CVE-2026-58093: TIOCSCTTY Race Condition Vulnerability

CVE-2026-58093 Overview

CVE-2026-58093 is a race condition in the FreeBSD kernel's TIOCSCTTY ioctl handler. The handler drops the tty lock to acquire the process tree lock. After reacquiring the tty lock, it fails to revalidate terminal state. An unprivileged local user can exploit this window to link a terminal that is concurrently being destroyed to the calling process' session. The result is local privilege escalation on affected FreeBSD systems.

Critical Impact
An unprivileged local user can win a kernel race between tty acquisition and destruction to escalate privileges, gaining high impact on confidentiality, integrity, and availability.

Affected Products

FreeBSD operating system (see FreeBSD Security Advisory SA-26:62 for affected branches and patch levels)
Systems exposing the TIOCSCTTY ioctl to unprivileged local users
Multi-user FreeBSD hosts and jails where local shell access is available

Discovery Timeline

2026-08-26 - CVE-2026-58093 published to NVD
2026-08-27 - Last updated in NVD database

Technical Details for CVE-2026-58093

Vulnerability Analysis

The flaw resides in the FreeBSD kernel's handling of the TIOCSCTTY ioctl, which sets a controlling terminal for a session. To respect kernel lock ordering, the handler releases the tty lock before acquiring the process tree lock (proctree_lock). It then reacquires the tty lock to complete the operation.

During the interval when the tty lock is released, another thread can advance destruction of the same terminal. When the original handler resumes, it does not recheck whether the terminal is still valid. It proceeds to link a partially destroyed tty into the caller's session structure.

The result is a use-after-free style condition on kernel tty state. An attacker with a local shell can craft two threads: one repeatedly invoking TIOCSCTTY, and one triggering terminal teardown. Winning the race yields references to freed or transitioning kernel objects, which can be manipulated to corrupt kernel memory and gain elevated privileges.

Root Cause

The root cause is a missing revalidation check after lock re-acquisition, a classic time-of-check to time-of-use (CWE-362) concurrency defect. The handler assumes the terminal state observed before releasing the tty lock remains consistent, but concurrent destruction invalidates that assumption.

Attack Vector

Exploitation requires local access and low privileges. The attacker executes a multi-threaded program that races TIOCSCTTY invocations against terminal destruction paths. Attack complexity is high because the attacker must reliably interleave scheduler events across two kernel lock domains. No user interaction is required. See the FreeBSD Security Advisory SA-26:62 for the definitive technical description.

No verified public exploit code is available at the time of publication. The vulnerability mechanism is described in prose above; refer to the vendor advisory for kernel-level detail.

Detection Methods for CVE-2026-58093

Indicators of Compromise

Unexpected kernel panics or ttydev_leave-adjacent stack traces on FreeBSD hosts
Unprivileged processes acquiring UID 0 without corresponding su, sudo, or doas audit records
Repeated ioctl(TIOCSCTTY) calls from non-login shell processes visible in dtrace or audit logs

Detection Strategies

Enable FreeBSD auditd with pc and ex classes to record ioctl activity and privilege transitions
Use dtrace probes on fbt::tty_ioctl:entry filtered by TIOCSCTTY to baseline legitimate callers
Alert on any process performing sustained high-frequency TIOCSCTTY calls from multiple threads

Monitoring Recommendations

Forward FreeBSD audit trails and kernel messages to a centralized analytics platform for correlation
Monitor for unexplained UID transitions and new setuid-like behavior from user shells
Track kernel crash reports across the fleet and treat clustered tty-related panics as potential exploitation attempts

How to Mitigate CVE-2026-58093

Immediate Actions Required

Apply the kernel patches referenced in FreeBSD Security Advisory SA-26:62 and reboot affected hosts
Inventory FreeBSD systems that grant shell access to untrusted users, including jails and CI runners, and prioritize those for patching
Review recent kernel crash dumps for signatures consistent with tty race exploitation

Patch Information

FreeBSD has published fixes in Security Advisory SA-26:62. The corrected handler revalidates terminal state after reacquiring the tty lock, aborting the operation if the terminal is being destroyed. Administrators should use freebsd-update or rebuild from source at the patched revision, then reboot to load the fixed kernel.

Workarounds

Restrict local shell access to trusted users only on systems that cannot be patched immediately
Remove or restrict access to FreeBSD jails and shared build environments where untrusted code can execute
Increase kernel crash reporting verbosity to accelerate identification of exploitation attempts until patching completes

bash

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