CVE-2026-100231: Gstreamer1-plugins-bad-free: gstreamer1-plugins-bad-free: heap out-of-bounds write in asfmux due to incorrect utf-16 size calculation for non-bmp characters
A flaw was found in the GStreamer multimedia framework's ASF muxer plugin (asfmux). When writing string metadata tags containing Unicode characters outside the Basic Multilingual Plane (such as emoji) to an ASF container, the plugin incorrectly calculates the buffer size needed for the UTF-16LE representation. The size calculation assumes each Unicode code point requires 2 bytes, but non-BMP characters require 4 bytes (surrogate pairs). This leads to a heap out-of-bounds write when the actual UTF-16LE conversion is written into the undersized buffer. An attacker could exploit this by providing media with crafted metadata tags to an application that transcodes or remuxes to ASF format while preserving metadata, potentially causing a denial of service or limited memory corruption.
Other sources
GStreamer's asfmux plugin (gst-plugins-bad) contains a heap out-of-bounds write in the ASF header buffer allocation path. The functions contentdescriptioncalcsizefortag() (line 429) and addmetadatatagsize() (line 539) in subprojects/gst-plugins-bad/gst/asfmux/gstasfmux.c compute the UTF-16LE buffer size for string metadata tags using the formula 2 (1 + gutf8strlen(text, -1)). This counts Unicode code points and assumes each maps to one 16-bit UTF-16 code unit. However, non-BMP characters (U+10000 and above, e.g. emoji) require UTF-16 surrogate pairs (4 bytes per character). The write path in gstasfmuxwritestringwithsize() (line 881) uses gconvert() which produces the correct (larger) UTF-16LE byte count. The header buffer is allocated at gstasfmuxstartfile() (line 1332) using the pre-computed sizes. The resulting under-allocation is 2 bytes per non-BMP code point. The overflowing bytes are attacker-derived UTF-16LE content written via memcpy() at line 898.
The issue affects GStreamer versions containing the asfmux plugin in gst-plugins-bad. Reproduced at commit 4cb910d273bef5445c90e77e7b781ab99e236e1f (main branch). The affected size calculation is unchanged on current main.
Fix: Not yet available upstream (reported privately to GStreamer security team, awaiting triage).
Reporter: Bruno Fournier (xxtqqtx) PSIRT Ticket: PSIRTSUPT-24645
— Red Hat