From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: "Gregory P. Smith" <68491+gpshead@users.noreply.github.com>
Date: Tue, 20 Jan 2026 14:51:43 -0800
Subject: 00473: CVE-2026-0865

gh-143916: Reject control characters in wsgiref.headers.Headers  (GH-143917)

* Add 'test.support' fixture for C0 control characters
* gh-143916: Reject control characters in wsgiref.headers.Headers

(cherry picked from commit 2f840249550e082dc351743f474ba56da10478d2)

Co-authored-by: Seth Michael Larson <seth@python.org>
---
 Lib/test/support/__init__.py                          |  7 +++++++
 Lib/test/test_wsgiref.py                              | 11 +++++++++++
 Lib/wsgiref/headers.py                                |  3 +++
 .../2026-01-16-11-07-36.gh-issue-143916.dpWeOD.rst    |  2 ++
 4 files changed, 23 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Security/2026-01-16-11-07-36.gh-issue-143916.dpWeOD.rst

diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index da1f2ee719..7c260a3dfe 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -2902,3 +2902,10 @@ def adjust_int_max_str_digits(max_digits):
         yield
     finally:
         sys.set_int_max_str_digits(current)
+
+
+def control_characters_c0():
+    """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"]
diff --git a/Lib/test/test_wsgiref.py b/Lib/test/test_wsgiref.py
index 7708e20684..baac84d71c 100644
--- a/Lib/test/test_wsgiref.py
+++ b/Lib/test/test_wsgiref.py
@@ -1,5 +1,6 @@
 from unittest import mock
 from test import support
+from test.support import control_characters_c0
 from test.test_httpservers import NoLogRequestHandler
 from unittest import TestCase
 from wsgiref.util import setup_testing_defaults
@@ -517,6 +518,16 @@ class HeaderTests(TestCase):
             '\r\n'
         )
 
+    def testRaisesControlCharacters(self):
+        headers = Headers()
+        for c0 in control_characters_c0():
+            self.assertRaises(ValueError, headers.__setitem__, f"key{c0}", "val")
+            self.assertRaises(ValueError, headers.__setitem__, "key", f"val{c0}")
+            self.assertRaises(ValueError, headers.add_header, f"key{c0}", "val", param="param")
+            self.assertRaises(ValueError, headers.add_header, "key", f"val{c0}", param="param")
+            self.assertRaises(ValueError, headers.add_header, "key", "val", param=f"param{c0}")
+
+
 class ErrorHandler(BaseCGIHandler):
     """Simple handler subclass for testing BaseHandler"""
 
diff --git a/Lib/wsgiref/headers.py b/Lib/wsgiref/headers.py
index fab851c5a4..fd98e85d75 100644
--- a/Lib/wsgiref/headers.py
+++ b/Lib/wsgiref/headers.py
@@ -9,6 +9,7 @@ written by Barry Warsaw.
 # existence of which force quoting of the parameter value.
 import re
 tspecials = re.compile(r'[ \(\)<>@,;:\\"/\[\]\?=]')
+_control_chars_re = re.compile(r'[\x00-\x1F\x7F]')
 
 def _formatparam(param, value=None, quote=1):
     """Convenience function to format and return a key=value pair.
@@ -41,6 +42,8 @@ class Headers:
     def _convert_string_type(self, value):
         """Convert/check value type."""
         if type(value) is str:
+            if _control_chars_re.search(value):
+                raise ValueError("Control characters not allowed in headers")
             return value
         raise AssertionError("Header names/values must be"
             " of type str (got {0})".format(repr(value)))
diff --git a/Misc/NEWS.d/next/Security/2026-01-16-11-07-36.gh-issue-143916.dpWeOD.rst b/Misc/NEWS.d/next/Security/2026-01-16-11-07-36.gh-issue-143916.dpWeOD.rst
new file mode 100644
index 0000000000..44bd0b2705
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2026-01-16-11-07-36.gh-issue-143916.dpWeOD.rst
@@ -0,0 +1,2 @@
+Reject C0 control characters within wsgiref.headers.Headers fields, values,
+and parameters.