FreeBSD thr_kill2 PrivEsc via Unchecked Signal Delivery
CVE-2026-45256 Published on June 26, 2026
Missing permission check in thr_kill2(2)
When used to deliver a signal to a specific thread, thr_kill2(2) called p_cansignal() to determine whether the operation was permitted but did not check the result before delivering the signal. The signal was sent even when the permission check failed. The system call returned the resulting error to the caller, but by then the signal had already been delivered.
The missing check allows an unprivileged local user who knows or can guess a target's process and thread IDs to send any signal to a process they would not normally be permitted to signal, including processes owned by other users or by root. The same check enforces jail boundaries, so a jailed process can signal processes on the host or in other jails. Thread IDs are allocated globally and sequentially, and so can be discovered by brute force with no visibility into the target.
An attacker can stop or terminate arbitrary processes, including critical system daemons, resulting in a Denial of Service (DoS).
Vulnerability Analysis
CVE-2026-45256 is exploitable with local system access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Type
Improper Privilege Management
The software does not properly assign, modify, track, or check privileges for an actor, creating an unintended sphere of control for that actor.
Products Associated with CVE-2026-45256
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Affected Versions
FreeBSD:- Version 15.0-RELEASE and below p10 is affected.
- Version 14.4-RELEASE and below p6 is affected.
- Version 14.3-RELEASE and below p15 is affected.