CVE-2023-0266

What is the Linux Kernel ALSA Subsystem?

ALSA (Advanced Linux Sound Architecture) is the kernel subsystem that manages audio hardware drivers and provides the sound API for all Linux audio operations. The snd_ctl_elem_read function in sound/core/control.c handles reading the current value of audio control elements — mixer controls, PCM settings, and other audio parameters — on behalf of user-space applications. ALSA's control interface is accessible to users with audio group membership on most Linux distributions, and because the interface is backed by complex kernel state, concurrency bugs in its locking can be exploited to reach kernel-mode code execution.

Overview

CVE-2023-0266 is a use-after-free vulnerability (CWE-416) in the Linux kernel's ALSA sound subsystem that allows an attacker with access to the ALSA control interface to escalate privileges to ring0 — the highest kernel privilege level. The vulnerability arises from a race condition in snd_ctl_elem_read: the rwsem (read-write semaphore) protecting the control element was released too early, creating a window where a concurrent operation could free the control element while a stale pointer to it remained in use. An attacker winning the race can corrupt kernel memory and achieve privilege escalation.

The CVSS vector reflects the nuanced attack surface: AV:A (adjacent — requires access to the ALSA interface, not arbitrary local code execution), AC:H (high complexity — race condition requires precise timing), and S:C (scope changed — exploit transitions from user-mode to kernel ring0). Despite these constraints, CISA's March 2023 KEV addition confirms real-world weaponization.

Affected Versions

Technical Details

The use-after-free occurs in snd_ctl_elem_read in the kernel's sound/core/control.c. The function reads audio control element values under a read lock (rwsem), but the lock was released at a point where subsequent accesses to the control element object could still occur. The race window between lock release and final object access allows a concurrent thread to:

1. Free the control element — a parallel snd_ctl_remove or similar operation frees the kernel object
2. Reallocate the freed memory — a separate allocation occupies the same memory region with attacker-controlled content
3. Use the stale pointer — snd_ctl_elem_read continues executing and accesses the now-freed memory through the dangling pointer

The controlled read or write through the dangling pointer enables kernel heap exploitation — overwriting function pointers, token privilege bitmasks, or other kernel control structures to achieve ring0 code execution.

The fix moves the rwsem lock to encompass the full duration of the snd_ctl_elem_read operation, eliminating the race window entirely.

Discovery

The vulnerability was confirmed as actively exploited at the time CISA added it to the KEV catalog on March 30, 2023 — two months after the CVE was published on January 30, 2023. The two-month gap between publication and KEV addition suggests exploitation was either detected in incident response activity or observed in threat intelligence after the patch existed but was not yet widely deployed.

Exploitation Context

Linux kernel ALSA UAF vulnerabilities are exploited in contexts where an attacker has already achieved local code execution at a reduced privilege level and needs to escalate to root or kernel. Typical exploitation chains include:

• Container escape — containerized workloads running as non-root inside a container escaping to host kernel privileges via a kernel UAF
• Local privilege escalation — a local user account with audio group membership (standard on desktop Linux) exploiting the ALSA interface to escalate to root
• Post-compromise persistence — an attacker with limited shell access using the kernel UAF to disable security mechanisms that require root

The AV:A (adjacent) designation reflects the ALSA interface requirement, but on shared Linux systems with audio hardware (typical servers, workstations, cloud VMs with audio pass-through), this is accessible to standard users.

Remediation

1. Apply Linux kernel updates from your distribution — the fix is a stable-queue patch moving the rwsem lock inside snd_ctl_elem_read; distribution kernels (RHEL, Ubuntu, Debian, etc.) include this fix in their security updates.
2. Prioritize systems with audio hardware — CVE-2023-0266 requires access to the ALSA control interface, which is active on systems with audio hardware; servers without audio are less exposed.
3. Restrict audio group membership — on multi-user systems, limit membership in the audio group to users who genuinely need hardware audio access.
4. Monitor for kernel exploitation patterns — detection of unexpected privilege escalations, unusual kernel module loading, or processes spawning children at elevated privileges are behavioral indicators of kernel UAF exploitation.