gh-87451: Apply CVE-2021-4189 PASV fix to ftplib.ftpcp()

ftpcp() called parse227() directly and passed the source server's
self-reported PASV IPv4 address to the target server's PORT command,
bypassing the CVE-2021-4189 fix that was applied only to FTP.makepasv().
A malicious source FTP server could use this to redirect the target
server's data connection to an arbitrary host:port (SSRF).

ftpcp() now uses the source server's actual peer address, honoring the
existing trust_server_pasv_ipv4_address opt-out, the same as makepasv().

Thanks to Qi Ding (AKA ikow) for the report.

Author: gpshead

Lib/ftplib.py

@@ -883,7 +883,16 @@ def ftpcp(source, sourcename, target, targetname = '', type = 'I'):
     type = 'TYPE ' + type
     source.voidcmd(type)
     target.voidcmd(type)
-    sourcehost, sourceport = parse227(source.sendcmd('PASV'))
+    # Don't trust the IPv4 address the source server advertises in its PASV
+    # reply: a malicious source could otherwise point the target's data
+    # connection at an arbitrary host (SSRF).  A caller that needs the old
+    # behavior can set trust_server_pasv_ipv4_address on the source FTP
+    # object.  See FTP.makepasv(), which applies the same rule.
+    untrusted_host, sourceport = parse227(source.sendcmd('PASV'))
+    if source.trust_server_pasv_ipv4_address:
+        sourcehost = untrusted_host
+    else:
+        sourcehost = source.sock.getpeername()[0]
     target.sendport(sourcehost, sourceport)
     # RFC 959: the user must "listen" [...] BEFORE sending the
     # transfer request.

Lib/test/test_ftplib.py

@@ -1145,6 +1145,60 @@ def testTimeoutDirectAccess(self):
         ftp.close()
 
 
+class TestFtpcpSecurity(TestCase):
+    """ftpcp() must not trust the host a source server advertises in PASV.
+
+    A malicious source server can otherwise redirect the target server's
+    data connection to an arbitrary host:port (SSRF), so ftpcp() uses the
+    source server's actual peer address instead, the same as FTP.makepasv().
+    """
+
+    class _FakeSock:
+        def __init__(self, peer_host):
+            self._peer = (peer_host, 21)
+        def getpeername(self):
+            return self._peer
+
+    class _FakeSource:
+        trust_server_pasv_ipv4_address = False
+        def __init__(self, advertised_host, real_host):
+            self.sock = TestFtpcpSecurity._FakeSock(real_host)
+            self._advertised = advertised_host.replace('.', ',')
+        def voidcmd(self, cmd):
+            pass
+        def sendcmd(self, cmd):
+            if cmd == 'PASV':
+                return '227 Entering Passive Mode (%s,1,2).' % self._advertised
+            return '150 ok'
+        def voidresp(self):
+            pass
+
+    class _FakeTarget:
+        def __init__(self):
+            self.sendport_args = None
+        def voidcmd(self, cmd):
+            pass
+        def sendport(self, host, port):
+            self.sendport_args = (host, port)
+        def sendcmd(self, cmd):
+            return '150 ok'
+        def voidresp(self):
+            pass
+
+    def test_ftpcp_ignores_untrusted_pasv_host(self):
+        source = self._FakeSource('10.0.0.5', '198.51.100.7')
+        target = self._FakeTarget()
+        ftplib.ftpcp(source, 'a', target, 'b')
+        self.assertEqual(target.sendport_args, ('198.51.100.7', 258))
+
+    def test_ftpcp_trust_server_pasv_ipv4_address(self):
+        source = self._FakeSource('10.0.0.5', '198.51.100.7')
+        source.trust_server_pasv_ipv4_address = True
+        target = self._FakeTarget()
+        ftplib.ftpcp(source, 'a', target, 'b')
+        self.assertEqual(target.sendport_args, ('10.0.0.5', 258))
+
+
 class MiscTestCase(TestCase):
     def test__all__(self):
         not_exported = {

Misc/NEWS.d/next/Security/2026-05-10-18-05-32.gh-issue-87451.XkKB6M.rst

@@ -0,0 +1,6 @@
+The :mod:`ftplib` module's undocumented ``ftpcp`` function no longer trusts
+the IPv4 address value returned from the source server in response to the
+``PASV`` command by default, completing the fix for CVE-2021-4189.  As with
+:class:`ftplib.FTP`, the former behavior can be re-enabled by setting the
+``trust_server_pasv_ipv4_address`` attribute on the source :class:`ftplib.FTP`
+instance to ``True``.  Thanks to Qi Ding (AKA ikow) for the report.