CVE-2026-12892: GStreamer Buffer Overflow Vulnerability

CVE-2026-12892 Overview

CVE-2026-12892 is a heap out-of-bounds read vulnerability in the GStreamer gst-plugins-bad package. The flaw resides in the H.264 parser when processing malformed Multiview Video Coding (MVC) or Scalable Video Coding (SVC) extension slice Network Abstraction Layer (NAL) units. The parser checks slice boundary information without first verifying that the NAL unit contains sufficient data beyond the extension header. An attacker who convinces a user to open a crafted H.264 video file can trigger a 1-byte heap read past the buffer, leaking a single byte of heap memory or crashing the application.

Critical Impact
A crafted H.264 file containing malformed MVC/SVC extension slice NAL units triggers a 1-byte heap out-of-bounds read, enabling memory disclosure or denial of service in any application using gst-plugins-bad for video parsing.

Affected Products

GStreamer gst-plugins-bad package (H.264 parser component)
Linux distributions packaging affected gst-plugins-bad versions, including Red Hat Enterprise Linux
Downstream applications relying on GStreamer for H.264 media playback or transcoding

Discovery Timeline

2026-06-23 - CVE-2026-12892 published to NVD
2026-06-24 - Last updated in NVD database

Technical Details for CVE-2026-12892

Vulnerability Analysis

The defect is classified as [CWE-125] Out-of-Bounds Read. It triggers inside the H.264 NAL unit parser within gst-plugins-bad when handling extension slice headers used by MVC (Annex H) and SVC (Annex G) profiles. These profiles extend the base H.264 syntax with a small per-slice extension that precedes the slice payload.

During parsing, the code dereferences a slice boundary indicator byte that sits immediately after the extension header. The parser does not validate that the NAL unit buffer extends one byte past the extension header before reading. When the supplied NAL unit terminates exactly at the extension header boundary, the read targets the byte directly after the heap allocation.

The consequence is a single byte of heap memory disclosed to the parser logic, or a crash if the read lands on an unmapped page. The attack requires local user interaction to open a malicious file and the impact is limited to the privileges of the GStreamer-consuming process.

Root Cause

The root cause is a missing length check before reading the slice boundary field of MVC/SVC NAL unit extensions. The parser assumes that any NAL unit advertised as an MVC or SVC extension slice contains at least one byte beyond the 3-byte extension header. A truncated NAL unit violates this assumption and the read proceeds without bounds validation.

Attack Vector

An attacker crafts an H.264 elementary stream or container (MP4, MKV, TS) holding a NAL unit with nal_unit_type 20 (coded slice extension) or 14 (prefix NAL unit) whose payload terminates immediately after the extension header bytes. The attacker delivers the file by email, web download, or messaging and persuades the user to open it in a GStreamer-backed player such as Totem, Rhythmbox, or any application embedding playbin. Parsing begins automatically and the out-of-bounds read fires before playback starts. The CVSS vector indicates a local attack with required user interaction and low confidentiality and availability impact.

No public proof-of-concept code is available. Refer to the Red Hat CVE-2026-12892 Advisory and the GStreamer Work Item #5108 for upstream technical details.

Detection Methods for CVE-2026-12892

Indicators of Compromise

Unexpected crashes or SIGSEGV terminations in processes loading libgstcodecparsers or the H.264 parser plugin
Core dumps referencing gst_h264_parser_parse_nal or related slice-extension parsing functions
Media files with H.264 NAL units of type 14 or 20 whose payload length is at or below the extension header size

Detection Strategies

Inventory installed gst-plugins-bad package versions across Linux endpoints and compare against fixed releases listed in the vendor advisory
Inspect untrusted H.264 streams with a NAL parser to flag MVC/SVC extension slices with truncated payloads before passing them to GStreamer
Enable AddressSanitizer (ASan) builds of GStreamer in test environments to catch the 1-byte over-read during fuzzing or QA

Monitoring Recommendations

Forward application crash telemetry from media-handling processes to a central log store and alert on repeated GStreamer-related faults
Monitor execution of media players opening files from email attachments, browser downloads, or removable media
Track package update status for gst-plugins-bad through configuration management tooling and flag hosts running unpatched versions

How to Mitigate CVE-2026-12892

Immediate Actions Required

Apply distribution updates for gst-plugins-bad once vendors publish patched packages tracking the upstream fix referenced in GStreamer Work Item #5108
Restrict opening of H.264 video files from untrusted sources until the patched package is deployed
Subscribe to vendor security feeds, including the Red Hat CVE-2026-12892 Advisory, for fixed package version announcements

Patch Information

The upstream fix is tracked in GStreamer Work Item #5108. The corrective change adds a length check that verifies the NAL unit buffer contains data beyond the MVC/SVC extension header before reading the slice boundary byte. Red Hat tracks the issue in Bug #2491321 and will publish errata with fixed package versions for supported releases.

Workarounds

Disable or remove the H.264 parser plugin from gst-plugins-bad on systems that do not require H.264 playback
Use sandboxed media players such as those running under Flatpak or Firejail to contain potential memory disclosure or crashes
Block delivery of unsolicited video attachments at the mail gateway and require user-initiated retrieval from trusted repositories

bash

# Check installed gst-plugins-bad version on Red Hat-based systems
rpm -q gstreamer1-plugins-bad-free

# Check on Debian/Ubuntu-based systems
dpkg -l | grep gstreamer1.0-plugins-bad

# After vendor patch is published, update the package
sudo dnf update gstreamer1-plugins-bad-free   # RHEL/Fedora
sudo apt update && sudo apt install --only-upgrade gstreamer1.0-plugins-bad   # Debian/Ubuntu