CVE-2026-63932: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-63932 Overview

CVE-2026-63932 is a buffer overflow vulnerability in the Linux kernel's Industrial I/O (IIO) chemical sensor driver for the MH-Z19B CO2 sensor. The flaw resides in the mhz19b_receive_buf() function, which handles serial device (serdev) callbacks. The function appends each serdev chunk into a fixed-size receive buffer of MHZ19B_CMD_SIZE and advances buf_idx by the chunk length without validating that the chunk fits in the remaining space. A large callback can overflow st->buf before the command path validates the reply.

Critical Impact
An oversized serial reply from the sensor path can overflow the fixed receive buffer in kernel memory, leading to memory corruption within the IIO subsystem.

Affected Products

Linux kernel versions containing the iio: chemical: mhz19b driver prior to the fix
Systems using the MH-Z19B CO2 sensor via serdev
Distributions shipping the affected stable kernel branches

Discovery Timeline

2026-07-19 - CVE-2026-63932 published to NVD
2026-07-19 - Last updated in NVD database

Technical Details for CVE-2026-63932

Vulnerability Analysis

The MH-Z19B driver uses the serial device bus (serdev) to communicate with a CO2 sensor connected via UART. When serial data arrives, the kernel invokes mhz19b_receive_buf() with a chunk of bytes. The driver copies each chunk into the fixed-size buffer st->buf of MHZ19B_CMD_SIZE bytes and increments buf_idx by the chunk length. The function performs no bounds check comparing buf_idx + len against MHZ19B_CMD_SIZE.

A large or crafted callback delivering data that exceeds the remaining space corrupts adjacent kernel memory. The command path validates the reply only after copying is complete, so validation cannot prevent the overwrite. This is an out-of-bounds write in kernel space triggered from a peripheral data path.

Root Cause

The root cause is missing length validation in the receive callback. The driver treats the fixed buffer as if the peripheral will always send well-formed, size-bounded replies. Any deviation from the expected reply size, whether from a faulty device, an attacker-controlled peripheral, or a fuzzed serial stream, can overrun st->buf.

Attack Vector

Exploitation requires the ability to deliver oversized data to the MH-Z19B serdev driver. This typically means physical or local access to the UART interface backing the sensor, or control over a peripheral that speaks the sensor protocol. The vulnerability is not remotely exploitable over the network in a standard configuration. The upstream fix resets the reply state before each command and rejects oversized serial replies before copying them into st->buf. When an oversized reply is detected, the driver wakes the waiter and reports -EMSGSIZE instead of overwriting the buffer.

Detection Methods for CVE-2026-63932

Indicators of Compromise

Unexpected kernel oops or panic messages referencing mhz19b_receive_buf or the IIO chemical subsystem
KASAN reports flagging out-of-bounds writes in st->buf within the mhz19b driver
Sensor read failures accompanied by unusually large serdev traffic bursts on the associated UART

Detection Strategies

Audit kernel versions across Linux fleets to identify hosts running vulnerable iio/chemical/mhz19b code paths
Enable KASAN or KFENCE on test kernels to catch out-of-bounds writes originating in serdev callbacks
Monitor dmesg for repeated warnings tied to the IIO chemical driver or -EMSGSIZE reports after patching

Monitoring Recommendations

Track kernel patch levels against the fixing commits 673478b, a5a0541, and ea69e7a on git.kernel.org
Alert on serial device driver crashes on embedded systems using CO2 sensors
Include kernel version telemetry in endpoint inventory data to prioritize patch rollout

How to Mitigate CVE-2026-63932

Immediate Actions Required

Update Linux kernels to a stable release containing the mhz19b bounds-check fix
Restrict physical and local access to systems exposing MH-Z19B sensors on UART interfaces
If the driver is not required, unload or blacklist the mhz19b module

Patch Information

The fix is available in the upstream Linux kernel across three stable commits: Kernel Git Commit 673478b, Kernel Git Commit a5a0541, and Kernel Git Commit ea69e7a. The patch resets reply state before each command and rejects oversized serial replies by returning -EMSGSIZE and waking the waiter instead of writing past st->buf.

Workarounds

Blacklist the mhz19b kernel module on systems that do not use the MH-Z19B sensor
Physically isolate UART interfaces bound to the mhz19b driver from untrusted peripherals
Rebuild kernels without CONFIG_MHZ19B where the sensor is not deployed

bash

# Blacklist the vulnerable driver until the kernel is patched
echo "blacklist mhz19b" | sudo tee /etc/modprobe.d/blacklist-mhz19b.conf
sudo rmmod mhz19b 2>/dev/null || true
sudo update-initramfs -u