CVE-2026-58083 in FreeBSD
Summary
by MITRE • 08/19/2026
While the kernel was copying knotes during fork, a knote with a timer-based filter could fire and be enqueued on the kqueue's active list before the copy was complete. The copy routine did not account for this and could enqueue the new knote a second time, corrupting the active list. In addition, the copy routine did not hold the appropriate locks while reading knote state, allowing further races.
An unprivileged local user can trigger a use-after-free in the kernel, potentially leading to privilege escalation.
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Analysis
by VulDB Data Team • 08/19/2026
The vulnerability described involves a critical race condition within the FreeBSD kernel's kqueue implementation during process forking operations. Specifically, when the kernel attempts to duplicate file descriptor tables and associated notification structures from a parent process to its child, it must copy all registered knotes (kernel notes) that are attached to kqueues. The flaw arises because certain filters, particularly those based on timers or other asynchronous events, can trigger their filter routines during this copying phase. If such an event fires while the copy is in progress, the associated knote may be enqueued onto the active list of the original kqueue before the duplication routine has finished processing that specific note.
This timing discrepancy leads to a logic error where the copy routine, unaware that the knote has already been processed and added to the active queue by the filter callback, proceeds to enqueue it again as part of its standard copying procedure. This results in duplicate entries within the kqueue's active list structure. The presence of these duplicates corrupts the internal data structures used for managing pending events. Because the kernel relies on this linked list or similar structure to track which notifications are ready for delivery to user space, corruption here can lead to memory safety violations when the system attempts to traverse and process these entries later.
Compounding the structural issue is a lack of proper synchronization primitives during the read phase of the copy operation. The routine does not hold the necessary locks while inspecting the state of individual knotes. This absence of mutual exclusion allows for further race conditions where the state of a knote can change unpredictably between checks and updates. These concurrent modifications exacerbate the risk of memory corruption, specifically creating scenarios that lead to use-after-free vulnerabilities. In such cases, kernel memory previously allocated for a knote structure might be freed or reused while pointers to it are still being accessed due to the corrupted list state or race conditions in the copy logic.
The operational impact of this vulnerability is severe because it can be exploited by an unprivileged local user. By carefully crafting applications that utilize kqueues with timer-based filters and triggering rapid fork operations, a malicious actor can induce the kernel into executing invalid memory accesses. The primary consequence of exploiting this flaw is privilege escalation. An attacker who successfully triggers the use-after-free condition may gain the ability to execute arbitrary code within the kernel space, thereby obtaining root-level privileges on the affected system. This transforms a local denial-of-service or stability issue into a critical security breach that compromises the integrity and confidentiality of the entire operating environment.
From an industry standards perspective, this vulnerability aligns with CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization), specifically highlighting race conditions in kernel memory management. It also relates to CWE-416 (Use After Free) due to the potential for accessing freed memory through corrupted list pointers. In terms of attack vectors, this falls under ATT&CK technique T1059 (Command and Scripting Interpreter) if leveraged for initial access or more accurately T1068 (Exploitation for Privilege Escalation) as it allows moving from user space to kernel space with elevated rights. The lack of locking during state inspection also reflects CWE-362 in the context of insufficient synchronization mechanisms protecting shared resources.
Mitigation strategies primarily involve applying vendor-provided security patches that address the race condition by ensuring atomicity and proper lock acquisition during the knote copying process. System administrators should ensure that all kernel updates are applied promptly to close this window of exploitation. For environments where immediate patching is not feasible, monitoring for unusual patterns of fork operations combined with heavy kqueue usage might help in detecting potential exploitation attempts, although such detection is challenging due to the low-level nature of the flaw. Ultimately, restricting untrusted code execution and enforcing strict privilege separation models can reduce the attack surface available to local users attempting to leverage this kernel vulnerability.