support new TCPConnector param fingerprint

enables ssl certificate pinning

Author: requiredfield

CHANGES.txt

@@ -4,6 +4,9 @@ CHANGES
 0.16.0 (XX-XX-XXXX)
 -------------------
 
+- Support new `fingerprint` param of TCPConnector to enable verifying
+  ssl certificates via md5, sha1, or sha256 digest
+
 - Setup uploaded filename if field value is binary and transfer
   encoding is not specified #349
 

CONTRIBUTORS.txt

@@ -38,6 +38,7 @@ Olaf Conradi
 Paul Colomiets
 Philipp A.
 Raúl Cumplido
+"Required Field" <requiredfield256@gmail.com>
 Robert Lu
 Sebastian Hanula
 Simon Kennedy

aiohttp/connector.py

@@ -9,6 +9,7 @@
 import warnings
 
 from collections import defaultdict
+from hashlib import md5, sha1, sha256
 from itertools import chain
 from math import ceil
 
@@ -17,6 +18,7 @@
 from .errors import ServerDisconnectedError
 from .errors import HttpProxyError, ProxyConnectionError
 from .errors import ClientOSError, ClientTimeoutError
+from .errors import FingerprintMismatch
 from .helpers import BasicAuth
 
 
@@ -25,6 +27,12 @@
 PY_34 = sys.version_info >= (3, 4)
 PY_343 = sys.version_info >= (3, 4, 3)
 
+HASHFUNC_BY_DIGESTLEN = {
+    16: md5,
+    20: sha1,
+    32: sha256,
+}
+
 
 class Connection(object):
 
@@ -347,13 +355,17 @@ class TCPConnector(BaseConnector):
     """TCP connector.
 
     :param bool verify_ssl: Set to True to check ssl certifications.
+    :param bytes fingerprint: Pass the binary md5, sha1, or sha256
+        digest of the expected certificate in DER format to verify
+        the cert the server presents matches. See also
+        https://en.wikipedia.org/wiki/Transport_Layer_Security#Certificate_pinning
     :param bool resolve: Set to True to do DNS lookup for host name.
     :param family: socket address family
     :param args: see :class:`BaseConnector`
     :param kwargs: see :class:`BaseConnector`
     """
 
-    def __init__(self, *, verify_ssl=True,
+    def __init__(self, *, verify_ssl=True, fingerprint=None,
                  resolve=False, family=socket.AF_INET, ssl_context=None,
                  **kwargs):
         super().__init__(**kwargs)
@@ -364,6 +376,15 @@ def __init__(self, *, verify_ssl=True,
                 "verify_ssl=False or specify ssl_context, not both.")
 
         self._verify_ssl = verify_ssl
+
+        if fingerprint:
+            digestlen = len(fingerprint)
+            hashfunc = HASHFUNC_BY_DIGESTLEN.get(digestlen)
+            if not hashfunc:
+                raise ValueError('fingerprint has invalid length')
+            self._hashfunc = hashfunc
+        self._fingerprint = fingerprint
+
         self._ssl_context = ssl_context
         self._family = family
         self._resolve = resolve
@@ -374,6 +395,11 @@ def verify_ssl(self):
         """Do check for ssl certifications?"""
         return self._verify_ssl
 
+    @property
+    def fingerprint(self):
+        """Expected ssl certificate fingerprint."""
+        return self._fingerprint
+
     @property
     def ssl_context(self):
         """SSLContext instance for https requests.
@@ -464,11 +490,25 @@ def _create_connection(self, req):
 
         for hinfo in hosts:
             try:
-                return (yield from self._loop.create_connection(
-                    self._factory, hinfo['host'], hinfo['port'],
+                host = hinfo['host']
+                port = hinfo['port']
+                conn = yield from self._loop.create_connection(
+                    self._factory, host, port,
                     ssl=sslcontext, family=hinfo['family'],
                     proto=hinfo['proto'], flags=hinfo['flags'],
-                    server_hostname=hinfo['hostname'] if sslcontext else None))
+                    server_hostname=hinfo['hostname'] if sslcontext else None)
+                transport = conn[0]
+                has_cert = transport.get_extra_info('sslcontext')
+                if has_cert and self._fingerprint:
+                    sock = transport.get_extra_info('socket')
+                    # gives DER-encoded cert as a sequence of bytes (or None)
+                    cert = sock.getpeercert(binary_form=True)
+                    assert cert
+                    got = self._hashfunc(cert).digest()
+                    expected = self._fingerprint
+                    if got != expected:
+                        raise FingerprintMismatch(expected, got, host, port)
+                return conn
             except OSError as e:
                 exc = e
         else:

aiohttp/errors.py

@@ -13,6 +13,7 @@
     'ClientError', 'ClientHttpProcessingError', 'ClientConnectionError',
     'ClientOSError', 'ClientTimeoutError', 'ProxyConnectionError',
     'ClientRequestError', 'ClientResponseError',
+    'FingerprintMismatch',
 
     'WSServerHandshakeError', 'WSClientDisconnectedError')
 
@@ -170,3 +171,18 @@ class LineLimitExceededParserError(ParserError):
     def __init__(self, msg, limit):
         super().__init__(msg)
         self.limit = limit
+
+
+class FingerprintMismatch(ClientConnectionError):
+    """SSL certificate does not match expected fingerprint."""
+
+    def __init__(self, expected, got, host, port):
+        self.expected = expected
+        self.got = got
+        self.host = host
+        self.port = port
+
+    def __repr__(self):
+        return '<{} expected={} got={} host={} port={}>'.format(
+            self.__class__.__name__, self.expected, self.got,
+            self.host, self.port)

docs/client.rst

@@ -396,20 +396,37 @@ By default it uses strict checks for HTTPS protocol. Certification
 checks can be relaxed by passing ``verify_ssl=False``::
 
   >>> conn = aiohttp.TCPConnector(verify_ssl=False)
-  >>> r = yield from aiohttp.request(
-  ...     'get', 'https://example.com', connector=conn)
+  >>> session = aiohttp.ClientSession(connector=conn)
+  >>> r = yield from session.get('https://example.com')
 
 
 If you need to setup custom ssl parameters (use own certification
 files for example) you can create a :class:`ssl.SSLContext` instance and
 pass it into the connector::
 
-  >>> sslcontext = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
-  >>> sslcontext.verify_mode = ssl.CERT_REQUIRED
-  >>> sslcontext.load_verify_locations("/etc/ssl/certs/ca-bundle.crt")
+  >>> sslcontext = ssl.create_default_context(cafile='/path/to/ca-bundle.crt')
   >>> conn = aiohttp.TCPConnector(ssl_context=sslcontext)
-  >>> r = yield from aiohttp.request(
-  ...     'get', 'https://example.com', connector=conn)
+  >>> session = aiohttp.ClientSession(connector=conn)
+  >>> r = yield from session.get('https://example.com')
+
+You may also verify certificates via fingerprint::
+
+  >>> # Attempt to connect to https://www.python.org
+  >>> # with a pin to a bogus certificate:
+  >>> bad_md5 = b'\xa2\x06G\xad\xaa\xf5\xd8\\J\x99^by;\x06='
+  >>> conn = aiohttp.TCPConnector(fingerprint=bad_md5)
+  >>> session = aiohttp.ClientSession(connector=conn)
+  >>> exc = None
+  >>> try:
+  ...     r = yield from session.get('https://www.python.org')
+  ... except FingerprintMismatch as e:
+  ...     exc = e
+  >>> exc is not None
+  True
+  >>> exc.expected == bad_md5
+  True
+  >>> exc.got  # www.python.org cert's actual md5
+  b'\xca;I\x9cuv\x8es\x138N$?\x15\xca\xcb'
 
 
 Unix domain sockets

tests/sample.crt.der

@@ -12,6 +12,7 @@
 import aiohttp
 from aiohttp import client
 from aiohttp import test_utils
+from aiohttp.errors import FingerprintMismatch
 from aiohttp.client import ClientResponse, ClientRequest
 from aiohttp.connector import Connection
 
@@ -452,10 +453,57 @@ def test_cleanup3(self):
     def test_tcp_connector_ctor(self):
         conn = aiohttp.TCPConnector(loop=self.loop)
         self.assertTrue(conn.verify_ssl)
+        self.assertIs(conn.fingerprint, None)
         self.assertFalse(conn.resolve)
         self.assertEqual(conn.family, socket.AF_INET)
         self.assertEqual(conn.resolved_hosts, {})
 
+    def test_tcp_connector_ctor_fingerprint_valid(self):
+        valid = b'\xa2\x06G\xad\xaa\xf5\xd8\\J\x99^by;\x06='
+        conn = aiohttp.TCPConnector(loop=self.loop, fingerprint=valid)
+        self.assertEqual(conn.fingerprint, valid)
+
+    def test_tcp_connector_fingerprint_invalid(self):
+        invalid = b'\x00'
+        with self.assertRaises(ValueError):
+            aiohttp.TCPConnector(loop=self.loop, fingerprint=invalid)
+
+    def test_tcp_connector_fingerprint(self):
+        # The even-index fingerprints below are "expect success" cases
+        # for ./sample.crt.der, the cert presented by test_utils.run_server.
+        # The odd-index fingerprints are "expect fail" cases.
+        testcases = (
+            # md5
+            b'\xa2\x06G\xad\xaa\xf5\xd8\\J\x99^by;\x06=',
+            b'\x00' * 16,
+
+            # sha1
+            b's\x93\xfd:\xed\x08\x1do\xa9\xaeq9\x1a\xe3\xc5\x7f\x89\xe7l\xf9',
+            b'\x00' * 20,
+
+            # sha256
+            b'0\x9a\xc9D\x83\xdc\x91\'\x88\x91\x11\xa1d\x97\xfd\xcb~7U\x14D@L'
+            b'\x11\xab\x99\xa8\xae\xb7\x14\xee\x8b',
+            b'\x00' * 32,
+        )
+        for i, fingerprint in enumerate(testcases):
+            expect_fail = i % 2
+            conn = aiohttp.TCPConnector(loop=self.loop, verify_ssl=False,
+                                        fingerprint=fingerprint)
+            with test_utils.run_server(self.loop, use_ssl=True) as httpd:
+                coro = client.request('get', httpd.url('method', 'get'),
+                                      connector=conn, loop=self.loop)
+                if expect_fail:
+                    with self.assertRaises(FingerprintMismatch) as cm:
+                        self.loop.run_until_complete(coro)
+                    exc = cm.exception
+                    self.assertEqual(exc.expected, fingerprint)
+                    # the previous test case should be what we actually got
+                    self.assertEqual(exc.got, testcases[i-1])
+                else:
+                    # should not raise
+                    self.loop.run_until_complete(coro)
+
     def test_tcp_connector_clear_resolved_hosts(self):
         conn = aiohttp.TCPConnector(loop=self.loop)
         info = object()