CVE-2026-89873: Linux Kernel HEVC Privilege Escalation

CVE-2026-89873 Overview

CVE-2026-89873 is a Linux kernel vulnerability in the Video4Linux2 (V4L2) controls subsystem. The flaw affects the HEVC (High Efficiency Video Coding) Sequence Parameter Set (SPS) control handling in v4l2-ctrls. Userspace applications can supply short-term and long-term Reference Picture Set (RPS) counts that exceed HEVC specification limits. Decoder drivers then use these unvalidated counts to walk EXT SPS dynamic arrays, leading to out-of-bounds memory access. The vulnerability requires local access with low privileges and no user interaction.

Critical Impact
A local attacker with access to V4L2 HEVC decoder devices can trigger out-of-bounds reads or writes in kernel memory, potentially achieving privilege escalation or kernel information disclosure.

Affected Products

Linux kernel versions containing the vulnerable HEVC v4l2-ctrls implementation prior to the patches referenced in commits 30f85a7c5911 and 796b5c6d4f16
Linux distributions shipping affected kernel builds with V4L2 HEVC stateless decoder support enabled
Systems using hardware-accelerated HEVC decoding through V4L2 stateless decoder drivers

Discovery Timeline

2026-09-16 - CVE-2026-89873 published to the National Vulnerability Database
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-89873

Vulnerability Analysis

The vulnerability resides in the V4L2 controls validation logic for HEVC SPS controls. The HEVC SPS control carries short-term and long-term RPS counts that decoder drivers reference when walking matching EXT SPS dynamic arrays. Before the patch, the kernel accepted RPS counts that exceeded HEVC specification limits of 64 short-term RPS sets and 32 long-term references.

Drivers subsequently used these unchecked counts as loop bounds when iterating dynamic arrays sized to the specification maximums. The mismatch enabled out-of-bounds memory access during decoder operations. Additionally, EXT SPS short-term RPS entries whose negative or positive picture counts exceeded the 16-entry arrays were not rejected, nor were combined delta-POC counts that exceeded the HEVC Decoded Picture Buffer (DPB) maximum.

Root Cause

The root cause is missing input validation [CWE-20] on user-supplied control values passed through the V4L2 ioctl interface. The v4l2-ctrls framework did not enforce HEVC specification bounds on RPS count fields before passing them to decoder drivers. Downstream drivers assumed the framework had validated these values and used them directly as array indices and loop counters.

Attack Vector

Exploitation requires local access to a V4L2 HEVC decoder device node, typically /dev/videoN. An attacker with permission to open the device submits a crafted HEVC SPS control structure containing RPS counts exceeding specification maximums. When a decoder driver processes the malformed control, it iterates past the fixed-size EXT SPS arrays. The out-of-bounds access can corrupt adjacent kernel memory or leak kernel data to userspace. The fix rejects SPS values exceeding 64 short-term sets, 32 long-term references, 16-entry negative or positive picture count arrays, and the HEVC DPB maximum for combined delta-POC counts. See the Kernel Git Commit 30f85a7 and Kernel Git Commit 796b5c6 for the upstream fixes.

Detection Methods for CVE-2026-89873

Indicators of Compromise

Unexpected kernel oops or panic messages referencing v4l2-ctrls or HEVC decoder driver functions in dmesg output
KASAN (Kernel Address Sanitizer) reports indicating out-of-bounds access in HEVC SPS control handling paths
Unusual process activity involving non-privileged users opening /dev/video* device nodes and issuing VIDIOC_S_EXT_CTRLS ioctls with HEVC control identifiers

Detection Strategies

Audit kernel logs for crashes or warnings originating from HEVC stateless decoder drivers or v4l2_ctrl_new_std_compound paths
Monitor for local privilege escalation attempts following interaction with V4L2 media device nodes
Deploy kernel runtime protections such as KASAN in test environments to identify exploitation attempts against the vulnerable code path

Monitoring Recommendations

Track access patterns to V4L2 device nodes by non-media applications and flag anomalous ioctl usage
Correlate V4L2 device access events with subsequent privilege elevation or unexpected kernel module behavior
Ensure kernel version telemetry is collected across the fleet to identify unpatched hosts vulnerable to CVE-2026-89873

How to Mitigate CVE-2026-89873

Immediate Actions Required

Apply the upstream Linux kernel patches referenced in commits 30f85a7c5911 and 796b5c6d4f16 as soon as vendor-backported updates are available
Inventory systems with V4L2 HEVC stateless decoder support enabled and prioritize patching multi-user hosts and shared workstations
Restrict access to /dev/video* device nodes to trusted users and processes through group membership and udev rules

Patch Information

The fix adds validation in v4l2-ctrls to reject HEVC SPS control values that exceed specification limits: 64 short-term RPS sets, 32 long-term references, 16 entries per negative or positive picture count array, and the HEVC DPB maximum for combined delta-POC counts. Consult your Linux distribution's security advisories for backported updates. Reference the upstream commits at Kernel Git Commit 30f85a7 and Kernel Git Commit 796b5c6.

Workarounds

Remove or blacklist HEVC stateless decoder kernel modules on systems that do not require hardware-accelerated HEVC decoding
Tighten permissions on /dev/video* device nodes so only trusted service accounts can open them
Apply mandatory access control policies (SELinux, AppArmor) to confine applications that legitimately access V4L2 decoder interfaces

bash

# Restrict V4L2 device access to the video group only
sudo chown root:video /dev/video*
sudo chmod 660 /dev/video*

# Blacklist HEVC stateless decoder modules if unused (example)
echo "blacklist hantro_vpu" | sudo tee /etc/modprobe.d/blacklist-hevc.conf
echo "blacklist rkvdec"     | sudo tee -a /etc/modprobe.d/blacklist-hevc.conf
sudo update-initramfs -u