@@ -650,6 +650,23 @@ def socketpair(family=AF_INET, type=SOCK_STREAM, proto=0):
                 raise
         finally:
             lsock.close()
+
+        # Authenticating avoids using a connection from something else
+        # able to connect to {host}:{port} instead of us.
+        # We expect only AF_INET and AF_INET6 families.
+        try:
+            if (
+                ssock.getsockname() != csock.getpeername()
+                or csock.getsockname() != ssock.getpeername()
+            ):
+                raise ConnectionError("Unexpected peer connection")
+        except:
+            # getsockname() and getpeername() can fail
+            # if either socket isn't connected.
+            ssock.close()
+            csock.close()
+            raise
+
         return (ssock, csock)
     __all__.append("socketpair")
 
 
@@ -558,19 +558,27 @@ class SocketPairTest(unittest.TestCase, ThreadableTest):
     def __init__(self, methodName='runTest'):
         unittest.TestCase.__init__(self, methodName=methodName)
         ThreadableTest.__init__(self)
+        self.cli = None
+        self.serv = None
+
+    def socketpair(self):
+        # To be overridden by some child classes.
+        return socket.socketpair()
 
     def setUp(self):
-        self.serv, self.cli = socket.socketpair()
+        self.serv, self.cli = self.socketpair()
 
     def tearDown(self):
-        self.serv.close()
+        if self.serv:
+            self.serv.close()
         self.serv = None
 
     def clientSetUp(self):
         pass
 
     def clientTearDown(self):
-        self.cli.close()
+        if self.cli:
+            self.cli.close()
         self.cli = None
         ThreadableTest.clientTearDown(self)
 
@@ -4786,6 +4794,120 @@ def _testSend(self):
         self.assertEqual(msg, MSG)
 
 
+class PurePythonSocketPairTest(SocketPairTest):
+
+    # Explicitly use socketpair AF_INET or AF_INET6 to ensure that is the
+    # code path we're using regardless platform is the pure python one where
+    # `_socket.socketpair` does not exist.  (AF_INET does not work with
+    # _socket.socketpair on many platforms).
+    def socketpair(self):
+        # called by super().setUp().
+        try:
+            return socket.socketpair(socket.AF_INET6)
+        except OSError:
+            return socket.socketpair(socket.AF_INET)
+
+    # Local imports in this class make for easy security fix backporting.
+
+    def setUp(self):
+        import _socket
+        self._orig_sp = getattr(_socket, 'socketpair', None)
+        if self._orig_sp is not None:
+            # This forces the version using the non-OS provided socketpair
+            # emulation via an AF_INET socket in Lib/socket.py.
+            del _socket.socketpair
+            import importlib
+            global socket
+            socket = importlib.reload(socket)
+        else:
+            pass  # This platform already uses the non-OS provided version.
+        super().setUp()
+
+    def tearDown(self):
+        super().tearDown()
+        import _socket
+        if self._orig_sp is not None:
+            # Restore the default socket.socketpair definition.
+            _socket.socketpair = self._orig_sp
+            import importlib
+            global socket
+            socket = importlib.reload(socket)
+
+    def test_recv(self):
+        msg = self.serv.recv(1024)
+        self.assertEqual(msg, MSG)
+
+    def _test_recv(self):
+        self.cli.send(MSG)
+
+    def test_send(self):
+        self.serv.send(MSG)
+
+    def _test_send(self):
+        msg = self.cli.recv(1024)
+        self.assertEqual(msg, MSG)
+
+    def test_ipv4(self):
+        cli, srv = socket.socketpair(socket.AF_INET)
+        cli.close()
+        srv.close()
+
+    def _test_ipv4(self):
+        pass
+
+    @unittest.skipIf(not hasattr(_socket, 'IPPROTO_IPV6') or
+                     not hasattr(_socket, 'IPV6_V6ONLY'),
+                     "IPV6_V6ONLY option not supported")
+    @unittest.skipUnless(socket_helper.IPV6_ENABLED, 'IPv6 required for this test')
+    def test_ipv6(self):
+        cli, srv = socket.socketpair(socket.AF_INET6)
+        cli.close()
+        srv.close()
+
+    def _test_ipv6(self):
+        pass
+
+    def test_injected_authentication_failure(self):
+        orig_getsockname = socket.socket.getsockname
+        inject_sock = None
+
+        def inject_getsocketname(self):
+            nonlocal inject_sock
+            sockname = orig_getsockname(self)
+            # Connect to the listening socket ahead of the
+            # client socket.
+            if inject_sock is None:
+                inject_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+                inject_sock.setblocking(False)
+                try:
+                    inject_sock.connect(sockname[:2])
+                except (BlockingIOError, InterruptedError):
+                    pass
+                inject_sock.setblocking(True)
+            return sockname
+
+        sock1 = sock2 = None
+        try:
+            socket.socket.getsockname = inject_getsocketname
+            with self.assertRaises(OSError):
+                sock1, sock2 = socket.socketpair()
+        finally:
+            socket.socket.getsockname = orig_getsockname
+            if inject_sock:
+                inject_sock.close()
+            if sock1:  # This cleanup isn't needed on a successful test.
+                sock1.close()
+            if sock2:
+                sock2.close()
+
+    def _test_injected_authentication_failure(self):
+        # No-op.  Exists for base class threading infrastructure to call.
+        # We could refactor this test into its own lesser class along with the
+        # setUp and tearDown code to construct an ideal; it is simpler to keep
+        # it here and live with extra overhead one this _one_ failure test.
+        pass
+
+
 class NonBlockingTCPTests(ThreadedTCPSocketTest):
 
     def __init__(self, methodName='runTest'):
 
@@ -0,0 +1,5 @@
+Authenticate the socket connection for the ``socket.socketpair()`` fallback
+on platforms where ``AF_UNIX`` is not available like Windows.
+
+Patch by Gregory P. Smith <greg@krypto.org> and Seth Larson <seth@python.org>. Reported by Ellie
+<el@horse64.org>