Make ftplib not trust the PASV response. (#846)

Author: ivanaclairineirsan

Src/StdLib/Lib/ftplib.py

@@ -107,7 +107,9 @@ class FTP:
     sock = None
     file = None
     welcome = None
-    passiveserver = 1
+    passiveserver = True
+    # 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 +312,13 @@ 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

Src/StdLib/Lib/test/test_ftplib.py

@@ -66,7 +66,11 @@ def __init__(self, conn):
         self.next_response = ''
         self.rest = None
         self.next_retr_data = RETR_DATA
-        self.push('220 welcome')
+        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)
@@ -109,7 +113,8 @@ def cmd_pasv(self, arg):
         sock.bind((self.socket.getsockname()[0], 0))
         sock.listen(5)
         sock.settimeout(10)
-        ip, port = sock.getsockname()[:2]
+        port = sock.getsockname()[1]
+        ip = self.fake_pasv_server_ip
         ip = ip.replace('.', ',')
         p1, p2 = divmod(port, 256)
         self.push('227 entering passive mode (%s,%d,%d)' %(ip, p1, p2))
@@ -576,6 +581,26 @@ def test_makepasv(self):
         conn.close()
         # 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_line_too_long(self):
         self.assertRaises(ftplib.Error, self.client.sendcmd,