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============================================================================= FreeBSD-SA-26:25.thr Security Advisory
The FreeBSD Project

Topic: Missing permission check in thr_kill2(2)

Category: core
Module: thr
Announced: 2026-06-09
Credits: Yuxiang Yang, Yizhou Zhao, Ao Wang, Xuewei Feng, Qi Li,
and Ke Xu from Tsinghua University using GLM-5.1 from Z.ai Credits: Igor Gabriel Sousa e Souza
Affects: All supported versions of FreeBSD
Corrected: 2026-06-09 19:17:27 UTC (stable/15, 15.1-STABLE)
2026-06-09 19:20:05 UTC (releng/15.1, 15.1-RC3-p1)
2026-06-09 19:19:42 UTC (releng/15.0, 15.0-RELEASE-p10)
2026-06-09 19:17:45 UTC (stable/14, 14.4-STABLE)
2026-06-09 19:19:04 UTC (releng/14.4, 14.4-RELEASE-p6)
2026-06-09 19:18:34 UTC (releng/14.3, 14.3-RELEASE-p15)
CVE Name: CVE-2026-45256

This vulnerability was independently reported by multiple parties prior to publication.

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the following sections, please visit <URL:https://security.FreeBSD.org/>.

I. Background

The thr_kill2(2) system call delivers a signal to a specific thread of a process identified by its process and thread IDs. As with kill(2), the
kernel verifies that the calling process is permitted to signal the target before the signal is delivered.

II. Problem Description

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.

III. Impact

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).

IV. Workaround

No workaround is available.

V. Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date, and
reboot the system.

Perform one of the following:

1) To update your vulnerable system installed from base system packages:

Systems running a 15.0-RELEASE version of FreeBSD on the amd64 or arm64 platforms, which were installed using base system packages, can be updated
via the pkg(8) utility:

# pkg upgrade -r FreeBSD-base
# shutdown -r +10min "Rebooting for a security update"

2) To update your vulnerable system installed from binary distribution sets:

Systems running a RELEASE version of FreeBSD on the amd64 or arm64 platforms which were not installed using base system packages can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for a security update"

3) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-26:25/thr.patch
# fetch https://security.FreeBSD.org/patches/SA-26:25/thr.patch.asc
# gpg --verify thr.patch.asc

b) Apply the patch. Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in <URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI. Correction details

This issue is corrected as of the corresponding Git commit hash in the following stable and release branches:

Branch/path Hash Revision
- ------------------------------------------------------------------------- stable/15/ afa0c67a1ba3 stable/15-n283881 releng/15.1/ 068168fefd4b releng/15.1-n283549 releng/15.0/ 6f6c7b996719 releng/15.0-n281051 stable/14/ 72ad7baa99c7 stable/14-n274310 releng/14.4/ 31f6086db8fe releng/14.4-n273713 releng/14.3/ fa5581c379fe releng/14.3-n271513
- -------------------------------------------------------------------------

Run the following command to see which files were modified by a
particular commit:

# git show --stat <commit hash>

Or visit the following URL, replacing NNNNNN with the hash:

<URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>

To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:

# git rev-list --count --first-parent HEAD

VII. References

<URL:https://www.cve.org/CVERecord?id=CVE-2026-45256>

The latest revision of this advisory is available at <URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-26:25.thr.asc> -----BEGIN PGP SIGNATURE-----

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