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

Index: Python-3.15.0a3/Lib/poplib.py
===================================================================
--- Python-3.15.0a3.orig/Lib/poplib.py	2026-02-04 20:53:38.757990472 +0100
+++ Python-3.15.0a3/Lib/poplib.py	2026-02-04 20:53:46.764343586 +0100
@@ -122,6 +122,8 @@
     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)
 
 
Index: Python-3.15.0a3/Lib/test/test_poplib.py
===================================================================
--- Python-3.15.0a3.orig/Lib/test/test_poplib.py	2026-02-04 20:53:40.553673802 +0100
+++ Python-3.15.0a3/Lib/test/test_poplib.py	2026-02-04 20:53:46.764523568 +0100
@@ -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 @@
         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()
Index: Python-3.15.0a3/Misc/NEWS.d/next/Security/2026-01-16-11-43-47.gh-issue-143923.DuytMe.rst
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ Python-3.15.0a3/Misc/NEWS.d/next/Security/2026-01-16-11-43-47.gh-issue-143923.DuytMe.rst	2026-02-04 20:53:46.764674776 +0100
@@ -0,0 +1 @@
+Reject control characters in POP3 commands.