8000 CVE-2021-4189 cherry-pick Fix 4134f154ae2f621f25c5d698cc0f1748035a1b88 · ActiveState/cpython@8ebb54f · GitHub
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CVE-2021-4189 cherry-pick Fix 4134f15
[3.6] bpo-43285 Make ftplib not trust the PASV response. (pythonGH-24838) (G… …H-24881) (pythonGH-24882) 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)
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‎Doc/whatsnew/2.7.rst

Lines changed: 9 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2777,6 +2777,15 @@ It has been replaced by the new ``make regen-all`` target.
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.. _acks27:
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2780+
Security fix for FTP
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================================
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A security fix alters the :class:`ftplib.FTP` behavior to not trust the
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IPv4 address sent from the remote server when setting up a passive data
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channel. We reuse the ftp server IP address instead. For unusual code
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requiring the old behavior, set a ``trust_server_pasv_ipv4_address``
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attribute on your FTP instance to ``True``. (See :issue:`43285`)
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Acknowledgements
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================
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‎Lib/ftplib.py

Lines changed: 33 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -108,30 +108,55 @@ class FTP:
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file = None
109109
welcome = None
110110
passiveserver = 1
111+
encoding = "latin-1"
112+
# # Disables https://bugs.python.org/issue43285 security if set to True.
113+
# trust_server_pasv_ipv4_address = False
111114

112115
# Initialization method (called by class instantiation).
113116
# Initialize host to localhost, port to standard ftp port
114117
# Optional arguments are host (for connect()),
115118
# and user, passwd, acct (for login())
116119
def __init__(self, host='', user='', passwd='', acct='',
117-
timeout=_GLOBAL_DEFAULT_TIMEOUT):
120+
timeout=_GLOBAL_DEFAULT_TIMEOUT, source_address=None):
121+
self.source_address = source_address
122+
# Disables https://bugs.python.org/issue43285 security if set to True.
123+
self.trust_server_pasv_ipv4_address = False
118124
self.timeout = timeout
119125
if host:
120126
self.connect(host)
121127
if user:
122128
self.login(user, passwd, acct)
123129

124-
def connect(self, host='', port=0, timeout=-999):
130+
def __enter__(self):
131+
return self
132+
133+
# Context management protocol: try to quit() if active
134+
def __exit__(self, *args):
135+
if self.sock is not None:
136+
try:
137+
self.quit()
138+
except (OSError, EOFError):
139+
pass
140+
finally:
141+
if self.sock is not None:
142+
self.close()
143+
144+
def connect(self, host='', port=0, timeout=-999, source_address=None):
125145
'''Connect to host. Arguments are:
126146
- host: hostname to connect to (string, default previous host)
127147
- port: port to connect to (integer, default previous port)
148+
- timeout: the timeout to set against the ftp socket(s)
149+
- source_address: a 2-tuple (host, port) for the socket to bind
150+
to as its source address before connecting.
128151
'''
129152
if host != '':
130153
self.host = host
131154
if port > 0:
132155
self.port = port
133156
if timeout != -999:
134157
self.timeout = timeout
158+
if source_address is not None:
159+
self.source_address = source_address
135160
self.sock = socket.create_connection((self.host, self.port), self.timeout)
136161
self.af = self.sock.family
137162
self.file = self.sock.makefile('rb')
@@ -310,8 +335,13 @@ def makeport(self):
310335
return sock
311336

312337
def makepasv(self):
338+
"""Internal: Does the PASV or EPSV handshake -> (address, port)"""
313339
if self.af == socket.AF_INET:
314-
host, port = parse227(self.sendcmd('PASV'))
340+
untrusted_host, port = parse227(self.sendcmd('PASV'))
341+
if self.trust_server_pasv_ipv4_address:
342+
host = untrusted_host
343+
else:
344+
host = self.sock.getpeername()[0]
315345
else:
316346
host, port = parse229(self.sendcmd('EPSV'), self.sock.getpeername())
317347
return host, port

‎Lib/test/test_ftplib.py

Lines changed: 110 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -15,6 +15,7 @@
1515
except ImportError:
1616
ssl = None
1717

18+
from contextlib import closing
1819
from unittest import TestCase, SkipTest, skipUnless
1920
from test import test_support
2021
from test.test_support import HOST, HOSTv6
@@ -67,6 +68,10 @@ def __init__(self, conn):
6768
self.rest = None
6869
self.next_retr_data = RETR_DATA
6970
self.push('220 welcome')
71+
# We use this as the string IPv4 address to direct the client
72+
# to in response to a PASV command. To test security behavior.
73+
# https://bugs.python.org/issue43285/.
74+
self.fake_pasv_server_ip = '252.253.254.255'
7075

7176
def collect_incoming_data(self, data):
7277
self.in_buffer.append(data)
@@ -109,13 +114,13 @@ def cmd_pasv(self, arg):
109114
sock.bind((self.socket.getsockname()[0], 0))
110115
sock.listen(5)
111116
sock.settimeout(10)
112-
ip, port = sock.getsockname()[:2]
113-
ip = ip.replace('.', ',')
114-
p1, p2 = divmod(port, 256)
117+
port = sock.getsockname()[1]
118+
ip = self.fake_pasv_server_ip
119+
ip = ip.replace('.', ','); p1 = port / 256; p2 = port % 256
115120
self.push('227 entering passive mode (%s,%d,%d)' %(ip, p1, p2))
116121
conn, addr = sock.accept()
117122
self.dtp = self.dtp_handler(conn, baseclass=self)
118-
123+
119124
def cmd_eprt(self, arg):
120125
af, ip, port = arg.split(arg[0])[1:-1]
121126
port = int(port)
@@ -577,6 +582,107 @@ def test_makepasv(self):
577582
# IPv4 is in use, just make sure send_epsv has not been used
578583
self.assertEqual(self.server.handler_instance.last_received_cmd, 'pasv')
579584

585+
def test_makepasv_issue43285_security_disabled(self):
586+
"""Test the opt-in to the old vulnerable behavior."""
587+
self.client.trust_server_pasv_ipv4_address = True
588+
bad_host, port = self.client.makepasv()
589+
self.assertEqual(
590+
bad_host, self.server.handler_instance.fake_pasv_server_ip)
591+
# Opening and closing a connection keeps the dummy server happy
592+
# instead of timing out on accept.
593+
socket.create_connection((self.client.sock.getpeername()[0], port),
594+
timeout=TIMEOUT).close()
595+
596+
def test_makepasv_issue43285_security_enabled_default(self):
597+
self.assertFalse(self.client.trust_server_pasv_ipv4_address)
598+
trusted_host, port = self.client.makepasv()
599+
self.assertNotEqual(
600+
trusted_host, self.server.handler_instance.fake_pasv_server_ip)
601+
# Opening and closing a connection keeps the dummy server happy
602+
# instead of timing out on accept.
603+
socket.create_connection((trusted_host, port), timeout=TIMEOUT).close()
604+
605+
def test_with_statement(self):
606+
self.client.quit()
607+
608+
def is_client_connected():
609+
if self.client.sock is None:
610+
return False
611+
try:
612+
self.client.sendcmd('noop')
613+
except (OSError, EOFError):
614+
return False
615+
return True
616+
617+
# base test
618+
with ftplib.FTP(timeout=TIMEOUT) as self.client:
619+
self.client.connect(self.server.host, self.server.port)
620+
self.client.sendcmd('noop')
621+
self.assertTrue(is_client_connected())
622+
self.assertEqual(self.server.handler_instance.last_received_cmd, 'quit')
623+
self.assertFalse(is_client_connected())
624+
625+
# QUIT sent inside the with block
626+
with ftplib.FTP(timeout=TIMEOUT) as self.client:
627+
self.client.connect(self.server.host, self.server.port)
628+
self.client.sendcmd('noop')
629+
self.client.quit()
630+
self.assertEqual(self.server.handler_instance.last_received_cmd, 'quit')
631+
self.assertFalse(is_client_connected())
632+
633+
# force a wrong response code to be sent on QUIT: error_perm
634+
# is expected and the connection is supposed to be closed
635+
try:
636+
with ftplib.FTP(timeout=TIMEOUT) as self.client:
637+
self.client.connect(self.server.host, self.server.port)
638+
self.client.sendcmd('noop')
639+
self.server.handler_instance.next_response = '550 error on quit'
640+
except ftplib.error_perm as err:
641+
self.assertEqual(str(err), '550 error on quit')
642+
else:
643+
self.fail('Exception not raised')
644+
# needed to give the threaded server some time to set the attribute
645+
# which otherwise would still be == 'noop'
646+
time.sleep(0.1)
647+
self.assertEqual(self.server.handler_instance.last_received_cmd, 'quit')
648+
self.assertFalse(is_client_connected())
649+
650+
def test_source_address(self):
651+
self.client.quit()
652+
port = test_support.find_unused_port()
653+
try:
654+
self.client.connect(self.server.host, self.server.port,
655+
source_address=(HOST, port))
656+
self.assertEqual(self.client.sock.getsockname()[1], port)
657+
self.client.quit()
658+
except OSError as e:
659+
if e.errno == errno.EADDRINUSE:
660+
self.skipTest("couldn't bind to port %d" % port)
661+
raise
662+
663+
def test_source_address_passive_connection(self):
664+
port = test_support.find_unused_port()
665+
self.client.source_address = (HOST, port)
666+
try:
667+
with closing(self.client.transfercmd('list')) as sock:
668+
self.assertEqual(sock.getsockname()[1], port)
669+
except OSError as e:
670+
if e.errno == errno.EADDRINUSE:
671+
self.skipTest("couldn't bind to port %d" % port)
672+
raise
673+
674+
def test_parse257(self):
675+
self.assertEqual(ftplib.parse257('257 "/foo/bar"'), '/foo/bar')
676+
self.assertEqual(ftplib.parse257('257 "/foo/bar" created'), '/foo/bar')
677+
self.assertEqual(ftplib.parse257('257 ""'), '')
678+
self.assertEqual(ftplib.parse257('257 "" created'), '')
679+
self.assertRaises(ftplib.error_reply, ftplib.parse257, '250 "/foo/bar"')
680+
# The 257 response is supposed to include the directory
681+
# name and in case it contains embedded double-quotes
682+
# they must be doubled (see RFC-959, chapter 7, appendix 2).
683+
self.assertEqual(ftplib.parse257('257 "/foo/b""ar"'), '/foo/b"ar')
684+
self.assertEqual(ftplib.parse257('257 "/foo/b""ar" created'), '/foo/b"ar')
685+
580686
def test_line_too_long(self):
581687
self.assertRaises(ftplib.Error, self.client.sendcmd,
582688
'x' * self.client.maxline * 2)

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

Lines changed: 8 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,8 @@
1+
:mod:`ftplib` no longer trusts the IP address value returned from the server
2+
in response to the PASV command by default. This prevents a malicious FTP
3+
server from using the response to probe IPv4 address and port combinations
4+
on the client network.
5+
6+
Code that requires the former vulnerable behavior may set a
7+
``trust_server_pasv_ipv4_address`` attribute on their
8+
:class:`ftplib.FTP` instances to ``True`` to re-enable it.

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