On 1 Dec 2013 04:32, "Christian Heimes" <christ...@python.org> wrote: > > Hi, > > Larry has granted me a special pardon to add an outstanding fix for SSL, > http://bugs.python.org/issue19509 . Right now most stdlib modules > (ftplib, imaplib, nntplib, poplib, smtplib) neither support server name > indication (SNI) nor check the subject name of the peer's certificate > properly. The second issue is a major loop-hole because it allows > man-in-the-middle attack despite CERT_REQUIRED. > > With CERT_REQUIRED OpenSSL verifies that the peer's certificate is > directly or indirectly signed by a trusted root certification authority. > With Python 3.4 the ssl module is able to use/load the system's trusted > root certs on all major systems (Linux, Mac, BSD, Windows). On Linux and > BSD it requires a properly configured system openssl to locate the root > certs. This usually works out of the box. On Mac Apple's openssl build > is able to use the keychain API of OSX. I have added code for Windows' > system store. > > SSL socket code usually looks like this: > > context = ssl.SSLContext(ssl.PROTOCOL_TLSv1) > context.verify_mode = ssl.CERT_REQUIRED > # new, by default it loads certs trusted for Purpose.SERVER_AUTH > context.load_default_certs() > > sock = socket.create_connection(("example.net", 443)) > sslsock = context.wrap_socket(sock) > > SSLContext.wrap_socket() wraps an ordinary socket into a SSLSocket. With > verify_mode = CERT_REQUIRED OpenSSL ensures that the peer's SSL > certificate is signed by a trusted root CA. In this example one very > important step is missing. The peer may return *ANY* signed certificate > for *ANY* hostname. These lines do NOT check that the certificate's > information match "example.net". An attacker can use any arbitrary > certificate (e.g. for "www.evil.net"), get it signed and abuse it for > MitM attacks on "mail.python.org". > http://docs.python.org/3/library/ssl.html#ssl.match_hostname must be > used to verify the cert. It's easy to forget it... > > > I have thought about multiple ways to fix the issue. At first I added a > new argument "check_hostname" to all affected modules and implemented > the check manually. For every module I had to modify several places for > SSL and STARTTLS and add / change about 10 lines. The extra lines are > required to properly shutdown and close the connection when the cert > doesn't match the hostname. I don't like the solution because it's > tedious. Every 3rd party author has to copy the same code, too. > > Then I came up with a better solution: > > context = ssl.SSLContext(ssl.PROTOCOL_TLSv1) > context.verify_mode = ssl.CERT_REQUIRED > context.load_default_certs() > context.check_hostname = True # <-- NEW > > sock = socket.create_connection(("example.net", 443)) > # server_hostname is already used for SNI > sslsock = context.wrap_socket(sock, server_hostname="example.net") > > > This fix requires only a new SSLContext attribute and a small > modification to SSLSocket.do_handshake(): > > if self.context.check_hostname: > try: > match_hostname(self.getpeercert(), self.server_hostname) > except Exception: > self.shutdown(_SHUT_RDWR) > self.close() > raise > > > Pros: > > * match_hostname() is done in one central place > * the cert is matched as early as possible > * no extra arguments for APIs, a context object is enough > * library developers just have to add server_hostname to get SNI and > hostname checks at the same time > * users of libraries can configure cert verification and checking on the > same object > * missing checks will not pass silently > > Cons: > > * Doesn't work with OpenSSL < 0.9.8f (released 2007) because older > versions lack SNI support. The ssl module raises an exception for > server_hostname if SNI is not supported. > > > The default settings for all stdlib modules will still be verify_mode = > CERT_NONE and check_hostname = False for maximum backward compatibility. > Python 3.4 comes with a new function ssl.create_default_context() that > returns a new context with best practice settings and loaded root CA > certs. The settings are TLS 1.0, no weak and insecure ciphers (no MD5, > no RC4), no compression (CRIME attack), CERT_REQUIRED and check_hostname > = True (for client side only). > > http://bugs.python.org/issue19509 has a working patch for ftplib. > > Comments?
If Larry is OK with it as RM (and it sounds like he is), +1 from me as well. Cheers, Nick. > > Christian > > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: https://mail.python.org/mailman/options/python-dev/ncoghlan%40gmail.com
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