: Administrators with access can enable debug ssh on a Cisco device to view the exact exchange of identification strings during connection attempts, as seen in the official Cisco documentation:
: Refers to the version of the cryptographic application engine code compiled into the operating system. It does not map directly to the operating system's version (such as Cisco IOS 12.4 or 15.1). 2. Associated Vulnerabilities and Flaws
: The simplest way to identify devices is to run an Nmap script ( -sV ) on port 22. Any response containing SSH-2.0-Cisco-1.25 should be documented for review.
Devices reporting SSH-2.0-Cisco-1.25 are often running software that has reached End-of-Life. This means they no longer receive security patches for newly discovered vulnerabilities, making them a persistent security liability. ssh-2.0-cisco-1.25 vulnerability
Log into the device and run:
The most critical step is to keep your Cisco software up to date.
SSH0: Exchanging versions - SSH-2.0-Cisco-1.25 SSH0: send SSH message: outdated is NULL server version string: SSH-2.0-Cisco-1.25 : Administrators with access can enable debug ssh
Vulnerabilities related to SSH host key validation have also been identified. CVE-2025-20163 in the Cisco Nexus Dashboard Fabric Controller (NDFC) allows an unauthenticated, remote attacker to impersonate NDFC-managed devices. The flaw is due to insufficient SSH host key validation, which enables a machine-in-the-middle (MitM) attack. An attacker in a position to intercept network traffic could capture and decrypt SSH sessions meant for the legitimate device.
Use ACLs to restrict SSH access to only trusted source IP addresses and networks. This limits the attack surface and can mitigate many remote vulnerabilities. For Cisco devices, ACLs are a fundamental tool for management plane protection.
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Data source: Security Operations Center informative findings. Step-by-Step Remediation Playbook
The appearance of this string in security reports usually indicates the device is running a version of Cisco software that has not yet been hardened against recent SSH exploits. There are two primary security concerns currently associated with this banner: 1. The Terrapin Attack (CVE-2023-48795)
By intercepting the initial handshake, a Man-in-the-Middle (MitM) attacker can drop specific protocol messages without the client or server realizing a change occurred. Associated Vulnerabilities and Flaws : The simplest way