Ssh20cisco125 Vulnerability

Improper input validation during the SSH2 key exchange or algorithm negotiation phase. Specifically, when the controller received a malformed SSH_MSG_KEXINIT packet, it failed to handle the error gracefully, leading to a memory corruption or process crash.

Imagine a regional power utility still using Cisco 3825 routers from 2008, running IOS 12.4(24)T. The network admin generated an RSA key in 2012 using modulus 1000. An external attacker scans Shodan for "Cisco IOS" port:22 and filters by weak key exchange. They find 1,200 devices. Using a GPU cluster, they factor 500 keys in 48 hours. They then decrypt captured traffic and retrieve SNMP community strings, enabling remote control of substation breakers.

This is not science fiction – it’s a mathematical certainty. Factorization of 1000-bit RSA is doable today. ssh20cisco125 vulnerability

From an external Linux host:

nmap --script ssh2-enum-algos -p 22 <cisco-ip>

Then use a tool like ssh-audit:

ssh-audit <cisco-ip> | grep -i "modulus"

If output shows rsa 1000 or modulus size: 125, you are vulnerable.

To verify if a device is exposing this banner, a penetration tester or administrator can perform a simple banner grab using standard tools like Netcat or Telnet on port 22. Improper input validation during the SSH2 key exchange

Using Netcat:

$ nc -v <target_ip> 22

Expected Vulnerable Response:

SSH-2.0-Cisco125

Secure/Generic Response Example:

SSH-2.0-OpenSSH_8.9p1
SSH-2.0-Cisco-1.25