gh-143921: Narrow the control character check in imaplib commands (GH-153067)

Only NUL, CR and LF are rejected now.  Other control characters are
valid in quoted strings and can occur in mailbox names returned by
the server, so they are now accepted and sent quoted.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Lib/imaplib.py

@@ -129,7 +129,9 @@ Untagged_status = re.compile(
 # We compile these in _mode_xxx.
 _Literal = br'.*{(?P<size>\d+)}$'
 _Untagged_status = br'\* (?P<data>\d+) (?P<type>[A-Z-]+)( (?P<data2>.*))?'
-_control_chars = re.compile(b'[\x00-\x1F\x7F]')
+# Only NUL, CR and LF are unsafe (they cannot be represented even in
+# a quoted string); other control characters are sent quoted.
+_control_chars = re.compile(b'[\x00\r\n]')
 _non_astring_char = re.compile(br'[(){ \x00-\x1f\x7f-\xff%*\\"]')
 _non_list_char = re.compile(br'[(){ \x00-\x1f\x7f-\xff\\"]')
 _quoted = re.compile(br'"(?:[^"\\]|\\.)*+"')
@@ -1170,7 +1172,7 @@ class IMAP4:
             if isinstance(arg, str):
                 arg = bytes(arg, self._encoding)
             if _control_chars.search(arg):
-                raise ValueError("Control characters not allowed in commands")
+                raise ValueError("NUL, CR and LF not allowed in commands")
             data = data + b' ' + arg
 
         literal = self.literal

Lib/test/test_imaplib.py

@@ -1751,10 +1751,20 @@ class NewIMAPTestsMixin:
             client.NONEXISTENT
 
     def test_control_characters(self):
-        client, _ = self._setup(SimpleIMAPHandler)
-        for c0 in support.control_characters_c0():
+        client, server = self._setup(SimpleIMAPHandler)
+        client.login('user', 'pass')
+        for c in '\0\r\n':
             with self.assertRaises(ValueError):
-                client.login(f'user{c0}', 'pass')
+                client.select(f'a{c}b')
+        # Other control characters are valid in a quoted string and can
+        # occur in mailbox names returned by the server, so the client
+        # must be able to send them back.
+        for c in support.control_characters_c0():
+            if c in '\0\r\n':
+                continue
+            typ, _ = client.select(f'a{c}b')
+            self.assertEqual(typ, 'OK')
+            self.assertEqual(server.is_selected, [f'"a{c}b"'])
 
     # property tests
 

Misc/NEWS.d/next/Library/2026-07-05-10-24-30.gh-issue-143921.wQx3Tn.rst

@@ -0,0 +1,4 @@
+Narrow the control character check in :mod:`imaplib` commands: only NUL, CR
+and LF are now rejected. Other control characters are valid in quoted
+strings and can occur in mailbox names returned by the server, so they are
+now accepted and sent quoted.