CVE-2026-20009: Cisco Secure Firewall Adaptive Security Appliance SSH Partial Private Key Authentication Bypass Vulnerability

What it is

A vulnerability in the implementation of the proprietary SSH stack with SSH key-based authentication in Cisco Secure Firewall Adaptive Security Appliance (ASA) Software could allow an unauthenticated, remote attacker to log in to a Cisco Secure Firewall ASA device and execute commands as a specific user.

This vulnerability is due to insufficient validation of user input during the SSH authentication phase. An attacker could exploit this vulnerability by submitting crafted input during SSH authentication to an affected device. A successful exploit could allow the attacker to log in to the device as a specific user without the private SSH key of that user. To exploit this vulnerability, the attacker must possess a valid username and the associated public key. The private key is not required. Notes:

Exploitation of this vulnerability does not provide the attacker with root access. The authentication, authorization, and accounting (AAA) configuration command auto-enable is not affected by this vulnerability.  

Exploitation

The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.