On 18 February 2025 Debian released DSA-5868-1 for openssh on Debian 12 (bookworm). It fixes CVE-2025-26465, found by the Qualys Threat Research Unit: the OpenSSH client could be fooled by a machine-in-the-middle when the VerifyHostKeyDNS option is enabled. That option is off by default, so most machines were never exposed, but admin workstations and automation hosts that trust SSHFP records in DNS should check.
At a glance
| Item | Detail |
|---|---|
| Source package | openssh (the fix matters for the openssh-client binary package, which provides ssh) |
| Releases | Debian 12 (bookworm); Debian 11 (bullseye) received DLA-4057-1 the same day |
| Fixed versions | bookworm 1:9.2p1-2+deb12u5; bullseye 1:8.4p1-5+deb11u4 (historical) |
| CVE | CVE-2025-26465 |
| Severity | Medium: CNA (Red Hat) CVSS 3.1 base 6.8 |
| Advisory | DSA-5868-1, 18 February 2025 |
What the vulnerability is
With VerifyHostKeyDNS set, the ssh client checks the server's host key against SSHFP records published in DNS. The OpenSSH 9.9p2 release notes describe a logic error in versions 6.8p1 to 9.9p1 that allowed an on-path attacker to impersonate any server when this option is enabled. Qualys notes that the attack works whether the option is set to yes or ask, and without user interaction. A successful attacker could read or alter what you send over that session, including passwords typed into it.
The same Qualys report covers a second issue, CVE-2025-26466, a pre-authentication memory and CPU denial of service in OpenSSH 9.5p1 to 9.9p1. Debian's security tracker marks bookworm and bullseye as not affected because the vulnerable code was introduced in a later upstream version. Debian 13 (trixie) ships OpenSSH 10.0p1, which is newer than the fixed 9.9p2, and the tracker lists it as fixed for both CVEs.
Are you affected?
Check the installed client version and compare it with the fixed one:
dpkg-query -W -f='${Package} ${Version}\n' openssh-client openssh-server
apt policy openssh-clientThen check whether any configuration enables the option. ssh -G prints the configuration the client would use for a host after evaluating Host and Match blocks:
ssh -G server.example.com | grep -i verifyhostkeydns
grep -ri verifyhostkeydns /etc/ssh/ssh_config /etc/ssh/ssh_config.d/ ~/.ssh/configIf the value shown is not yes/true or ask for the hosts you connect to (the default is off), the flaw did not apply to you. Remember to check the accounts used by scripts, CI runners and backup jobs, not only your own user.
How to update
sudo apt update
sudo apt install --only-upgrade openssh-client openssh-server
apt policy openssh-clientA system that has received regular apt upgrade runs since February 2025 already has this fix: the bookworm version available today is newer than 1:9.2p1-2+deb12u5. Debian 11 (bullseye) reached the end of its LTS period on 31 August 2026, so bullseye machines should be upgraded to a supported release rather than patched package by package.
Restart or reboot?
No reboot is needed. The fix is in the client, so it applies the next time ssh starts. Close and reopen long-lived client processes such as tunnels, autossh sessions and ControlMaster connections so they use the updated binary.
Sources
- security-tracker.debian.org/tracker/DSA-5868-1
- lists.debian.org/debian-security-announce/2025/msg00030.html
- debian.org/security/2025/dsa-5868
- security-tracker.debian.org/tracker/DLA-4057-1
- lists.debian.org/debian-lts-announce/2025/02/msg00020.html
- security-tracker.debian.org/tracker/CVE-2025-26465
- security-tracker.debian.org/tracker/CVE-2025-26466
- cve.org/CVERecord?id=CVE-2025-26465
- cve.org/CVERecord?id=CVE-2025-26466
- openssh.com/releasenotes.html#9.9p2
- qualys.com/2025/02/18/openssh-mitm-dos.txt
- manpages.debian.org/bookworm/openssh-client/ssh.1.en.html
- manpages.debian.org/bookworm/openssh-client/ssh_config.5.en.html
- debian.org/releases
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