Zephyr Project up to 4.4.x Wait Queue kernel/include/wait_q.h z_waitq_head wait_q use after free

Summaryinfo

A vulnerability, which was classified as very critical, was found in Zephyr Project Zephyr up to 4.4.x. This impacts the function z_waitq_head of the file kernel/include/wait_q.h of the component Wait Queue. Executing a manipulation of the argument wait_q can lead to use after free. This vulnerability is tracked as CVE-2026-78118. The attack is restricted to local execution. No exploit exists. You should upgrade the affected component.

Detailsinfo

A vulnerability classified as very critical was found in Zephyr Project Zephyr up to 4.4.x. This vulnerability affects the function z_waitq_head of the file kernel/include/wait_q.h of the component Wait Queue. The manipulation of the argument wait_q with an unknown input leads to a use after free vulnerability. The CWE definition for the vulnerability is CWE-416. Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

The kernel helper z_waitq_head() (kernel/include/wait_q.h) reads the head of a thread wait queue, but on SMP systems it was called from several places without holding the scheduler spinlock _sched_spinlock that serializes wait-queue mutation. The consequential call site is z_timer_expiration_handler() in kernel/timer.c, which held only the timer subsystem lock timer_lock while reading timer->wait_q and then unpending, setting the return value of, and readying the thread it obtained. halt_thread() in kernel/sched.c removes an aborted thread from its wait queue while holding only _sched_spinlock, so the two paths are not mutually exclusive. When the last waiter is removed on another CPU inside that window, sys_dlist_peek_head() can return the list sentinel — a pointer into the k_timer object itself — typed as a struct k_thread * (with CONFIG_WAITQ_SCALABLE, rb_get_min() can return an arbitrary node of a tree being restructured), or it can return a pointer to a thread that has already been unpended and, for dynamically allocated threads, freed. The handler then calls unpend_thread_no_timeout(), which performs a sys_dlist_remove() through pointers read from that object, and arch_thread_return_value_set(), which writes unconditionally through the pointer. The result is out-of-bounds and use-after-free writes in supervisor context, plus scheduler state corruption; a milder outcome is a thread that has re-pended elsewhere being woken with a success return value from an unrelated blocking call. All of the primitives needed to drive the interleaving — k_timer_start(), k_timer_status_sync(), k_thread_create() and k_thread_abort() — are system calls, so with CONFIG_USERSPACE enabled an unprivileged user-mode thread can attempt the race against kernel-owned data. Exploitation requires a multi-CPU build (CONFIG_MP_MAX_NUM_CPUS greater than one; the affected region is a no-op on uniprocessor targets), local code execution, and winning a narrow timing window, so the practical outcome is most often a kernel crash rather than controlled corruption. The fix splits the helper into z_waitq_head_locked() for callers that already hold the scheduler spinlock and a z_waitq_head() that acquires it, and brings the timer expiration handler's head read, unpend and ready under a single LOCK_SCHED_SPINLOCK region.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-78118 since 08/22/2026. The exploitation appears to be easy. The attack needs to be approached locally. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 10/21/2026).

Upgrading to version 4.5.0 eliminates this vulnerability. Applying the patch a9b21fb354e8468dd15db307c3e599dc68b7ef33 is able to eliminate this problem. The best possible mitigation is suggested to be upgrading to the latest version.

Once again VulDB remains the best source for vulnerability data.

Productinfo

Type

• Project Management Software

Vendor

• Zephyr Project

Name

• Zephyr

Version

• 4.0
• 4.1
• 4.2
• 4.3
• 4.4

CPE 2.3info

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CPE 2.2info

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