gh-143923: Reject control characters in POP3 commands

Lib/poplib.py

@@ -122,6 +122,8 @@ def _putline(self, line):
     def _putcmd(self, line):
         if self._debugging: print('*cmd*', repr(line))
         line = bytes(line, self.encoding)
+        if re.search(b'[\x00-\x1F\x7F]', line):
+            raise ValueError('Control characters not allowed in commands')
         self._putline(line)
 
 

Lib/test/support/__init__.py

@@ -3303,3 +3303,10 @@ def linked_to_musl():
         return _linked_to_musl
     _linked_to_musl = tuple(map(int, version.split('.')))
     return _linked_to_musl
+
+
+def control_characters_c0() -> list[str]:
+    """Returns a list of C0 control characters as strings.
+    C0 control characters defined as the byte range 0x00-0x1F, and 0x7F.
+    """
+    return [chr(c) for c in range(0x00, 0x20)] + ["\x7F"]

Lib/test/test_poplib.py

@@ -17,6 +17,7 @@
 from test.support import threading_helper
 from test.support import asynchat
 from test.support import asyncore
+from test.support import control_characters_c0
 
 
 test_support.requires_working_socket(module=True)
@@ -395,6 +396,13 @@ def test_quit(self):
         self.assertIsNone(self.client.sock)
         self.assertIsNone(self.client.file)
 
+    def test_control_characters(self):
+        for c0 in control_characters_c0():
+            with self.assertRaises(ValueError):
+                self.client.user(f'user{c0}')
+            with self.assertRaises(ValueError):
+                self.client.pass_(f'{c0}pass')
+
     @requires_ssl
     def test_stls_capa(self):
         capa = self.client.capa()

Misc/NEWS.d/next/Security/2026-01-16-11-43-47.gh-issue-143923.DuytMe.rst

@@ -0,0 +1 @@
+Reject control characters in POP3 commands.