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.

1 files changed, 9 insertions, 2 deletions

diff --git a/Lib/ftplib.py b/Lib/ftplib.py
index 1f760ed1ce0..7c5a50715f6 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 @@ 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