diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 503e3c9cb8b7770..0223a4ad9e2b027 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -132,6 +132,9 @@
 # We compile these in _mode_xxx.
 _Literal = br'.*{(?P<size>\d+)}$'
 _Untagged_status = br'\* (?P<data>\d+) (?P<type>[A-Z-]+)( (?P<data2>.*))?'
+# 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'"(?:[^"\\]|\\.)*+"')
@@ -1047,6 +1050,8 @@ def _command(self, name, *args):
             if arg is None: continue
             if isinstance(arg, str):
                 arg = bytes(arg, self._encoding)
+            if _control_chars.search(arg):
+                raise ValueError("NUL, CR and LF not allowed in commands")
             data = data + b' ' + arg
 
         literal = self.literal
diff --git a/Lib/test/test_imaplib.py b/Lib/test/test_imaplib.py
index 9c1dcb1e299bf73..49a8c1a1b817c82 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -1626,6 +1626,22 @@ def test_uppercase_command_names(self):
         with self.assertRaises(AttributeError):
             client.NONEXISTENT
 
+    def test_control_characters(self):
+        client, server = self._setup(SimpleIMAPHandler)
+        client.login('user', 'pass')
+        for c in '\0\r\n':
+            with self.assertRaises(ValueError):
+                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"'])
+
 
 class NewIMAPTests(NewIMAPTestsMixin, unittest.TestCase):
     imap_class = imaplib.IMAP4
diff --git a/Misc/NEWS.d/next/Security/2026-01-16-11-41-06.gh-issue-143921.AeCOor.rst b/Misc/NEWS.d/next/Security/2026-01-16-11-41-06.gh-issue-143921.AeCOor.rst
new file mode 100644
index 000000000000000..de62e020a95e6b2
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2026-01-16-11-41-06.gh-issue-143921.AeCOor.rst
@@ -0,0 +1,2 @@
+Reject NUL, CR and LF characters in IMAP commands. Other control
+characters are allowed and sent quoted.