CVE-2021-4180 cherry-pick Fix 4134f15

[3.6] bpo-43285 Make ftplib not trust the PASV response. (pythonGH-24838) (G…

…H-24881) (pythonGH-24882)

The IPv4 address value returned from the server in response to the PASV command
should not be trusted.  This prevents a malicious FTP server from using the
response to probe IPv4 address and port combinations on the client network.

Instead of using the returned address, we use the IP address we're
already connected to.  This is the strategy other ftp clients adopted,
and matches the only strategy available for the modern IPv6 EPSV command
where the server response must return a port number and nothing else.

For the rare user who _wants_ this ugly behavior, set a `trust_server_pasv_ipv4_address`
attribute on your `ftplib.FTP` instance to True..
(cherry picked from commit 0ab152c)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
(cherry picked from commit 664d1d1)

Doc/whatsnew/2.7.rst

@@ -2777,6 +2777,15 @@ It has been replaced by the new ``make regen-all`` target.
 
 .. _acks27:
 
+Security fix for FTP
+================================
+
+A security fix alters the :class:`ftplib.FTP` behavior to not trust the
+IPv4 address sent from the remote server when setting up a passive data
+channel.  We reuse the ftp server IP address instead.  For unusual code
+requiring the old behavior, set a ``trust_server_pasv_ipv4_address``
+attribute on your FTP instance to ``True``.  (See :issue:`43285`)
+
 Acknowledgements
 ================
 

Lib/ftplib.py

@@ -108,6 +108,9 @@ class FTP:
     file = None
     welcome = None
     passiveserver = 1
+    encoding = "latin-1"
+    # Disables https://bugs.python.org/issue43285 security if set to True.
+    trust_server_pasv_ipv4_address = False
 
     # Initialization method (called by class instantiation).
     # Initialize host to localhost, port to standard ftp port
@@ -310,8 +313,13 @@ def makeport(self):
         return sock
 
     def makepasv(self):
+        """Internal: Does the PASV or EPSV handshake -> (address, port)"""
         if self.af == socket.AF_INET:
-            host, port = parse227(self.sendcmd('PASV'))
+            untrusted_host, port = parse227(self.sendcmd('PASV'))
+            if self.trust_server_pasv_ipv4_address:
+                host = untrusted_host
+            else:
+                host = self.sock.getpeername()[0]
         else:
             host, port = parse229(self.sendcmd('EPSV'), self.sock.getpeername())
         return host, port

Lib/test/test_ftplib.py

@@ -67,6 +67,10 @@ def __init__(self, conn):
         self.rest = None
         self.next_retr_data = RETR_DATA
         self.push('220 welcome')
+        # We use this as the string IPv4 address to direct the client
+        # to in response to a PASV command.  To test security behavior.
+        # https://bugs.python.org/issue43285/.
+        self.fake_pasv_server_ip = '252.253.254.255'
 
     def collect_incoming_data(self, data):
         self.in_buffer.append(data)
@@ -109,13 +113,13 @@ def cmd_pasv(self, arg):
         sock.bind((self.socket.getsockname()[0], 0))
         sock.listen(5)
         sock.settimeout(10)
-        ip, port = sock.getsockname()[:2]
-        ip = ip.replace('.', ',')
-        p1, p2 = divmod(port, 256)
+        port = sock.getsockname()[1]
+        ip = self.fake_pasv_server_ip
+        ip = ip.replace('.', ','); p1 = port / 256; p2 = port % 256
         self.push('227 entering passive mode (%s,%d,%d)' %(ip, p1, p2))
         conn, addr = sock.accept()
         self.dtp = self.dtp_handler(conn, baseclass=self)
-
+        
     def cmd_eprt(self, arg):
         af, ip, port = arg.split(arg[0])[1:-1]
         port = int(port)
@@ -577,6 +581,107 @@ def test_makepasv(self):
         # IPv4 is in use, just make sure send_epsv has not been used
         self.assertEqual(self.server.handler_instance.last_received_cmd, 'pasv')
 
+    def test_makepasv_issue43285_security_disabled(self):
+        """Test the opt-in to the old vulnerable behavior."""
+        self.client.trust_server_pasv_ipv4_address = True
+        bad_host, port = self.client.makepasv()
+        self.assertEqual(
+                bad_host, self.server.handler_instance.fake_pasv_server_ip)
+        # Opening and closing a connection keeps the dummy server happy
+        # instead of timing out on accept.
+        socket.create_connection((self.client.sock.getpeername()[0], port),
+                                 timeout=TIMEOUT).close()
+
+    def test_makepasv_issue43285_security_enabled_default(self):
+        self.assertFalse(self.client.trust_server_pasv_ipv4_address)
+        trusted_host, port = self.client.makepasv()
+        self.assertNotEqual(
+                trusted_host, self.server.handler_instance.fake_pasv_server_ip)
+        # Opening and closing a connection keeps the dummy server happy
+        # instead of timing out on accept.
+        socket.create_connection((trusted_host, port), timeout=TIMEOUT).close()
+
+    def test_with_statement(self):
+        self.client.quit()
+
+        def is_client_connected():
+            if self.client.sock is None:
+                return False
+            try:
+                self.client.sendcmd('noop')
+            except (OSError, EOFError):
+                return False
+            return True
+
+        # base test
+        with ftplib.FTP(timeout=TIMEOUT) as self.client:
+            self.client.connect(self.server.host, self.server.port)
+            self.client.sendcmd('noop')
+            self.assertTrue(is_client_connected())
+        self.assertEqual(self.server.handler_instance.last_received_cmd, 'quit')
+        self.assertFalse(is_client_connected())
+
+        # QUIT sent inside the with block
+        with ftplib.FTP(timeout=TIMEOUT) as self.client:
+            self.client.connect(self.server.host, self.server.port)
+            self.client.sendcmd('noop')
+            self.client.quit()
+        self.assertEqual(self.server.handler_instance.last_received_cmd, 'quit')
+        self.assertFalse(is_client_connected())
+
+        # force a wrong response code to be sent on QUIT: error_perm
+        # is expected and the connection is supposed to be closed
+        try:
+            with ftplib.FTP(timeout=TIMEOUT) as self.client:
+                self.client.connect(self.server.host, self.server.port)
+                self.client.sendcmd('noop')
+                self.server.handler_instance.next_response = '550 error on quit'
+        except ftplib.error_perm as err:
+            self.assertEqual(str(err), '550 error on quit')
+        else:
+            self.fail('Exception not raised')
+        # needed to give the threaded server some time to set the attribute
+        # which otherwise would still be == 'noop'
+        time.sleep(0.1)
+        self.assertEqual(self.server.handler_instance.last_received_cmd, 'quit')
+        self.assertFalse(is_client_connected())
+
+    def test_source_address(self):
+        self.client.quit()
+        port = support.find_unused_port()
+        try:
+            self.client.connect(self.server.host, self.server.port,
+                                source_address=(HOST, port))
+            self.assertEqual(self.client.sock.getsockname()[1], port)
+            self.client.quit()
+        except OSError as e:
+            if e.errno == errno.EADDRINUSE:
+                self.skipTest("couldn't bind to port %d" % port)
+            raise
+
+    def test_source_address_passive_connection(self):
+        port = support.find_unused_port()
+        self.client.source_address = (HOST, port)
+        try:
+            with self.client.transfercmd('list') as sock:
+                self.assertEqual(sock.getsockname()[1], port)
+        except OSError as e:
+            if e.errno == errno.EADDRINUSE:
+                self.skipTest("couldn't bind to port %d" % port)
+            raise
+
+    def test_parse257(self):
+        self.assertEqual(ftplib.parse257('257 "/foo/bar"'), '/foo/bar')
+        self.assertEqual(ftplib.parse257('257 "/foo/bar" created'), '/foo/bar')
+        self.assertEqual(ftplib.parse257('257 ""'), '')
+        self.assertEqual(ftplib.parse257('257 "" created'), '')
+        self.assertRaises(ftplib.error_reply, ftplib.parse257, '250 "/foo/bar"')
+        # The 257 response is supposed to include the directory
+        # name and in case it contains embedded double-quotes
+        # they must be doubled (see RFC-959, chapter 7, appendix 2).
+        self.assertEqual(ftplib.parse257('257 "/foo/b""ar"'), '/foo/b"ar')
+        self.assertEqual(ftplib.parse257('257 "/foo/b""ar" created'), '/foo/b"ar')
+
     def test_line_too_long(self):
         self.assertRaises(ftplib.Error, self.client.sendcmd,
                           'x' * self.client.maxline * 2)

Misc/NEWS.d/next/Security/2021-03-13-03-48-14.bpo-43285.g-Hah3.rst

@@ -0,0 +1,8 @@
+:mod:`ftplib` no longer trusts the IP address value returned from the server
+in response to the PASV command by default.  This prevents a malicious FTP
+server from using the response to probe IPv4 address and port combinations
+on the client network.
+
+Code that requires the former vulnerable behavior may set a
+``trust_server_pasv_ipv4_address`` attribute on their
+:class:`ftplib.FTP` instances to ``True`` to re-enable it.