Debian OpenSSH GSSAPI key exchange flaw (DSA-6204-1, CVE-2026-3497): who is affected and how to update

On 9 April 2026 Debian published DSA-6204-1 for openssh on Debian 12 (bookworm) and Debian 13 (trixie). It fixes CVE-2026-3497, a flaw in the GSSAPI key exchange patch that Debian (and several other distributions) apply on top of upstream OpenSSH. The default configuration is not affected: the problem only matters on servers where GSSAPIKeyExchange has been switched on, which is usually done for Kerberos single sign-on.

At a glance

What the vulnerabilities are

CVE-2026-3497 (server side). The GSSAPI key exchange code added by distributions handled an error by sending a disconnect message without ending the process. According to the CVE record, an attacker can send an unexpected GSSAPI message type during key exchange, after which the server keeps running and later reads connection variables that were never initialised. Debian's advisory says a remote attacker can use this to cause a denial of service, or potentially the execution of arbitrary code. The CVE record notes that the real impact depends heavily on the compiler hardening used. Upstream OpenSSH is not affected because it does not contain this patch.

The scores differ widely: the CNA (Canonical) rated it CVSS 4.0 base 2.7 (Low), while NVD rated it CVSS 3.1 base 7.5 (High). The practical rule is simple: if your server has GSSAPIKeyExchange yes, treat this update as urgent; if it does not, apply it with your normal updates.

CVE-2025-61984 and CVE-2025-61985 (client side, bookworm only). The bookworm update also includes two fixes that Debian had queued for the 12.14 point release. Both concern the ssh client: control characters in usernames from untrusted sources, and a NUL character in an ssh:// URI, which could lead to code execution when a ProxyCommand is used. Each has a CNA score of 3.6 (Low).

Are you affected?

First check the installed version and compare it with the fixed version for your release:

dpkg-query -W -f='${Package} ${Version}\n' openssh-server openssh-client
apt policy openssh-server

Then check whether GSSAPI key exchange is enabled. sshd -T prints the effective server configuration:

sudo sshd -T | grep -i gssapikeyexchange
grep -ri gssapi /etc/ssh/sshd_config /etc/ssh/sshd_config.d/

The default for GSSAPIKeyExchange in Debian's sshd_config manual page is no. If the output shows gssapikeyexchange yes, the server was exposed until it is updated.

How to update

sudo apt update
sudo apt install --only-upgrade openssh-server openssh-client openssh-sftp-server
apt policy openssh-server

A regular sudo apt upgrade also picks the update up. Your release may already offer a newer version than the one in the advisory; anything at or above the fixed version contains the fix.

On trixie you may also see openssh-server-gssapi and openssh-client-gssapi. The trixie release notes explain that on trixie these are empty packages that depend on openssh-server and openssh-client, so updating the main packages is what matters. If you rely on GSSAPI, the same notes ask you to install them now, because Debian's main OpenSSH packages will stop supporting GSS-API from forky.

Restart or reboot?

No reboot is needed. Restart the SSH service so new connections are handled by the fixed daemon, and keep your current session open while you test a fresh login from a second terminal:

sudo systemctl restart ssh

The client-side fixes take effect the next time ssh is started.

Sources

• security-tracker.debian.org/tracker/DSA-6204-1
• lists.debian.org/debian-security-announce/2026/msg00114.html
• debian.org/security/2026/dsa-6204
• security-tracker.debian.org/tracker/CVE-2026-3497
• cve.org/CVERecord?id=CVE-2026-3497
• nvd.nist.gov/vuln/detail/CVE-2026-3497
• cve.org/CVERecord?id=CVE-2025-61984
• cve.org/CVERecord?id=CVE-2025-61985
• packages.debian.org/source/trixie/openssh
• debian.org/releases/trixie/release-notes/issues.en.html
• manpages.debian.org/trixie/openssh-server/sshd_config.5.en.html
• manpages.debian.org/trixie/openssh-server/sshd.8.en.html