From 7485ee5e2cf81d3e5ad0d9c3be73cecd2ab4eec7 Mon Sep 17 00:00:00 2001
From: Seth Michael Larson <seth@python.org>
Date: Fri, 16 Jan 2026 10:54:09 -0600
Subject: [PATCH 1/2] Add 'test.support' fixture for C0 control characters

---
 Lib/test/support/__init__.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index 3a639497fa1272..9c113a6c137e52 100644
--- a/Lib/test/support/__init__.py
+++ b/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"]

From 997a64a52cc0802a7692a82a8810c8b32f3758f6 Mon Sep 17 00:00:00 2001
From: Seth Michael Larson <seth@python.org>
Date: Wed, 7 Jan 2026 14:10:08 -0600
Subject: [PATCH 2/2] gh-143921: Disallow control characters in IMAP commands

---
 Lib/imaplib.py                                              | 4 +++-
 Lib/test/test_imaplib.py                                    | 6 ++++++
 .../Security/2026-01-16-11-41-06.gh-issue-143921.AeCOor.rst | 1 +
 3 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Security/2026-01-16-11-41-06.gh-issue-143921.AeCOor.rst

diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 22a0afcd981519..cb3edceae0d9f1 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -129,7 +129,7 @@
 # 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]')
 
 
 class IMAP4:
@@ -1105,6 +1105,8 @@ 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("Control characters 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 430fa71fa29f59..cb5454b40eccf9 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -657,6 +657,12 @@ def test_unselect(self):
         self.assertEqual(data[0], b'Returned to authenticated state. (Success)')
         self.assertEqual(client.state, 'AUTH')
 
+    def test_control_characters(self):
+        client, _ = self._setup(SimpleIMAPHandler)
+        for c0 in support.control_characters_c0():
+            with self.assertRaises(ValueError):
+                client.login(f'user{c0}', 'pass')
+
     # property tests
 
     def test_file_property_should_not_be_accessed(self):
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 00000000000000..4e13fe92bc60fb
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2026-01-16-11-41-06.gh-issue-143921.AeCOor.rst
@@ -0,0 +1 @@
+Reject control characters in IMAP commands.