For HTTP, logging and fingerprinting browser user agents is standard practice. Many anti-automation tricks use the user agent and compare it to other browser artifacts, for example, supported JavaScript APIs, to detect bots. SSH offers an "identification string" with a format mandated by RFC 4253.

The RFC mandates this format:

SSH-protoversion-softwareversion SP comments CR LF

For example:

SSH-2.0-billsSSH_3.6.3q3<CR><LF>

With SSH 2.0 being the only SSH version that should be in use currently (I know... IoT... ), all identification strings should start with SSH-2.0-. The server and the client will send an identification string. But let's first focus on clients.

Why should you care about SSH identification strings: Just like user agents, looking for anomalies can be useful to detect compromise, or outdated clients and servers that may be present in your network. The identification string, as well as the ciphers offered, are exchanged in the clear. Tools like Zeek are nicely able to extract this data for analysis.

Let's go over some of the top different ssh clients that I see in our honeypot data.

I looked through some of our honeypot data to identify common patterns in identification strings. As a quick breakdown before diving into details, the main implementations I saw:

libssh

One of the most common libraries to implement ssh clients is libssh. Currently, versions 0.9 and 0.10 are supported, with 0.10.6 and 0.9.8 being the most recent versions. There is also a very similar libssh2, which appears to be updated more frequently. The most recent libssh2 version is 1.11.0.

Here are the top 10 "libssh" identification strings from our honeypots:

4331094  SSH-2.0-libssh-0.6.3
4270423  SSH-2.0-libssh_0.9.6
1137866  SSH-2.0-libssh2_1.10.0
 425204  SSH-2.0-libssh2_1.4.3
  99634  SSH-2.0-libssh2_1.9.0
  83101  SSH-2.0-libssh-0.6.0
  82085  SSH-2.0-libssh_0.4.8
  81603  SSH-2.0-libssh_0.5.5
  81380  SSH-2.0-libssh_0.11
  80949  SSH-2.0-libssh-0.3.4

Note that many of the common versions are very much out of date. This could be an artifact of scanning tools using either hard-coded identification strings or them using outdated libraries. Remember that they may use the library present on the random (outdated) compromised IoT device they infected.

The most common libssh version in my home network is 0.9.3.

OpenSSH

Another very common SSH implementation is OpenSSH. In particular, on Linux systems, it is the preferred SSH implementation. 

Top 10 identification strings for OpenSSH:

 339860  SSH-2.0-OpenSSH_7.4
 213486  SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.5
 162726  SSH-2.0-OpenSSH_5.3
  81907  SSH-2.0-OpenSSH_5.2
  81697  SSH-2.0-OpenSSH_4.3
  81595  SSH-2.0-OpenSSH_6.2
  81575  SSH-2.0-OpenSSH_3.9p1
  81467  SSH-2.0-OpenSSH_5.9
  81440  SSH-2.0-OpenSSH_6.0p
  81332  SSH-2.0-OpenSSH_6.1

Ubuntu 22.04 uses SSH-2.0-OpenSSH_8.9p1 Ubuntu-3ubuntu0.5 as its identification string. The second one is close and could be a slightly older Ubuntu version. Current Macs use SSH-2.0-OpenSSH_9.4. 

Putty

Using a lot of Windows systems? You may have users using Putty. Of course, bots and scanning tools will use this identification strings to "blend in":

 250595  SSH-2.0-PuTTY
 163900  SSH-2.0-PuTTY_Release_0.63
 162709  SSH-2.0-PuTTY_Release_0.62
  81663  SSH-2.0-PuTTY_Release_0.61
  81477  SSH-2.0-PuTTY_Snapshot_2010_02_20
  81420  SSH-2.0-PuTTY_Release_0.62.1
  81391  SSH-2.0-PuTTY_KiTTY
  81349  SSH-2.0-PuTTY_Release_0.58
  81267  SSH-2.0-PuTTY_Release_0.67
  81189  SSH-2.0-PuTTY_Release_0.59

Go

Go is a language that has made a name for itself for creating efficient, multi threated network servers and clients. So no surprise that it is used for scanning tools. My random sample only had one specific identification string:

SSH-2.0-Go

paramiko

Of course Python users want to play with ssh as well. Paramiko is a Python module implementing ssh.

 163066  SSH-2.0-paramiko_1.8.1
  82258  SSH-2.0-paramiko_1.16.0
  81923  SSH-2.0-paramiko_2.0.0
  81786  SSH-2.0-paramiko_2.1.2
  81674  SSH-2.0-paramiko_1.15.1
  81568  SSH-2.0-paramiko_1.10.1
  81532  SSH-2.0-paramiko_1.7.7.1
  81490  SSH-2.0-paramiko_1.16.1
  81410  SSH-2.0-paramiko_2.1.1
  81234  SSH-2.0-paramiko_1.17.1

Other odd once:

FlowSsh (SSH-2.0-8.35 FlowSsh)
Another ssh library. See https://www.bitvise.com/flowssh

HELLOWORLD (SSH-2.0-HELLOWORLD)
probably not good. Not sure which exact tool this is.

ZGrab 9 (SSH-2.0-ZGrab ZGrab SSH Survey and SSH-2.0-zgrab2 opt-out available at http://195.37.190.89/)
part of the ZMap scanner. See https://zmap.io/

check_ssh (SSH-2.0-check_ssh_2.3.1)
the "check_ssh" plugin used by network monitoring tools like Nagios and Icinga.

---
Johannes B. Ullrich, Ph.D. , Dean of Research, SANS.edu
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