CVE-2025-26465: OpenBSD OpenSSH DoS Vulnerability

CVE-2025-26465 Overview

CVE-2025-26465 affects OpenSSH clients when the VerifyHostKeyDNS option is enabled. The flaw allows a machine-in-the-middle (MITM) attacker to impersonate a legitimate SSH server because OpenSSH mishandles error codes during host key verification. The Qualys Threat Research Unit (TRU) disclosed the vulnerability on February 18, 2025, alongside CVE-2025-26466. Exploitation requires the attacker to first exhaust the SSH client's memory resources, increasing attack complexity. Affected products include OpenBSD OpenSSH, NetApp ONTAP, NetApp Active IQ Unified Manager, Red Hat Enterprise Linux, Red Hat OpenShift Container Platform, and Debian Linux.

Critical Impact
A successful attack lets an adversary intercept SSH sessions and capture credentials or session data without triggering host key validation warnings.

Affected Products

OpenBSD OpenSSH (versions up to 9.9p1, fixed in 9.9p2)
NetApp ONTAP 9 and Active IQ Unified Manager for VMware vSphere
Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4, and Debian Linux 11/12

Discovery Timeline

2025-02-18 - CVE CVE-2025-26465 published to NVD following coordinated disclosure by the Qualys Threat Research Unit
2025-02-18 - OpenSSH 9.9p2 released with the fix
2026-05-12 - Last updated in NVD database

Technical Details for CVE-2025-26465

Vulnerability Analysis

The vulnerability is classified as [CWE-390] (Detection of Error Condition Without Action). It resides in the OpenSSH client's host key verification logic when VerifyHostKeyDNS is set to yes or ask. Under this configuration, the SSH client retrieves SSHFP records from DNS to validate the server's host key fingerprint. The client incorrectly handles certain error return codes during this verification process. An attacker positioned between the client and server can exploit this mishandling to bypass host authenticity checks. The EPSS score of 61.222% (98.336 percentile) indicates a high probability of exploitation activity relative to other published CVEs.

Root Cause

The root cause is improper error code handling in the host key verification routine. When memory allocation failures occur during key validation, the client treats certain failure paths as if verification succeeded. This logic gap stems from the function not propagating allocation errors back to the caller, allowing the SSH connection to proceed despite an invalid host key.

Attack Vector

The attack vector is network-based and requires user interaction along with high attack complexity. An attacker must first exhaust the SSH client's memory resources to trigger the error condition. Once the memory pressure is induced, the attacker presents a forged host key from a malicious server impersonating the legitimate destination. The client accepts the connection without warning, exposing authentication credentials and session traffic to interception.

No public exploit code is currently available. Refer to the Qualys TRU technical writeup for in-depth analysis of the error code handling flaw.

Detection Methods for CVE-2025-26465

Indicators of Compromise

Unexpected SSH client memory exhaustion or repeated ssh process crashes preceding successful connections
SSH connections to hosts where the presented host key does not match historical fingerprints stored in known_hosts
Anomalous DNS queries for SSHFP records originating from SSH client hosts to untrusted resolvers

Detection Strategies

Inventory all hosts running OpenSSH client versions prior to 9.9p2 and identify systems with VerifyHostKeyDNS yes or VerifyHostKeyDNS ask in /etc/ssh/ssh_config or user-level ~/.ssh/config
Monitor SSH client telemetry for abnormal memory consumption immediately preceding outbound SSH session establishment
Correlate DNS resolver logs with SSH connection attempts to flag SSHFP lookups against unauthorized name servers

Monitoring Recommendations

Centralize SSH client and authentication logs in a SIEM and alert on host key changes for production assets
Track package versions of openssh-clients across the fleet and generate alerts when vulnerable versions appear
Capture network flow data on egress SSH traffic to detect connections to unexpected destinations

How to Mitigate CVE-2025-26465

Immediate Actions Required

Upgrade OpenSSH clients to version 9.9p2 or apply distribution-specific patches released by Red Hat, Debian, Ubuntu, SUSE, and NetApp
Disable VerifyHostKeyDNS on all SSH clients until patches are deployed by setting VerifyHostKeyDNS no in ssh_config
Audit known_hosts files for unexpected entries that could indicate prior MITM activity

Patch Information

The OpenSSH project addressed the issue in release 9.9p2. See the OpenSSH 9.9p2 release notes and the OpenSSH announcement. Distribution patches are tracked in Red Hat CVE-2025-26465, Debian Security Tracker, Ubuntu Security, and NetApp Advisory NTAP-20250228-0003. Siemens product guidance is published in SSA-082556.

Workarounds

Set VerifyHostKeyDNS no globally in /etc/ssh/ssh_config to disable DNS-based host key validation
Rely on known_hosts files distributed through configuration management for trusted host key pinning
Restrict SSH client egress to known-good destinations using network ACLs or jump host architectures

bash

# Configuration example - disable DNS host key verification
# /etc/ssh/ssh_config
Host *
    VerifyHostKeyDNS no
    StrictHostKeyChecking yes
    UserKnownHostsFile /etc/ssh/known_hosts