diff --git a/Doc/whatsnew/3.6.rst b/Doc/whatsnew/3.6.rst
index 296935adadd0ca..561fb67d6696a6 100644
--- a/Doc/whatsnew/3.6.rst
+++ b/Doc/whatsnew/3.6.rst
@@ -2472,3 +2472,12 @@ separator key, with ``&`` as the default. This change also affects
functions internally. For more details, please see their respective
documentation.
(Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in :issue:`42967`.)
+
+Notable changes in Python 3.6.14
+================================
+
+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`)
diff --git a/Lib/ftplib.py b/Lib/ftplib.py
index 2ff251a0f7d995..385e4324c6f5d8 100644
--- 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
index 4ff2f71afb837d..3ca7cc1c30cacd 100644
--- 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
new file mode 100644
index 00000000000000..8312b7e885441d
--- /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.
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