From b6f733b285b1c4f27dacb5c2e1f292c914e8b933 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 58591540fc2140bd1a79e292af1ceea607ba957a Mon Sep 17 00:00:00 2001
From: Seth Michael Larson <seth@python.org>
Date: Tue, 30 Dec 2025 11:03:02 -0600
Subject: [PATCH 2/2] gh-143923: Reject control characters in POP3 commands

---
 Lib/poplib.py                                             | 2 ++
 Lib/test/test_poplib.py                                   | 8 ++++++++
 .../2026-01-16-11-43-47.gh-issue-143923.DuytMe.rst        | 1 +
 3 files changed, 11 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Security/2026-01-16-11-43-47.gh-issue-143923.DuytMe.rst

diff --git a/Lib/poplib.py b/Lib/poplib.py
index 4469bff44b4c45..b97274c5c32ee6 100644
--- a/Lib/poplib.py
+++ b/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)
 
 
diff --git a/Lib/test/test_poplib.py b/Lib/test/test_poplib.py
index ef2da97f86734a..18ca7cb556836e 100644
--- a/Lib/test/test_poplib.py
+++ b/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()
diff --git a/Misc/NEWS.d/next/Security/2026-01-16-11-43-47.gh-issue-143923.DuytMe.rst b/Misc/NEWS.d/next/Security/2026-01-16-11-43-47.gh-issue-143923.DuytMe.rst
new file mode 100644
index 00000000000000..3cde4df3e0069f
--- /dev/null
+++ b/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.