By [Your Name/Security Team] Date: [Current Date]
In the world of enterprise networking, few things send shivers down an administrator's spine faster than the phrase "critical vulnerability in Cisco IOS." Late in 2023, the security community was rocked by the disclosure of a severe vulnerability tracked as CVE-2023-20273, which has since become colloquially associated with the search term "ssh20cisco125" due to its impact on SSH interfaces and specific hardware series.
If you are running Cisco IOS XE, this is not a drill. This blog post breaks down what this vulnerability is, how attackers are exploiting it via SSH, and what you need to do immediately to secure your network. ssh20cisco125 vulnerability
When an SSH client initiates a connection to a server, the server responds with a protocol banner before encryption is negotiated. This handshake is defined in RFC 4253 (The Secure Shell Protocol). The banner format is typically:
SSH-protoversion-softwareversion SP comments CR LF
In vulnerable Cisco devices, the software version field is overly specific. Instead of returning a generic string like SSH-2.0-Cisco, the device returns:
SSH-2.0-Cisco125 By [Your Name/Security Team] Date: [Current Date] In
This reveals that the device is likely a Cisco Aironet 1250 or 1200 series (or the software version specifically correlates to the 12.x train for wireless). This specific identifier acts as a "fingerprint."
Log into the device and run:
show crypto key mypubkey rsa
Look for output like:
% Key pair was generated at: 00:00:00 UTC Jan 1 2015
Key name: myrouter.cisco.com
Storage Device: private-config
Usage: General Purpose Key
Key Data:
Modulus Length (bits): 1000 <--- DANGER
Key is not exportable.
[Critical] SSH20Cisco125 Vulnerability
Please confirm remediation by [Date].
Since past sessions could have been decrypted, assume all credentials are compromised. Look for output like: % Key pair was