SSL: CERTIFICATE_VERIFY_FAILED when connecting to a company Exchange Server

I try to send mails from out company exchange server via Python. My problem is, that I get a SSL error. I read a lot of pem-, cer-, crt-files, but I can not connect all these information to have success. I am sure, that if one can fix the issue in my little example script, one could fix the exchange script, too.

I read a lot about certificates and ca_bundles, but I don't know how to apply all of that to my issue.

Example-Script:

import requests
requests.get('https://mail.ourserver.loc')

Error:

C:\Python\python.exe "PATHtoMYproject/testing.py"
Traceback (most recent call last):
  File "C:\Python\lib\site-packages\urllib3\contrib\pyopenssl.py", line 441, in wrap_socket
    cnx.do_handshake()
  File "C:\Python\lib\site-packages\OpenSSL\SSL.py", line 1806, in do_handshake
    self._raise_ssl_error(self._ssl, result)
  File "C:\Python\lib\site-packages\OpenSSL\SSL.py", line 1546, in _raise_ssl_error
    _raise_current_error()
  File "C:\Python\lib\site-packages\OpenSSL\_util.py", line 54, in exception_from_error_queue
    raise exception_type(errors)
OpenSSL.SSL.Error: [('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')]

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  [.. a lot of text..]
ssl.SSLError: ("bad handshake: Error([('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')],)",)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  [.. a lot of text..]
urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='mail.ourserver.loc', port=443): Max retries exceeded with url: / (Caused by SSLError(SSLError("bad handshake: Error([('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')],)",),))

Exchange-Script:

from exchangelib import DELEGATE, Account, Credentials, Configuration, Message

import os

import urllib3
urllib3.disable_warnings()

creds = Credentials(username='EX\\GE-USR', password='secret')
config = Configuration(server='mail.ourserver.loc', credentials=creds)
account = Account(primary_smtp_address="[email protected]", autodiscover=False, config=config,
                  access_type=DELEGATE)

def send(email_receiver):
    m = Message(
        account=account,
        subject='This is a test!',
        body='Hallo',
        to_recipients=[email_receiver])
    m.send_and_save()

receiver = '[email protected]'

send(email_receiver=receiver)

print('Finish')

Error:

C:\Python\python.exe "PATHtoMYproject//exchange_main.py"
Traceback (most recent call last):
  File "C:\Python\lib\site-packages\urllib3\contrib\pyopenssl.py", line 441, in wrap_socket
    cnx.do_handshake()
  File "C:\Python\lib\site-packages\OpenSSL\SSL.py", line 1806, in do_handshake
    self._raise_ssl_error(self._ssl, result)
  File "C:\Python\lib\site-packages\OpenSSL\SSL.py", line 1546, in _raise_ssl_error
    _raise_current_error()
  File "C:\Python\lib\site-packages\OpenSSL\_util.py", line 54, in exception_from_error_queue
    raise exception_type(errors)
OpenSSL.SSL.Error: [('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')]

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
[.. a lot of text..]
ssl.SSLError: ("bad handshake: Error([('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')],)",)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Python\lib\site-packages\requests\adapters.py", line 440, in send
    timeout=timeout
  File "C:\Python\lib\site-packages\urllib3\connectionpool.py", line 639, in urlopen
    _stacktrace=sys.exc_info()[2])
  File "C:\Python\lib\site-packages\urllib3\util\retry.py", line 388, in increment
    raise MaxRetryError(_pool, url, error or ResponseError(cause))
urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='mail.ourserver.loc', port=443): Max retries exceeded with url: /EWS/Exchange.asmx (Caused by SSLError(SSLError("bad handshake: Error([('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')],)",),))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
[.. a lot of text..]
requests.exceptions.ConnectionError: HTTPSConnectionPool(host='mail.ourserver.loc', port=443): Max retries exceeded with url: /EWS/Exchange.asmx (Caused by SSLError(SSLError("bad handshake: Error([('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')],)",),))

Process finished with exit code 1url: /EWS/Exchange.asmx (Caused by SSLError(SSLError("bad handshake: Error([('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')],)",),))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  [.. a lot of text..]
requests.exceptions.ConnectionError: HTTPSConnectionPool(host='mail.ourserver.loc', port=443): Max retries exceeded with url: /EWS/Exchange.asmx (Caused by SSLError(SSLError("bad handshake: Error([('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')],)",),))

Process finished with exit code 1

I got it to work. I uninstalled pyopenssl and the error of my Example-Script changed to this:

requests.exceptions.SSLError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:777)

Then I found some useful commands to get into the pem-file topic. Basically, I just had to add the certificate in order to solve my problem. Most people said that I have to add my certificate to a pem-file like cacert.pem, but it is possible that you have several modules installed that use different files. I found this topic very useful to find out the location of my cacert.pem file: LINK_1 e. g.:

python -c "import requests; print requests.certs.where()"

Next I had the problem, that I didn't have a certificate to add to the pem-file. Somehow my browser was able to send a https request, so the browser was able to use a certificate from windows. This link brought the solution.

import ssl

context = ssl.create_default_context()
der_certs = context.get_ca_certs(binary_form=True)
pem_certs = [ssl.DER_cert_to_PEM_cert(der) for der in der_certs]

with open('wincacerts.pem', 'w') as outfile:
    for pem in pem_certs:
        outfile.write(pem + '\n')

I exported the certificate from windows and added the file to my request-script:

import os
import requests

root_path = os.getcwd()
path_pem=os.path.join(root_path, 'wincacerts.pem')
requests.get('https://mail.ourserver.loc', verify=path_pem)

Back to my exchange script I added these lines to the beginning and added my certificate. I simply renamed the file from .pem to .crt. I was then able to send an email via the exchange server to myself.

root_path = os.getcwd()
path_pem=os.path.join(root_path, 'files', 'wincacerts.crt')

class RootCAAdapter(requests.adapters.HTTPAdapter):
    # An HTTP adapter that uses a custom root CA certificate at a hard coded location
    def cert_verify(self, conn, url, verify, cert):
        cert_file = {
            'mail.ourserver.loc': path_pem,
            'mail.internal': '/path/to/mail.internal.crt'
            }[urlparse(url).hostname]
        super(RootCAAdapter, self).cert_verify(conn=conn, url=url, verify=cert_file, cert=cert)

# Tell exchangelib to use this adapter class instead of the default
BaseProtocol.HTTP_ADAPTER_CLS = RootCAAdapter

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