CVE-2026-58083: Kernel Privilege Escalation Vulnerability

CVE-2026-58083 Overview

CVE-2026-58083 is a use-after-free vulnerability [CWE-416] in the FreeBSD kernel's kqueue subsystem. The flaw resides in the routine that copies knotes during a fork() operation. A knote using a timer-based filter can fire and be enqueued on the kqueue active list before the copy routine completes. The copy logic does not detect this condition and can enqueue the new knote a second time, corrupting the active list. The copy routine also fails to hold the appropriate locks while reading knote state, opening additional race windows. An unprivileged local user can trigger the use-after-free to escalate privileges on affected systems.

Critical Impact
An unprivileged local user can trigger a kernel use-after-free through crafted kqueue and fork() sequences, potentially achieving root-level code execution.

Affected Products

FreeBSD operating system (kernel kqueue subsystem)
Systems using timer-based EVFILT_TIMER knotes across fork() boundaries
See the FreeBSD Security Advisory for specific affected releases and patch levels

Discovery Timeline

2026-08-19 - CVE-2026-58083 published to NVD
2026-08-20 - Last updated in NVD database

Technical Details for CVE-2026-58083

Vulnerability Analysis

The FreeBSD kqueue subsystem tracks registered events using knote structures attached to per-process kqueues. When a process calls fork(), the kernel duplicates the parent's kqueue state into the child by iterating the parent's registered knotes and cloning each one. This copy operation is not atomic with respect to knote activation.

A knote configured with EVFILT_TIMER is driven by callout routines that fire independently of the fork path. If the timer expires during the copy window, the firing path enqueues the knote onto the kqueue's active list. The copy routine then proceeds without observing the intermediate state and enqueues the newly cloned knote a second time.

Double insertion corrupts the active list, and the copy routine also reads knote fields without acquiring the knote lock. The resulting dangling pointer produces a use-after-free that an attacker can shape into arbitrary kernel memory writes and privilege escalation.

Root Cause

The root cause is missing synchronization in the fork-time knote copy routine. The routine assumes knote state is stable during iteration, but timer-driven filters can mutate the active list concurrently. Absent locks and absent enqueue checks combine to allow duplicated list membership and stale references.

Attack Vector

Exploitation requires only local, unprivileged access. An attacker registers one or more timer knotes with short expirations on a kqueue, then calls fork() to race the timer against the kernel copy path. Repeated attempts win the race and trigger the use-after-free. See the FreeBSD Security Advisory for the reference exploitation scenario.

Detection Methods for CVE-2026-58083

Indicators of Compromise

Unexpected kernel panics or page fault while in kernel mode messages referencing kqueue, knote_fork, or knote_enqueue frames
Processes issuing high-frequency kevent() registrations of EVFILT_TIMER followed by tight fork() loops from non-privileged users
New setuid binaries or root-owned processes spawned from unprivileged parent PIDs shortly after such syscall patterns

Detection Strategies

Monitor kernel logs and crash dumps for repeated faults in the kqueue code path, which typically precede a working exploit
Audit syscall telemetry for unusual combinations of kevent() timer registration and rapid fork() calls from the same process
Correlate local privilege escalation events with prior kernel warnings involving active-list corruption

Monitoring Recommendations

Enable and centralize dtrace or audit framework logging for kevent, fork, and process credential changes
Alert on unexpected UID transitions to 0 from processes that did not execute a known setuid binary
Track kernel message rate spikes on FreeBSD hosts to surface exploitation attempts that fail before succeeding

How to Mitigate CVE-2026-58083

Immediate Actions Required

Apply the FreeBSD kernel patch referenced in the FreeBSD Security Advisory and reboot affected hosts
Inventory FreeBSD systems and prioritize multi-user hosts, jails, and any system where untrusted local accounts exist
Restrict shell and local execution access on affected systems until patches are deployed

Patch Information

FreeBSD has released an official kernel patch that adds proper locking around knote state reads during fork and prevents duplicate active-list enqueue. Refer to the FreeBSD Security Advisory for supported release versions, patch commits, and freebsd-update instructions.

Workarounds

No supported workaround preserves kqueue functionality; patching is the required remediation
Where patching is delayed, remove interactive local access for untrusted users and disable non-essential jails
Monitor for the kernel-level indicators above and isolate hosts that exhibit kqueue-related faults