CVE-2026-101308 in FreeBSD
Summary
by MITRE • 10/15/2026
When an audio channel's buffer needs to be resized, the sound(4) driver checks that the buffer is not memory-mapped and then drops the channel lock to allocate a new buffer. An mmap(2) call on another thread can complete during this window, after which the resize path frees the now-mapped buffer without rechecking.
An unprivileged local user with access to a sound device can exploit this race to trigger a use-after-free in the kernel, potentially escalating privileges.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability described resides within the audio subsystem of the operating system's kernel, specifically affecting the sound driver implementation known as sound(4). This flaw represents a classic time-of-check-to-time-of-use (TOCTOU) race condition that arises during the dynamic resizing of an audio channel buffer. The core technical issue stems from improper synchronization mechanisms when handling memory allocation and deallocation in a multi-threaded environment. When the system determines that an existing audio buffer requires resizing, it performs an initial safety check to verify whether the current buffer is currently mapped into user space via mmap(2). This check is critical because freeing a buffer that is actively being accessed by user-space applications can lead to memory corruption or unauthorized access. However, after passing this verification step, the driver releases the channel lock before proceeding with the actual allocation of the new buffer and subsequent deallocation of the old one.
This gap between checking the mapping status and performing the free operation creates a significant window of vulnerability. During this interval, where the lock is dropped to allow for memory allocation operations that might block or take time, another thread executing an mmap(2) call can successfully map the buffer into its address space. Because the driver does not re-verify the mapping status after reacquiring locks and before freeing the old buffer, it proceeds to release memory that is now actively referenced by a user-space process. This sequence of events results in a use-after-free condition within kernel memory. The freed memory may be reallocated for other purposes while still being accessible through the stale pointer held by the mapped region in user space, allowing an attacker to read or write arbitrary kernel memory locations depending on how subsequent allocations are handled and what data is placed into that memory region.
The operational impact of this vulnerability is severe due to its potential for local privilege escalation. An unprivileged local user who has access to a sound device can exploit this race condition by carefully timing their mmap(2) calls relative to the buffer resize operations triggered by audio playback or recording activities. By successfully triggering the use-after-free, an attacker gains the ability to manipulate kernel memory structures. This capability can be leveraged to overwrite function pointers, modify security-critical data structures such as credential objects, or execute arbitrary code within the kernel context. Consequently, this leads to a complete compromise of system integrity and confidentiality, allowing the user to bypass all standard access controls and gain root-level privileges on the affected system.
From a classification perspective, this vulnerability aligns with CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition, as it involves checking a condition at one point in time and using that information later when the state may have changed. It also relates to CWE-416: Use After Free, which describes the error of accessing memory after it has been freed. In terms of adversarial tactics, this exploit technique corresponds to ATT&CK Tactic TA0004: Privilege Escalation and specifically aligns with techniques such as T1055: Process Injection or more broadly kernel exploitation patterns that leverage race conditions for privilege escalation like those seen in various historical Linux kernel vulnerabilities. The attack vector is classified as Local (AV:L) because it requires physical or logical access to the system, but its impact on confidentiality and integrity is high due to the resulting root compromise.
Mitigation strategies must address both immediate patching and long-term architectural improvements. The primary remediation involves applying vendor-provided security patches that correct the synchronization logic within the sound driver. These updates typically ensure that the channel lock is held continuously throughout the entire buffer resize operation, including the verification of mapping status, allocation of new buffers, and deallocation of old ones, thereby eliminating the race window entirely. Alternatively, if locking granularity cannot be increased due to performance constraints, additional checks should be implemented after reacquiring locks to verify that no user-space mappings exist before freeing memory. On a broader security architecture level, enabling kernel hardening features such as Kernel Page Table Isolation (KPTI), Stack Canaries, and Full Relocation Read-Only (RELR) can mitigate the exploitation of use-after-free vulnerabilities by making it significantly harder for attackers to achieve code execution even if they successfully corrupt memory. Regular auditing of driver code for proper lock ordering and atomicity is also essential to prevent similar race conditions in other subsystems.