CVE-2026-58098: FreeBSD semop(2) Privilege Escalation Flaw

CVE-2026-58098 Overview

CVE-2026-58098 is a kernel vulnerability in FreeBSD's System V semaphore subsystem. The flaw affects the semop(2) system call, which releases a per-set lock and sleeps when blocked on a semaphore condition. On wakeup, the kernel verifies the semaphore set still exists by comparing a 15-bit sequence number encoded in the IPC identifier. Because the counter is only 15 bits wide, rapid creation and destruction of semaphore sets in the same table slot can wrap it around. The blocked semop(2) caller then incorrectly concludes the original set is still valid and performs an out-of-bounds access on kernel heap memory [CWE-787]. An unprivileged local user can abuse this condition to escalate privileges.

Critical Impact
An unprivileged local user can trigger an out-of-bounds write on kernel heap memory, potentially leading to full privilege escalation on affected FreeBSD systems.

Affected Products

FreeBSD 14.4 (including patch levels p1 through p9)
FreeBSD 14.5
FreeBSD 15.0 (including patch levels p1 through p13)
FreeBSD 15.1 (including patch levels p1 through p3)

Discovery Timeline

2026-10-15 - CVE-2026-58098 published to NVD
2026-10-19 - Last updated in NVD database

Technical Details for CVE-2026-58098

Vulnerability Analysis

FreeBSD's System V semaphore implementation uses a per-set lock to serialize operations on each semaphore set. When semop(2) must block waiting for a semaphore condition, it releases this lock and sleeps. On wakeup, the kernel must determine whether the original semaphore set still exists or was destroyed and reused during the sleep window. This check relies on a sequence number embedded in the IPC identifier returned by semget(2). The implementation allocates only 15 bits for this sequence counter, which is insufficient to guarantee uniqueness under high churn. The bug falls under the kernel vulnerability and out-of-bounds write category.

Root Cause

The root cause is an insufficiently sized validation counter. The 15-bit sequence number embedded in the IPC identifier cannot uniquely identify a semaphore set across many allocation and deallocation cycles. When the counter wraps around to a prior value while a caller is blocked, the stale IPC identifier passes the validation check. The kernel then dereferences the semaphore set structure as if it were the original, even though the slot now holds a different set with potentially different dimensions.

Attack Vector

An unprivileged local user issues a blocking semop(2) call against a target semaphore set. In parallel, the attacker creates and destroys semaphore sets in a tight loop, targeting the same table slot. After roughly 2^15 allocation cycles, the sequence number wraps to match the blocked caller's cached identifier. When the semaphore condition is satisfied and the blocked caller wakes, it accesses a semaphore index that is valid for the original set but out of bounds for the current occupant. This out-of-bounds access on adjacent kernel heap memory can be shaped into a privilege escalation primitive. Exploitation requires local access with no privileges and no user interaction.

See the FreeBSD Security Advisory SA-26:64 for technical details.

Detection Methods for CVE-2026-58098

Indicators of Compromise

Unprivileged processes making unusually high-rate semget(2) and semctl(IPC_RMID) sequences, suggesting sequence number churn.
Kernel panics, page faults, or memory corruption traces originating in the sysvsem subsystem.
Unexpected privilege transitions from local user processes shortly after heavy IPC activity.

Detection Strategies

Audit ipcs -s output and process accounting to identify processes that create and destroy large numbers of semaphore sets.
Enable kernel audit logging via auditd and track AUE_SEMGET, AUE_SEMCTL, and AUE_SEMOP events for anomalous volume.
Correlate kernel crash reports or dtrace probes on sysvsem functions with suspicious user activity.

Monitoring Recommendations

Baseline normal System V semaphore usage per host and alert on deviations exceeding that baseline.
Forward FreeBSD audit logs to a centralized analytics platform for correlation with process execution and privilege change events.
Monitor for new setuid processes or shells spawned by accounts that recently performed heavy IPC operations.

How to Mitigate CVE-2026-58098

Immediate Actions Required

Apply the FreeBSD security update referenced in advisory FreeBSD-SA-26:64.sysvsem to all affected 14.4, 14.5, 15.0, and 15.1 systems.
Reboot systems after patching to ensure the updated kernel is loaded.
Restrict local shell access on multi-user systems until patches are deployed.

Patch Information

FreeBSD has published patched kernels through FreeBSD Security Advisory SA-26:64. Administrators should follow the advisory's freebsd-update or source-rebuild procedure for their release branch. Verify the patched kernel is active by checking uname -a and the kernel build identifier after reboot.

Workarounds

Disable System V semaphore support where applications do not require it by unloading the sysvsem kernel module with kldunload sysvsem.
Limit local user access and reduce the number of untrusted accounts on affected hosts.
Apply mandatory access control policies such as MAC or jails to constrain which users can invoke semget(2) and semop(2).

bash

# Check for and unload the sysvsem module where it is not required
kldstat | grep sysvsem
kldunload sysvsem

# Apply FreeBSD binary updates and reboot
freebsd-update fetch
freebsd-update install
shutdown -r now