Medium [CVE-2025-26465] machine-in-the-middle attack if verifyhostkeydns is enabled

A vulnerability was found in OpenSSH when the VerifyHostKeyDNS option is enabled. A machine-in-the-middle attack can be performed by a malicious machine impersonating a legit server.

This issue occurs due to how OpenSSH mishandles error codes in specific conditions when verifying the host key. For an attack to be considered successful, the attacker needs to manage to exhaust the client's memory resource first, turning the attack complexity high.

This issue is rated as a moderate severity vulnerability instead of important because it requires specific conditions to be exploitable. Second, the attacker must be able to exhaust SSH client memory and trigger out-of-memory condition.

Additionally, while the attack allows a machine-in-the-middle (MITM) adversary to trick the client into accepting an incorrect host key, it does not directly lead to code execution or immediate system compromise. Instead, the attack requires additional steps, such as credential interception or session hijacking to fully exploit the breach.

Red Hat severity: Moderate — CVSS 6.8 (CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N). Weakness: CWE-390.

Affected Red Hat products: Red Hat Enterprise Linux 8; Red Hat Enterprise Linux 9; Red Hat Enterprise Linux 9.4 Extended Update Support; Red Hat Discovery 1.14; Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7; Red Hat OpenShift Container Platform 4.