CVE-2026-53193: Linux Kernel Use-After-Free Vulnerability

CVE-2026-53193 Overview

CVE-2026-53193 is a use-after-free vulnerability in the Advanced Linux Sound Architecture (ALSA) timer subsystem of the Linux kernel. The flaw resides in the snd_timer_free() function, which unlinks pending snd_timer_instance objects and sets their ti->timer field to NULL before releasing resources. Slave timer instances associated with a master instance continue to reference the freed timer object after the master is unlinked, creating a use-after-free condition. The bug can be triggered through userspace-driven timers (CONFIG_SND_UTIMER), which allow creation and deletion of timer objects via simple file open and close operations while other applications retain access to the timer.

Critical Impact
Local attackers can trigger a use-after-free condition in the kernel's ALSA timer subsystem, potentially leading to memory corruption or local privilege escalation.

Affected Products

Linux Kernel (ALSA timer subsystem)
Kernel builds with CONFIG_SND_UTIMER enabled
Distributions shipping the affected upstream kernel versions

Discovery Timeline

2026-06-25 - CVE-2026-53193 published to NVD
2026-06-25 - Last updated in NVD database

Technical Details for CVE-2026-53193

Vulnerability Analysis

The vulnerability stems from incomplete cleanup logic in the ALSA timer release path. When snd_timer_free() releases a timer object, it iterates over pending snd_timer_instance objects, unlinks them, and clears their ti->timer pointer. This cleanup does not propagate to slave instances bound to a master instance. The slave instances retain dangling references to the freed timer structure.

Userspace-driven timers exacerbate the issue. The CONFIG_SND_UTIMER feature allows unprivileged callers to create and destroy timer objects through ordinary file descriptor operations. Other processes interacting with the same timer can race against the close operation and dereference freed memory.

Root Cause

The root cause is improper lifetime management of slave timer instances [CWE-416]. The cleanup routine in snd_timer_free() only addresses directly-linked instances and leaves slave instances pointing to memory that has been freed. The fix replaces the simple unlink with forced invocation of snd_timer_close() or snd_timer_close_locked() for each pending instance, ensuring slave instances are properly detached. A SNDRV_TIMER_IFLG_DEAD flag check is added at the start of snd_timer_close() to prevent double-close paths when drivers later call the function.

Attack Vector

An attacker with local access opens a userspace timer file descriptor and arranges slave instances tied to that timer. Closing the descriptor frees the timer object while slave instances retain references. Subsequent operations on the slave instances dereference freed kernel memory. Exploitation typically requires winning a race condition between the close path and concurrent timer access from another thread or process.

No verified public proof-of-concept code is available. Refer to the upstream kernel commits (586b21, 60e73a, da3039, f46093) for the patch details.

Detection Methods for CVE-2026-53193

Indicators of Compromise

Unexpected kernel oops or panic messages referencing snd_timer_free, snd_timer_close, or snd_timer_instance in dmesg output.
KASAN use-after-free reports involving the ALSA timer subsystem on kernels with KASAN enabled.
Repeated open and close operations on /dev/snd/timer or userspace timer devices from a single unprivileged process.

Detection Strategies

Enable Kernel Address Sanitizer (KASAN) in test environments to surface use-after-free conditions in the ALSA timer code paths.
Monitor auditd logs for unusual access patterns to /dev/snd/timer and process activity that combines rapid timer creation with concurrent timer reads.
Apply runtime kernel exploitation detection through eBPF-based behavioral monitoring focused on ALSA syscall sequences.

Monitoring Recommendations

Track kernel crash telemetry and forward dmesg output to centralized logging for analysis of ALSA-related faults.
Alert on local privilege escalation indicators such as unexpected setuid execution following sound subsystem activity.
Review installed kernel versions across the fleet to identify hosts still running vulnerable builds.

How to Mitigate CVE-2026-53193

Immediate Actions Required

Apply the upstream kernel patches referenced in commits 586b219a, 60e73ab8, da3039e9, and f46093dd from the stable kernel tree.
Update to the latest stable kernel release provided by your Linux distribution that incorporates these fixes.
Restrict access to ALSA timer devices by tightening permissions on /dev/snd/timer where the sound subsystem is not required.

Patch Information

The fix is available in the mainline and stable Linux kernel trees. The patch replaces the unlink-only cleanup with snd_timer_close[_locked]() calls for each pending instance and adds a SNDRV_TIMER_IFLG_DEAD check at the start of snd_timer_close(). See the upstream commits: 586b21, 60e73a, da3039, and f46093.

Workarounds

Disable CONFIG_SND_UTIMER in custom kernel builds where userspace-driven timers are not required.
Unload the snd_timer kernel module on systems that do not require audio timer functionality.
Restrict membership of the audio group and apply stricter device node permissions to reduce the local attack surface.

bash

# Verify current kernel version and check for the patched ALSA timer code
uname -r
modinfo snd_timer | grep -E 'filename|version'

# Restrict access to the ALSA timer device node
chmod 0660 /dev/snd/timer
chown root:audio /dev/snd/timer

# Optionally blacklist snd_timer if not required
echo 'blacklist snd_timer' | sudo tee /etc/modprobe.d/blacklist-snd-timer.conf