CVE-2026-58099 in Kqueue
Summary
by MITRE • 10/15/2026
When copying knotes from a parent kqueue to a child, the copy code did not correctly exclude marker knotes (used internally to track list traversal position) before marking them as in-flux and releasing the kqueue lock. If another thread freed a marker while the lock was dropped, the subsequent in-flux decrement operated on freed memory.
An unprivileged local user may be able to exploit these races to escalate privileges.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability described involves a critical race condition within the FreeBSD kqueue subsystem during the process of copying kernel notes from a parent queue to a child, typically occurring when a new process is spawned via fork or similar mechanisms that inherit file descriptors and associated event notification structures. The core technical flaw lies in the handling of internal marker knotes, which are not user-visible events but rather structural elements used by the kqueue implementation to track traversal positions within linked lists of registered notes. During the copy operation, the system is supposed to exclude these internal markers from being marked as in-flux and released under lock protection. However, due to a logic error, certain marker knotes were incorrectly included in this process. This oversight means that when the kqueue lock was dropped to allow for further processing or concurrent access, these specific kernel objects remained in an inconsistent state where they were considered active but lacked proper synchronization guarantees.
This inconsistency creates a window of opportunity for a race condition involving memory management and concurrency control. Specifically, if another thread attempts to free one of these improperly handled marker knotes while the lock is dropped, it can succeed because the object was not correctly flagged as being in-flux or protected by the current operation's scope. Consequently, when the original copying routine later proceeds to decrement the in-flux counter for that same object, it operates on memory that has already been freed and potentially reallocated for other purposes. This constitutes a classic use-after-free vulnerability, where the kernel accesses deallocated memory, leading to undefined behavior within the operating system's core subsystems.
The operational impact of this flaw is severe because kqueue is deeply integrated into the FreeBSD networking stack and general I/O event handling mechanisms. An unprivileged local user can exploit this race condition by crafting specific sequences of file descriptor operations that trigger the vulnerable code path repeatedly, increasing the probability of hitting the timing window where the memory corruption occurs. By carefully manipulating the allocation patterns in the kernel heap through subsequent system calls, an attacker may be able to control the contents of the freed memory region. This allows for arbitrary read or write primitives within the kernel address space, which can ultimately lead to privilege escalation from a standard user context to root-level access. The ability to execute code with elevated privileges compromises the integrity and confidentiality of all data on the affected system.
From a classification perspective, this vulnerability aligns closely with CWE-416, Use After Free, as it involves accessing memory after it has been returned to the free pool without proper validation or synchronization. Additionally, the exploitation technique leverages race conditions in concurrent execution paths, which relates to CWE-362, Concurrent Execution Using Shared Resource with Improper Synchronization. In terms of offensive security frameworks such as MITRE ATT&CK, this vulnerability facilitates lateral movement and privilege escalation techniques, specifically those involving kernel exploits or local privilege escalation via memory corruption vulnerabilities like CVE-2019-5548 which shares similar characteristics in the FreeBSD ecosystem.
Mitigation strategies primarily involve applying vendor-provided patches that correct the logic within the kqueue copy routine to ensure marker knotes are properly excluded from the in-flux tracking mechanism before any locks are released. System administrators should prioritize updating their systems to versions where this synchronization issue is resolved, ensuring that internal kernel structures maintain strict consistency during concurrent operations. For environments where immediate patching is not feasible, implementing mandatory access control policies such as SELinux or FreeBSD's TrustedBSD MAC framework can help restrict the impact of potential exploits by limiting the capabilities available to unprivileged processes, although these are compensating controls rather than fixes for the underlying code defect. Regular auditing of kernel module behavior and monitoring for unusual system call patterns related to file descriptor manipulation may also aid in detecting attempted exploitation activities.