CVE-2025-26466

A flaw was found in the OpenSSH package. For each ping packet the SSH server receives, a pong packet is allocated in a memory buffer and stored in a queue of packages. It is only freed when the server/client key exchange has finished. A malicious client may keep sending such packages, leading to an uncontrolled increase in memory consumption on the server side. Consequently, the server may become unavailable, resulting in a denial of service attack.

The versions of OpenSSH as shipped with Red Hat Enterprise Linux 8 and older are not affected by this vulnerability.

Also this issue marked as a moderate severity vulnerability rather than important because, while it enables a pre-authentication denial-of-service (DoS) attack, it does not allow remote code execution, privilege escalation, or data exfiltration.

The attack is asymmetric in resource consumption, meaning an attacker can force excessive CPU and memory usage, but it still requires repeated connection attempts to sustain the impact. Moreover, OpenSSH provides built-in mitigation mechanisms, such as LoginGraceTime, MaxStartups, and PerSourcePenalties, which can significantly limit the attack’s effectiveness if properly configured.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

This issue can be mitigated by setting the following three different options in the sshd configuration file located at: /etc/ssh/sshd_config

MaxStartups: Set to a reasonable value, this option controls the maximum number of concurrent unauthenticated connections the SSH server accepts;

PerSourcePenalties: Set its suboptions to a reasonable value, this option is used to help sshd to detect and drop connections that are potentially malicious for the SSH server;

LoginGraceTime: Set to a resonable value, this option controls how much time the SSH server will wait the client to authenticate before dropping its connection;

All the three option above needs to be set to implement a full mitigation for this vulnerability.