commit 0ab152c6b5d95caa2dc1a30fa96e10258b5f188e
Author: Gregory P. Smith <greg@krypto.org>
Date:   Mon Mar 15 11:39:31 2021 -0700

    bpo-43285 Make ftplib not trust the PASV response. (GH-24838)
    
    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.

---
 Doc/whatsnew/2.7.rst                                               |   10 +++
 Lib/ftplib.py                                                      |   11 +++-
 Lib/test/test_ftplib.py                                            |   27 +++++++++-
 Misc/NEWS.d/next/Security/2021-03-13-03-48-14.bpo-43285.g-Hah3.rst |    8 ++
 4 files changed, 53 insertions(+), 3 deletions(-)

--- a/Doc/whatsnew/2.7.rst
+++ b/Doc/whatsnew/2.7.rst
@@ -166,6 +166,16 @@ The ``unittest`` module also automatical
 when running tests.
 
 
+Post-EOS fixes
+==============
+
+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`)
+
+
 Python 3.1 Features
 =======================
 
--- a/Lib/ftplib.py
+++ b/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 @@ class FTP:
         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
--- a/Lib/test/test_ftplib.py
+++ b/Lib/test/test_ftplib.py
@@ -67,6 +67,10 @@ class DummyFTPHandler(asynchat.async_cha
         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)
@@ -109,7 +113,8 @@ class DummyFTPHandler(asynchat.async_cha
         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))
@@ -577,6 +582,26 @@ class TestFTPClass(TestCase):
         # 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,
                           'x' * self.client.maxline * 2)
--- /dev/null
+++ 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.