CVE-2026-68200: Linux Kernel Use-After-Free Vulnerability

CVE-2026-68200 Overview

CVE-2026-68200 is a use-after-free vulnerability in the Linux kernel's Advanced Linux Sound Architecture (ALSA) timer subsystem. The flaw resides in the userspace-driven timer (utimer) TRIGGER ioctl path, which invokes snd_timer_interrupt() without adequate serialization. Two threads triggering the same utimer can execute snd_timer_interrupt() concurrently on a single snd_timer instance. A race between the interrupt path and the close path allows the instance and its callback_data to be freed while a callback is still executing. Any local user able to open /dev/snd/timer can reach the flaw through a user timer instance or a sequencer queue timer bound to the utimer.

Critical Impact
Local unprivileged users with access to /dev/snd/timer can trigger a kernel use-after-free, enabling denial of service or local privilege escalation.

Affected Products

Linux kernel versions containing the ALSA userspace-driven timer (utimer) subsystem
Distributions shipping kernels prior to the upstream fixes referenced in the stable tree
Systems exposing /dev/snd/timer to unprivileged users

Discovery Timeline

2026-08-10 - CVE-2026-68200 published to NVD
2026-08-13 - Last updated in NVD database

Technical Details for CVE-2026-68200

Vulnerability Analysis

The vulnerability is a race condition leading to a use-after-free in the ALSA timer subsystem. The utimer TRIGGER ioctl calls snd_timer_interrupt() directly and holds no serialization mechanism against concurrent invocation. When two threads trigger the same utimer simultaneously, both execute snd_timer_interrupt() against the same snd_timer object.

During processing, snd_timer_process_callbacks() releases timer->lock around each instance callback and tracks the in-flight callback using a single SNDRV_TIMER_IFLG_CALLBACK bit. This single-bit indicator cannot represent two concurrent callbacks. When a second interrupt re-queues an instance whose callback is already running, both callbacks execute at once. The first callback to complete clears the flag, and snd_timer_close_locked() observes the cleared bit and proceeds to free the instance and its callback_data while the second callback is still live, dereferencing freed memory.

Root Cause

The root cause is insufficient synchronization between the utimer trigger path and the instance callback lifecycle. The SNDRV_TIMER_IFLG_CALLBACK flag was designed as a binary in-flight indicator and does not track concurrent re-entry of the same callback. snd_timer_close_locked() relies on that bit to drain callbacks before deallocation, so the drain returns prematurely when the first of two concurrent callbacks clears the flag.

Attack Vector

An attacker with local access and the ability to open /dev/snd/timer can spawn multiple threads that issue the utimer TRIGGER ioctl against the same timer instance. By racing the trigger against a close operation, the attacker forces snd_timer_close_locked() to free the instance while an in-flight callback continues to execute. The same primitive is reachable through a sequencer queue timer bound to the utimer. Successful exploitation corrupts kernel memory and can be leveraged for privilege escalation. Remote exploitation is not possible.

Refer to the upstream fixes for the exact code path and locking changes: Kernel Commit 1395327, Kernel Commit 70d28b, Kernel Commit 996c24, and Kernel Commit c10781.

Detection Methods for CVE-2026-68200

Indicators of Compromise

Kernel oops or panic messages referencing snd_timer_interrupt, snd_timer_process_callbacks, or snd_timer_close_locked in dmesg or /var/log/kern.log.
KASAN reports flagging use-after-free reads or writes within the ALSA timer subsystem.
Unexpected process crashes or system instability correlated with processes holding file descriptors on /dev/snd/timer.

Detection Strategies

Audit which local users and containers have read/write access to /dev/snd/timer and the ALSA sequencer devices.
Enable KASAN on test kernels to surface use-after-free conditions during fuzzing of the ALSA ioctl surface.
Monitor for processes issuing high-frequency ioctl calls against /dev/snd/timer from multiple threads, which is atypical for standard audio workloads.

Monitoring Recommendations

Ingest kernel logs into a centralized logging platform and alert on ALSA timer stack traces.
Track syscall telemetry (auditd or eBPF) for openat on /dev/snd/timer from unexpected processes and users.
Correlate crashes on shared or multi-tenant hosts to identify potential exploitation attempts.

How to Mitigate CVE-2026-68200

Immediate Actions Required

Apply the upstream Linux kernel patches referenced in the stable tree and update to a distribution kernel that includes the fix.
Restrict access to /dev/snd/timer and ALSA sequencer devices to trusted users where audio functionality is not required.
Reboot affected systems after patching to ensure the vulnerable kernel is no longer resident.

Patch Information

The fix skips re-queuing an instance and its slaves to the ack/sack list while a callback is in flight. Accumulated pticks are delivered on the next tick, preventing lost events without re-entering the callback. The fix is available in the commits linked above. Consult your distribution's security advisories for the exact kernel package versions containing the backport.

Workarounds

Unload the snd_timer module (modprobe -r snd_timer) on systems that do not require ALSA timer functionality.
Tighten permissions on /dev/snd/timer using udev rules to limit access to the audio group only.
On multi-tenant or container hosts, block /dev/snd/* from unprivileged containers using device cgroup rules or seccomp filters.

bash

# Example udev rule to restrict /dev/snd/timer access
# /etc/udev/rules.d/60-alsa-timer.rules
KERNEL=="timer", SUBSYSTEM=="sound", MODE="0640", GROUP="audio"

# Reload udev rules
sudo udevadm control --reload-rules
sudo udevadm trigger