CVE-2026-58093 in FreeBSD
Summary
by MITRE • 08/26/2026
The TIOCSCTTY ioctl handler drops the tty lock in order to acquire the process tree lock. After reacquiring the tty lock, the handler did not revalidate the state of the terminal, and could proceed to link a terminal that was concurrently being destroyed to the calling process' session.
An unprivileged local user can exploit this race condition to escalate privileges.
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Analysis
by VulDB Data Team • 08/26/2026
The vulnerability described involves a critical locking inconsistency within the Linux kernel's tty subsystem, specifically in the handler for the TIOCSCTTY ioctl command. This system call is used by processes to claim control of a terminal device as their controlling terminal. The core technical flaw lies in the sequence of lock acquisitions during this operation. To ensure data integrity and prevent race conditions involving process groups or sessions, the kernel must acquire specific locks such as the tty lock and the process tree lock. In this flawed implementation, the handler releases the tty lock temporarily to acquire the broader process tree lock. This release creates a window where the state of the terminal device is no longer protected by its dedicated synchronization primitive.
During this interval between releasing the tty lock and reacquiring it, an unprivileged local user can exploit the race condition through concurrent operations that modify or destroy the target terminal structure. Specifically, if another thread or process initiates the destruction of the same terminal while the ioctl handler is waiting for the process tree lock, the terminal's internal state may change significantly. When the ioctl handler finally reacquires the tty lock, it fails to revalidate whether the terminal is still in a valid, active state suitable for association with a session. Consequently, the code proceeds under the assumption that the terminal remains intact and usable, leading it to link a terminal object that is concurrently being freed or destroyed to the calling process's session structure.
This logic error results in use-after-free conditions or corruption of kernel data structures associated with session management. By manipulating this race condition, an attacker can cause the kernel to reference memory that has been deallocated or repurposed for other uses. This type of vulnerability typically falls under CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition) and often relates to CWE-416: Use After Free if the destruction process frees the underlying memory structures before they are safely dereferenced in the ioctl handler. The ATT&CK framework categorizes this behavior as part of Privilege Escalation techniques, specifically those involving kernel exploitation or local privilege escalation via race conditions (T1068).
The operational impact of this vulnerability is severe because it allows an unprivileged user to escalate privileges to root or other high-privilege levels. By corrupting the session structure or causing a kernel panic through invalid memory access, the attacker can gain unauthorized control over the system. This compromises the confidentiality, integrity, and availability of the host operating system. Attackers can leverage this flaw to bypass security controls, install persistent backdoors, or disrupt critical services running on the machine. The ability to escalate privileges without authentication makes this a high-severity issue that requires immediate attention from system administrators and kernel developers.
Mitigation strategies primarily involve applying vendor-provided patches that correct the locking logic within the tty subsystem. Developers must ensure that locks are held for the minimum necessary duration but also that state validation occurs immediately after reacquiring critical locks to prevent operations on stale or invalid objects. In environments where patching is not immediately feasible, restricting access to terminal devices and monitoring for unusual ioctl calls can provide limited defense-in-depth. However, given the nature of kernel-level race conditions, relying solely on perimeter defenses is insufficient; timely application of security updates remains the most effective remediation path to eliminate this vulnerability vector entirely.