CVE-2026-89876: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-89876 Overview

CVE-2026-89876 is an integer overflow vulnerability in the Linux kernel's tda18250 media tuner driver. The flaw occurs when the local variable exp equals zero. The subsequent shift operation 1 << (exp - 1) produces undefined behavior because exp - 1 underflows to a large unsigned value or a negative signed value. Kernel maintainers resolved the issue by adding a bounds check before the shift operation. The fix was distributed across multiple stable branches through a series of upstream commits.

Critical Impact
Undefined behavior in the tda18250 driver may lead to unpredictable kernel state, incorrect tuner configuration, or potential denial of service on systems that load the affected media module.

Affected Products

Linux kernel branches containing the drivers/media/tuners/tda18250.c driver
Systems using TDA18250 silicon tuner hardware for DVB media capture
Distributions shipping unpatched kernel versions prior to the fix commits

Discovery Timeline

2026-09-16 - CVE-2026-89876 published to the National Vulnerability Database
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-89876

Vulnerability Analysis

The vulnerability resides in the tda18250 DVB tuner driver within the Linux kernel media subsystem. The affected code path computes a rounding or masking value using the expression 1 << (exp - 1). When exp holds the value zero, the subtraction produces an out-of-range shift count. The C language specification classifies this as undefined behavior, and the kernel may generate an unexpected value or trigger a UBSAN report.

Because the driver operates in kernel context, any resulting logic error affects kernel state rather than a userspace process. Exploitability depends on whether an unprivileged actor can influence the value of exp through tuner configuration or media ioctl paths. Public information does not indicate a demonstrated privilege escalation or code execution primitive.

Root Cause

The root cause is missing input validation on the exponent variable exp before performing a left-shift operation [Integer Overflow]. The developer assumed exp would always be greater than zero. When calibration or channel selection produces a zero exponent, the arithmetic path exp - 1 yields an invalid shift amount for the target type width.

Attack Vector

The attack surface is limited to local users or processes with access to the media device nodes exposed by the DVB subsystem, typically under /dev/dvb/. Triggering the code path requires interaction with a tuner configuration that produces the zero-exponent condition. No remote network vector has been documented. The NVD entry does not currently assign a CVSS vector, and no exploit code is publicly available.

See the upstream fix commits for the exact code change and affected branches, including Kernel Commit 0e0fbdb and Kernel Commit f1ba602.

Detection Methods for CVE-2026-89876

Indicators of Compromise

Kernel log entries from UBSAN reporting shift exponent errors originating in tda18250.c
Unexpected DVB tuner initialization failures or repeated module reload attempts
Anomalous dmesg output referencing the tda18250 driver during device probe

Detection Strategies

Inventory running kernels against the fixed commit hashes and flag hosts running unpatched versions
Enable CONFIG_UBSAN in test builds to surface shift-related undefined behavior during driver exercise
Correlate DVB device access with process identity to detect unexpected users touching media nodes

Monitoring Recommendations

Forward dmesg and journald kernel messages to a centralized logging pipeline for review
Alert on repeated kernel warnings referencing tda18250 or shift-exponent UBSAN reports
Track kernel package versions across the fleet to identify hosts pending reboot after patching

How to Mitigate CVE-2026-89876

Immediate Actions Required

Apply the vendor-supplied kernel update from your Linux distribution as soon as it is available
Reboot affected hosts to activate the patched kernel image after installation
Restrict access to /dev/dvb/* device nodes to trusted users on multi-user systems

Patch Information

The fix has been merged into multiple stable Linux kernel branches. Reference commits include Kernel Commit 3e5568d, Kernel Commit 4e21f0e, Kernel Commit 593b117, Kernel Commit 6dd8e25, Kernel Commit 7c62bd6, and Kernel Commit cf8e3b3. Consult your distribution's security tracker for the exact package version that includes the backport.

Workarounds

Blacklist the tda18250 kernel module on systems that do not require DVB tuner functionality
Unload the module with modprobe -r tda18250 when the driver is not in active use
Apply strict file permissions on DVB device nodes to limit exposure of the vulnerable code path

bash

# Blacklist the tda18250 driver until patched kernel is deployed
echo "blacklist tda18250" | sudo tee /etc/modprobe.d/blacklist-tda18250.conf
sudo depmod -a
sudo modprobe -r tda18250