# OpenSSH on Debian: the GSSAPI Key Exchange patch opens the door to DoS and possible code execution (DSA-6204-1)

- **Published on:** 2026-04-09
- **Category:** Security
- **Original page:** https://linuxgratis.com/en/articles/dsa-6204-1-openssh-gssapi-key-exchange-rce-dos-debian

Debian fixes CVE-2026-3497, a flaw in its GSSAPI Key Exchange patch for OpenSSH that a remote attacker can use to crash SSH processes or, in the worst case, run code.

---

Debian shipped advisory DSA-6204-1 for OpenSSH on 9 April 2026. The flaw, tracked as CVE-2026-3497, does not live in the OpenSSH code maintained upstream. It sits in the GSSAPI Key Exchange patch that Debian (and other distributions such as Ubuntu) apply on their own. Researcher Jeremy Brown found it.

## What actually breaks

The bug is in the server loop that negotiates key exchange over GSSAPI. When an error happens during that negotiation, the code calls the wrong function: it uses `sshpkt_disconnect()` instead of `ssh_packet_disconnect()`. That difference matters. The second tears down the connection right away; the first only queues a disconnect message and lets the program keep running.

Because it keeps running, execution falls through into code that reads an uninitialized stack variable named `recv_tok`. Whatever garbage is left on the stack gets sent to the privileged monitor process over IPC and ends up passed to `gss_release_buffer()`, which can call `free()` on an invalid pointer. That is confirmed heap corruption.

In practice an attacker can reliably crash SSH child processes with a single crafted network packet, before authenticating. Debian's own advisory does not rule out that, with enough effort, the memory fault could lead to arbitrary code execution.

## Who is affected

Only systems that have explicitly enabled the `GSSAPIKeyExchange` option. This is not the default. It tends to show up in Kerberos environments, where GSSAPI handles domain authentication, so the typical targets are corporate and university networks running SSO over Kerberos.

If you have never touched that directive in your `sshd_config`, you are most likely not exposed. Check with:

```
grep -i gssapikeyexchange /etc/ssh/sshd_config
```

## Severity

Debian classes this as a flaw allowing denial of service and, potentially, code execution. The attack surface is remote and pre-authentication, which keeps it serious even though it depends on a non-default option. Since the bug lives in a Debian/Ubuntu-specific patch, OpenSSH built from upstream sources without that addition is not affected.

## Mitigation and patch

The fix is to update the `openssh` package:

- Debian 12 (Bookworm): fixed in `1:9.2p1-2+deb12u9`.
- Debian 13 (Trixie): fixed in `1:10.0p1-7+deb13u2`.

The Bookworm update also rolls in fixes for CVE-2025-61984 and CVE-2025-61985, slated for the 12.14 point release.

```
sudo apt update && sudo apt install --only-upgrade openssh-server openssh-client
sudo systemctl restart ssh
```

If you cannot patch right away and you do not need GSSAPI key exchange, turn off `GSSAPIKeyExchange` in `/etc/ssh/sshd_config` and restart the service. That closes the attack path until you apply the update.

For more on the distribution, see our [Debian](/en/debian) page.

## Source

- [Debian Security Announce — DSA-6204-1 openssh](https://lists.debian.org/debian-security-announce/2026/msg00114.html)
- [NVD — CVE-2026-3497](https://nvd.nist.gov/vuln/detail/CVE-2026-3497)