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FreeBSD-SA-26:62.tty                                        Security Advisory
                                                          The FreeBSD Project

Topic:          Kernel use-after-free via tty ioctls

Category:       core
Module:         tty
Announced:      2026-08-25
Credits:        tsune of GMO Cybersecurity by Ierae, Inc.
                working with TrendAI Zero Day Initiative
Affects:        All supported versions of FreeBSD.
Corrected:      2026-08-25 15:59:52 UTC (stable/15, 15.1-STABLE)
                2026-08-25 16:01:29 UTC (releng/15.1, 15.1-RELEASE-p3)
                2026-08-25 16:01:09 UTC (releng/15.0, 15.0-RELEASE-p13)
                2026-08-25 16:00:03 UTC (stable/14, 14.4-STABLE)
                2026-08-25 16:00:30 UTC (releng/14.4, 14.4-RELEASE-p9)
CVE Name:       CVE-2026-58093

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 tty subsystem manages terminal devices and their associated state.

The TIOCSCTTY ioctl(2) operation allows the session leader to make a
terminal its controlling terminal.

II.  Problem Description

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.

III. Impact

An unprivileged local user can exploit this race condition to escalate
privileges.

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 or later 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-26:62/tty.patch
# fetch https://security.FreeBSD.org/patches/SA-26:62/tty.patch.asc
# gpg --verify tty.patch.asc

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

# cd /usr/src
# patch -E -p0 < /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/                              c7cec6fa6e4a    stable/15-n285147
releng/15.1/                            b936c3417027  releng/15.1-n283609
releng/15.0/                            45a190c827e4  releng/15.0-n281112
stable/14/                              6269b9bce0e3    stable/14-n275048
releng/14.4/                            d7883b84b1e1  releng/14.4-n273764
- -------------------------------------------------------------------------

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-2026-58093>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-26:62.tty.asc>
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