Validate SSL certificates with Python


Question

I need to write a script that connects to a bunch of sites on our corporate intranet over HTTPS and verifies that their SSL certificates are valid; that they are not expired, that they are issued for the correct address, etc. We use our own internal corporate Certificate Authority for these sites, so we have the public key of the CA to verify the certificates against.

Python by default just accepts and uses SSL certificates when using HTTPS, so even if a certificate is invalid, Python libraries such as urllib2 and Twisted will just happily use the certificate.

Is there a good library somewhere that will let me connect to a site over HTTPS and verify its certificate in this way?

How do I verify a certificate in Python?

1
81
7/7/2014 5:53:21 AM

Accepted Answer

From release version 2.7.9/3.4.3 on, Python by default attempts to perform certificate validation.

This has been proposed in PEP 467, which is worth a read: https://www.python.org/dev/peps/pep-0476/

The changes affect all relevant stdlib modules (urllib/urllib2, http, httplib).

Relevant documentation:

https://docs.python.org/2/library/httplib.html#httplib.HTTPSConnection

This class now performs all the necessary certificate and hostname checks by default. To revert to the previous, unverified, behavior ssl._create_unverified_context() can be passed to the context parameter.

https://docs.python.org/3/library/http.client.html#http.client.HTTPSConnection

Changed in version 3.4.3: This class now performs all the necessary certificate and hostname checks by default. To revert to the previous, unverified, behavior ssl._create_unverified_context() can be passed to the context parameter.

Note that the new built-in verification is based on the system-provided certificate database. Opposed to that, the requests package ships its own certificate bundle. Pros and cons of both approaches are discussed in the Trust database section of PEP 476.

18
2/4/2015 7:15:18 PM

I have added a distribution to the Python Package Index which makes the match_hostname() function from the Python 3.2 ssl package available on previous versions of Python.

http://pypi.python.org/pypi/backports.ssl_match_hostname/

You can install it with:

pip install backports.ssl_match_hostname

Or you can make it a dependency listed in your project's setup.py. Either way, it can be used like this:

from backports.ssl_match_hostname import match_hostname, CertificateError
...
sslsock = ssl.wrap_socket(sock, ssl_version=ssl.PROTOCOL_SSLv3,
                      cert_reqs=ssl.CERT_REQUIRED, ca_certs=...)
try:
    match_hostname(sslsock.getpeercert(), hostname)
except CertificateError, ce:
    ...

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