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

…H-24881) bpo-43285: Make ftplib not trust the PASV response. 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) Co-authored-by: Gregory P. Smith <greg@krypto.org>
python · ned-deily · Mar 16, 2021 · Mar 15, 2021 · Mar 16, 2021 · Mar 16, 2021
commit 8759ca414288bea03f3c41287aa8db698aa1c7c8
diff --git a/Lib/ftplib.py b/Lib/ftplib.py
@@ -104,6 +104,8 @@ class FTP:
     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
@@ -333,8 +335,13 @@ 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

diff --git a/Lib/test/test_ftplib.py b/Lib/test/test_ftplib.py
@@ -94,6 +94,10 @@ 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)
@@ -136,7 +140,8 @@ def cmd_pasv(self, arg):
             sock.bind((self.socket.getsockname()[0], 0))
             sock.listen()
             sock.settimeout(TIMEOUT)
-            ip, port = sock.getsockname()[:2]
+            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()
@@ -694,6 +699,26 @@ 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()
 

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