FreeBSD Security Advisory FreeBSD-SA-25:01.openssh
29 January, 2025 by security-advisories@freebsd.org | freebsd
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FreeBSD-SA-25:01.openssh Security Advisory
The FreeBSD Project
Topic: OpenSSH Keystroke Obfuscation Bypass
Category: contrib
Module: openssh
Announced: 2025-01-29
Credits: Philippos Giavridis
Credits: Jacky Wei En Kung, Daniel Hugenroth and
Alastair Beresford (University of Cambridge)
Affects: FreeBSD 14.1
Corrected: 2024-07-15 18:45:16 UTC (stable/14, 14.2-STABLE)
2025-01-29 18:55:25 UTC (releng/14.1, 14.1-RELEASE-p7)
2024-08-01 15:03:50 UTC (stable/13, 13.4-STABLE)
CVE Name: CVE-2024-39894
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
OpenSSH is an implementation of the SSH protocol suite, providing an
encrypted and authenticated transport for a variety of services, including
remote shell access.
OpenSSH version 9.5 introduced a mechanism to mitigate keystroke timing
attacks by "sending interactive traffic at fixed intervals when there is
only a small amount of data being sent."
II. Problem Description
A logic error in the ssh(1) ObscureKeystrokeTiming feature (on by default)
rendered this feature ineffective.
III. Impact
A passive observer could detect which network packets contain real keystrokes,
and infer the specific characters being transmitted from packet timing.
IV. Workaround
No workaround is available. This bug does not affect connections when
ObscureKeystrokeTiming was disabled or sessions where no TTY was requested.
V. Solution
Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.
Perform one of the following:
1) To update your vulnerable system via a binary patch:
Systems running a RELEASE version of FreeBSD on the amd64 or arm64 platforms,
or the i386 platform on FreeBSD 13, can be updated via the freebsd-update(8)
utility:
# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for a security update"
2) 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.
[FreeBSD 14.1]
# fetch https://security.FreeBSD.org/patches/SA-25:01/openssh.patch
# fetch https://security.FreeBSD.org/patches/SA-25:01/openssh.patch.asc
# gpg --verify openssh.patch.asc
b) Apply the patch. Execute the following commands as root:
# cd /usr/src
# patch < /path/to/patch
c) Recompile the operating system using buildworld and installworld as
described in <URL:https://www.FreeBSD.org/handbook/makeworld.html>.
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/14/ bf9a275b24f6 stable/14-n268158
releng/14.1/ 88d5d8108711 releng/14.1-n267735
stable/13/ 79853e40abd8 stable/13-n258171
- -------------------------------------------------------------------------
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://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-39894>
The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-25:01.openssh.asc>
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