Disable SSL certificate validation in Python

Question

I'm writing a script that connects to a bunch of URLs over HTTPS, downloads their SSL certificate and extracts the CN. Everything works except when I stumble on a site with an invalid SSL certificate. I absolutely do not care if the certificate is valid or not. I just want the CN but Python stubbornly refuses to extract the certificate information if the certificate is not validated. Is there any way to get around this profoundly stupid behavior? Oh, I'm using the built-in socket and ssl libraries only. I don't want to use third-party libraries like M2Crypto or pyOpenSSL because I'm trying to keep the script as portable as possible.

Here's the relevant code:

    file = open("list.txt", "r")
    for x in file:
    server = socket.getaddrinfo(x.rstrip(), "443")[0][4][0]
    sslsocket = socket.socket()
    sslsocket.connect((server, 443))
    sslsocket = ssl.wrap_socket(sslsocket, cert_reqs=ssl.CERT_REQUIRED, ca_certs="cacerts.txt")
    certificate = sslsocket.getpeercert()`

Answer 1

The ssl.get_server_certificate can do it:

import ssl
ssl.get_server_certificate(("www.sefaz.ce.gov.br",443)) 

I think function doc string is more clear than python doc site:

"""Retrieve the certificate from the server at the specified address,
   and return it as a PEM-encoded string.
   If 'ca_certs' is specified, validate the server cert against it.
   If 'ssl_version' is specified, use it in the connection attempt."""

So you can extract common name from binary DER certificate searching for common name object identifier:

def get_commonname(host,port=443):
    oid='\x06\x03U\x04\x03' # Object Identifier 2.5.4.3 (COMMON NAME)
    pem=ssl.get_server_certificate((host,port))
    der=ssl.PEM_cert_to_DER_cert(pem)
    i=der.find(oid) # find first common name (certificate authority)
    if i!=-1:
        i=der.find(oid,i+1) # skip and find second common name
        if i!=-1:
            begin=i+len(oid)+2
            end=begin+ord(der[begin-1])
            return der[begin:end]
    return None

Answer 2

Ok. I cleaned up olivecoder's code to solve the problem that it assumes there will always be three CNs in the certificate chain (root, intermediate, server) and I condensed it. This is the final code I will be using.

cert = ssl.get_server_certificate(("www.google.com", 443)) #Retrieve SSL server certificate
cert = ssl.PEM_cert_to_DER_cert(cert) #Convert certificate to DER format
begin = cert.rfind('\x06\x03\x55\x04\x03') + 7 #Find the last occurence of this byte string indicating the CN, add 7 bytes to startpoint to account for length of byte string and padding
end = begin + ord(cert[begin - 1]) #Set endpoint to startpoint + the length of the CN
print cert[begin:end] #Retrieve the CN from the DER encoded certificate