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FreeBSD-SA-26:46.ktls Security Advisory
The FreeBSD Project
Topic: Remote DOS via uninitialized memory access in KTLS receive
Category: core
Module: ktls
Announced: 2026-06-30
Credits: Yuxiang Yang, Yizhou Zhao, Ao Wang, Xuewei Feng, Qi Li, and
Ke Xu from Tsinghua University using GLM-5.1 from Z.ai
Affects: All supported versions of FreeBSD.
Corrected: 2026-06-30 17:20:17 UTC (stable/15, 15.1-STABLE)
2026-06-30 17:22:06 UTC (releng/15.1, 15.1-RELEASE-p1)
2026-06-30 17:21:33 UTC (releng/15.0, 15.0-RELEASE-p11)
2026-06-30 17:19:58 UTC (stable/14, 14.4-STABLE)
2026-06-30 17:21:06 UTC (releng/14.4, 14.4-RELEASE-p7)
2026-06-30 17:20:39 UTC (releng/14.3, 14.3-RELEASE-p16)
CVE Name: CVE-2026-49423
For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit .
I. Background
Kernel TLS (KTLS) moves Transport Layer Security (TLS) record processing
into the kernel, allowing applications to encrypt and decrypt socket data
without copying it to and from userspace and to serve TLS data with
sendfile(2). When a connection uses software KTLS on the receive path,
the kernel decrypts each incoming TLS record in place within the socket
buffer.
II. Problem Description
When building the iovec array for a received TLS 1.2 CBC record,
ktls_ocf_tls_cbc_decrypt() incremented the iovec index for every
mbuf in the chain, including mbufs that were skipped because they
contained only TLS header bytes. This left uninitialized entries in
the iovec array. The iovec array was allocated without zeroing.
III. Impact
A remote TLS peer can cause the kernel to read from uninitialized
iovec entries during HMAC computation, resulting in a kernel panic.
The peer must be able to control TCP segmentation such that the
first mbuf of a CBC record contains only the 5-byte TLS record
header.
IV. Workaround
Only users running an application which enables receive-side KTLS
are affected. Systems with the kern.ipc.tls.enable sysctl set to 0
are unaffected.
The kern.ipc.tls.cbc_enable sysctl prevents applications from using
AES-CBC with KTLS. Setting it to 0 will prevent applications from
establishing new KTLS sessions using AES-CBC.
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-26:46/ktls.patch
# fetch https://security.FreeBSD.org/patches/SA-26:46/ktls.patch.asc
# gpg --verify ktls.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
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/ e4e6250999aa stable/15-n284332
releng/15.1/ 54372e3b56b7 releng/15.1-n283577
releng/15.0/ 5357f822416a releng/15.0-n281079
stable/14/ a7787f9f8b8e stable/14-n274457
releng/14.4/ 5f83a1c159a3 releng/14.4-n273739
releng/14.3/ f769a69b2da3 releng/14.3-n271539
- -------------------------------------------------------------------------
Run the following command to see which files were modified by a
particular commit:
# git show --stat
Or visit the following URL, replacing NNNNNN with the hash:
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
The latest revision of this advisory is available at
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