SMTP through Exchange using Integrated Windows Authentication (NTLM) using Python

Question

I want to use the credentials of the logged-in Windows user to authenticate an SMTP connection to an Exchange server using NTLM.

I\'m aware of the python-ntlm module

Answer 1

Great answer but as an update for python 3

def asbase64(msg):
    # encoding the message then convert to string
    return base64.b64encode(msg).decode("utf-8")

Answer 2

Although the solution below only uses the Python Win32 extensions (the sspi example code included with the Python Win32 extensions was very helpful), the python-ntlm IMAP & SMTP patches mentioned in the question also served as useful guides.

from smtplib import SMTPException, SMTPAuthenticationError
import string
import base64
import sspi

# NTLM Guide -- http://curl.haxx.se/rfc/ntlm.html

SMTP_EHLO_OKAY = 250
SMTP_AUTH_CHALLENGE = 334
SMTP_AUTH_OKAY = 235

def asbase64(msg):
    # encoding the message then convert to string
    return base64.b64encode(msg).decode("utf-8")

def connect_to_exchange_as_current_user(smtp):
    """Example:
    >>> import smtplib
    >>> smtp = smtplib.SMTP("my.smtp.server")
    >>> connect_to_exchange_as_current_user(smtp)
    """

    # Send the SMTP EHLO command
    code, response = smtp.ehlo()
    if code != SMTP_EHLO_OKAY:
        raise SMTPException("Server did not respond as expected to EHLO command")

    sspiclient = sspi.ClientAuth('NTLM')

    # Generate the NTLM Type 1 message
    sec_buffer=None
    err, sec_buffer = sspiclient.authorize(sec_buffer)
    ntlm_message = asbase64(sec_buffer[0].Buffer)

    # Send the NTLM Type 1 message -- Authentication Request
    code, response = smtp.docmd("AUTH", "NTLM " + ntlm_message)

    # Verify the NTLM Type 2 response -- Challenge Message
    if code != SMTP_AUTH_CHALLENGE:
        raise SMTPException("Server did not respond as expected to NTLM negotiate message")

    # Generate the NTLM Type 3 message
    err, sec_buffer = sspiclient.authorize(base64.decodebytes(response))
    ntlm_message = asbase64(sec_buffer[0].Buffer)

    # Send the NTLM Type 3 message -- Response Message
    code, response = smtp.docmd(ntlm_message)
    if code != SMTP_AUTH_OKAY:
        raise SMTPAuthenticationError(code, response)

Answer 3

Python 2.7.x will fail on sending the NTLM Type 3 message due to the blank cmd specified:

code, response = smtp.docmd("", ntlm_message)

This ends up sending the correct response back to the server, however it pre-pends a space due to the nature of docmd() calling putcmd().

smtplib.py:

def putcmd(self, cmd, args=""):
    """Send a command to the server."""
    if args == "":
        str = '%s%s' % (cmd, CRLF)
    else:
        str = '%s %s%s' % (cmd, args, CRLF)
    self.send(str)

# ...

def docmd(self, cmd, args=""):
    """Send a command, and return its response code."""
    self.putcmd(cmd, args)
    return self.getreply()

which as a result takes the path of the else condition, thereby sending str(' ' + ntlm_message + CRLF) which results in (501, 'Syntax error in parameters or arguments').

As such the fix is simply to send the NTLM message as the cmd.

code, response = smtp.docmd(ntlm_message)

A fix to the above answer was submitted, though who knows when it will be reviewed/accepted.