diff --git a/Lib/ftplib.py b/Lib/ftplib.py
index 1f760ed..7c5a507 100644
--- a/Lib/ftplib.py
+++ b/Lib/ftplib.py
@@ -102,7 +102,9 @@ 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
 
     def __init__(self, host='', user='', passwd='', acct='',
                  timeout=_GLOBAL_DEFAULT_TIMEOUT, source_address=None, *,
@@ -320,8 +322,13 @@ 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
index 2424911..154dce1 100644
--- a/Lib/test/test_ftplib.py
+++ b/Lib/test/test_ftplib.py
@@ -103,6 +103,10 @@ def __init__(self, conn, encoding=DEFAULT_ENCODING):
         self.next_retr_data = RETR_DATA
         self.push('220 welcome')
         self.encoding = encoding
+        # 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)
@@ -143,7 +147,8 @@ def cmd_port(self, arg):
     def cmd_pasv(self, arg):
         with socket.create_server((self.socket.getsockname()[0], 0)) as sock:
             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()
@@ -707,6 +712,26 @@ 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
new file mode 100644
index 0000000..8312b7e
--- /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.