within bounds, writing past three fixed-size arrays (sliceheightinctus, slicetopleftctux, slicetopleftctuy) in the GstH266PPS structure. While the initial proof-of-concept demonstrated a 4-byte out-of-bounds write, the code permits larger writes across multiple iterations. A crafted H.266/VVC media file can trigger this vulnerability.