CVE-2026-89898: Linux Kernel CEC Buffer Overflow Vulnerability

CVE-2026-89898 Overview

CVE-2026-89898 is a vulnerability in the Linux kernel's media: cec: extron-da-hd-4k-plus driver. The driver failed to validate the length of incoming Consumer Electronics Control (CEC) messages before writing them into the msg.msg[] buffer. Malformed data arriving over the CEC transport can overflow the message buffer inside the kernel. The upstream fix adds a sanity check to reject oversized or malformed messages before they reach the buffer.

Critical Impact
An adjacent-network attacker delivering crafted CEC traffic can corrupt kernel memory, leading to loss of confidentiality, integrity, and availability on affected Linux systems.

Affected Products

Linux kernel builds including the extron-da-hd-4k-plus CEC driver
Systems using the Extron DA HD 4K Plus CEC bridge hardware
Distributions shipping kernels prior to the patches referenced in the upstream stable commits

Discovery Timeline

2026-09-16 - CVE-2026-89898 published to the National Vulnerability Database
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-89898

Vulnerability Analysis

The vulnerability resides in the CEC receive path of the extron-da-hd-4k-plus driver in the Linux kernel media subsystem. CEC is a bidirectional serial bus carried over HDMI that allows connected devices to exchange short control messages. The driver parses inbound CEC frames from the Extron device and copies message bytes into a fixed-size msg.msg[] array inside a struct cec_msg.

Before the fix, the driver assumed the incoming payload length was within bounds. A malformed frame declaring or supplying more bytes than the destination buffer can hold triggers an out-of-bounds write into adjacent kernel memory. Because parsing happens in kernel context, corruption affects kernel data structures rather than a user process.

The upstream patch series (kernel commits 00c13b4a, 673611cc, 7ad2fec2, and abac9820) adds an explicit length check that rejects oversized data before it is written into msg.msg[].

Root Cause

The root cause is missing input validation on data sourced from an external hardware transport. The driver trusted length metadata from the incoming CEC stream without comparing it against the capacity of msg.msg[], producing a classic buffer overflow condition in kernel space.

Attack Vector

Exploitation requires access to the CEC transport reachable by the vulnerable driver, which maps to an adjacent-network attack vector. An attacker who can inject CEC frames — for example, through a connected HDMI device or a compromised sink — can send a malformed message that overflows the kernel buffer. No authentication or user interaction is required on the target host.

No verified public proof-of-concept is available at the time of writing. See the Kernel Commit 00c13b4a advisory for technical details of the fix.

// No verified exploit code available.
// The upstream fix adds a bounds check that rejects CEC messages
// whose declared length exceeds the capacity of msg.msg[]
// before any bytes are copied into the buffer.

Detection Methods for CVE-2026-89898

Indicators of Compromise

Unexpected kernel oops, panic, or KASAN slab-out-of-bounds reports referencing the extron-da-hd-4k-plus or cec modules
Kernel log entries indicating malformed CEC frames or driver state resets on HDMI-connected hosts
Unexplained reboots or media subsystem crashes on systems using Extron CEC hardware

Detection Strategies

Enable KASAN or equivalent memory-safety instrumentation on test kernels to surface out-of-bounds writes in the CEC receive path
Monitor kernel ring buffer output (dmesg, journalctl -k) for errors originating from the cec subsystem and the extron-da-hd-4k-plus driver
Track kernel package versions across the fleet to identify hosts still running pre-patch builds

Monitoring Recommendations

Forward kernel logs to a centralized logging platform and alert on repeated cec driver faults
Inventory endpoints and appliances that load the extron-da-hd-4k-plus module and prioritize them for patch tracking
Correlate physical-access or HDMI-switching events with kernel instability to detect abuse of the CEC transport

How to Mitigate CVE-2026-89898

Immediate Actions Required

Apply the upstream Linux kernel patches referenced in commits 00c13b4a, 673611cc, 7ad2fec2, and abac9820 as they land in your distribution
Update to a distribution kernel that incorporates the sanity check in the extron-da-hd-4k-plus driver
Restrict physical and logical access to HDMI ports and CEC-capable peripherals on sensitive systems

Patch Information

The fix is available in the upstream Linux stable tree. Refer to Kernel Commit 00c13b4a, Kernel Commit 673611cc, Kernel Commit 7ad2fec2, and Kernel Commit abac9820 for the patch content and affected stable branches.

Workarounds

Unload or blacklist the extron-da-hd-4k-plus kernel module on systems that do not require Extron CEC functionality
Disable CEC support on downstream HDMI devices where the feature is not needed
Physically isolate CEC-capable ports from untrusted equipment until the patched kernel is deployed

bash

# Blacklist the vulnerable driver until patched kernels are deployed
echo "blacklist extron-da-hd-4k-plus" | sudo tee /etc/modprobe.d/cve-2026-89898.conf
sudo update-initramfs -u
sudo reboot