diff --git a/Lib/email/_header_value_parser.py b/Lib/email/_header_value_parser.py
index 04035b2612d7af..91a01fd1f44b84 100644
--- a/Lib/email/_header_value_parser.py
+++ b/Lib/email/_header_value_parser.py
@@ -2641,7 +2641,7 @@ def _refold_parse_tree(parse_tree, *, policy):
                 want_encoding = False
                 last_ew = None
                 if part.syntactic_break:
-                    encoded_part = part.fold(policy=policy)[:-1] # strip nl
+                    encoded_part = part.fold(policy=policy)[:-len(policy.linesep)]
                     if policy.linesep not in encoded_part:
                         # It fits on a single line
                         if len(encoded_part) > maxlen - len(lines[-1]):
diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index 6deb95ba8a13c7..6b08d84ff4866d 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -22,6 +22,7 @@
 NLCRE = re.compile(r'\r\n|\r|\n')
 fcre = re.compile(r'^From ', re.MULTILINE)
 NEWLINE_WITHOUT_FWSP = re.compile(r'\r\n[^ \t]|\r[^ \n\t]|\n[^ \t]')
+NEWLINE_WITHOUT_FWSP_BYTES = re.compile(br'\r\n[^ \t]|\r[^ \n\t]|\n[^ \t]')
 
 
 
@@ -429,7 +430,19 @@ def _write_headers(self, msg):
         # This is almost the same as the string version, except for handling
         # strings with 8bit bytes.
         for h, v in msg.raw_items():
-            self._fp.write(self.policy.fold_binary(h, v))
+            folded = self.policy.fold_binary(h, v)
+            if self.policy.verify_generated_headers:
+                linesep = self.policy.linesep.encode()
+                if not folded.endswith(linesep):
+                    raise HeaderWriteError(
+                        f'folded header does not end with {linesep!r}: {folded!r}')
+                folded_no_linesep = folded
+                if folded.endswith(linesep):
+                    folded_no_linesep = folded[:-len(linesep)]
+                if NEWLINE_WITHOUT_FWSP_BYTES.search(folded_no_linesep):
+                    raise HeaderWriteError(
+                        f'folded header contains newline: {folded!r}')
+            self._fp.write(folded)
         # A blank line always separates headers from body
         self.write(self._NL)
 
diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 67b2cc02c40f18..abd530e98b484a 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -128,7 +128,7 @@
 # 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:
@@ -958,6 +958,8 @@ 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/poplib.py b/Lib/poplib.py
index 9796f0d2f9c552..64690628fec078 100644
--- a/Lib/poplib.py
+++ b/Lib/poplib.py
@@ -117,6 +117,8 @@ 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/support/__init__.py b/Lib/test/support/__init__.py
index da1f2ee719381e..7c260a3dfea85f 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_email/test_generator.py b/Lib/test/test_email/test_generator.py
index cdf1075bab8794..23adb06425988c 100644
--- a/Lib/test/test_email/test_generator.py
+++ b/Lib/test/test_email/test_generator.py
@@ -4,6 +4,7 @@
 from email import message_from_string, message_from_bytes
 from email.message import EmailMessage
 from email.generator import Generator, BytesGenerator
+from email.headerregistry import Address
 from email import policy
 import email.errors
 from test.test_email import TestEmailBase, parameterize
@@ -263,7 +264,7 @@ class TestGenerator(TestGeneratorBase, TestEmailBase):
     typ = str
 
     def test_verify_generated_headers(self):
-        """gh-121650: by default the generator prevents header injection"""
+        # gh-121650: by default the generator prevents header injection
         class LiteralHeader(str):
             name = 'Header'
             def fold(self, **kwargs):
@@ -284,6 +285,8 @@ def fold(self, **kwargs):
 
                 with self.assertRaises(email.errors.HeaderWriteError):
                     message.as_string()
+                with self.assertRaises(email.errors.HeaderWriteError):
+                    message.as_bytes()
 
 
 class TestBytesGenerator(TestGeneratorBase, TestEmailBase):
@@ -353,6 +356,27 @@ def test_smtputf8_policy(self):
         g.flatten(msg)
         self.assertEqual(s.getvalue(), expected)
 
+    def test_smtp_policy(self):
+        msg = EmailMessage()
+        msg["From"] = Address(addr_spec="foo@bar.com", display_name="Páolo")
+        msg["To"] = Address(addr_spec="bar@foo.com", display_name="Dinsdale")
+        msg["Subject"] = "Nudge nudge, wink, wink"
+        msg.set_content("oh boy, know what I mean, know what I mean?")
+        expected = textwrap.dedent("""\
+            From: =?utf-8?q?P=C3=A1olo?= <foo@bar.com>
+            To: Dinsdale <bar@foo.com>
+            Subject: Nudge nudge, wink, wink
+            Content-Type: text/plain; charset="utf-8"
+            Content-Transfer-Encoding: 7bit
+            MIME-Version: 1.0
+
+            oh boy, know what I mean, know what I mean?
+            """).encode().replace(b"\n", b"\r\n")
+        s = io.BytesIO()
+        g = BytesGenerator(s, policy=policy.SMTP)
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), expected)
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Lib/test/test_email/test_policy.py b/Lib/test/test_email/test_policy.py
index 76198392e5234e..815309732d04b6 100644
--- a/Lib/test/test_email/test_policy.py
+++ b/Lib/test/test_email/test_policy.py
@@ -267,7 +267,7 @@ def test_short_maxlen_error(self):
                     policy.fold("Subject", subject)
 
     def test_verify_generated_headers(self):
-        """Turning protection off allows header injection"""
+        # Turning protection off allows header injection
         policy = email.policy.default.clone(verify_generated_headers=False)
         for text in (
             'Header: Value\r\nBad: Injection\r\n',
@@ -290,6 +290,10 @@ def fold(self, **kwargs):
                     message.as_string(),
                     f"{text}\nBody",
                 )
+                self.assertEqual(
+                    message.as_bytes(),
+                    f"{text}\nBody".encode(),
+                )
 
     # XXX: Need subclassing tests.
     # For adding subclassed objects, make sure the usual rules apply (subclass
diff --git a/Lib/test/test_imaplib.py b/Lib/test/test_imaplib.py
index 0593a3756b0843..ec95577e96dc32 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -462,6 +462,12 @@ def test_login(self):
         self.assertEqual(data[0], b'LOGIN completed')
         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')
+
     def test_logout(self):
         client, _ = self._setup(SimpleIMAPHandler)
         typ, data = client.login('user', 'pass')
diff --git a/Lib/test/test_poplib.py b/Lib/test/test_poplib.py
index 234c855545c2bb..18b8c191ab22d9 100644
--- a/Lib/test/test_poplib.py
+++ b/Lib/test/test_poplib.py
@@ -12,6 +12,7 @@
 
 from unittest import TestCase, skipUnless
 from test import support as test_support
+from test.support import control_characters_c0
 threading = test_support.import_module('threading')
 
 HOST = test_support.HOST
@@ -349,6 +350,13 @@ 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/Lib/test/test_wsgiref.py b/Lib/test/test_wsgiref.py
index 7708e206840581..baac84d71cbcba 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 @@ def testExtras(self):
             '\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 fab851c5a44430..fd98e85d75492b 100644
--- a/Lib/wsgiref/headers.py
+++ b/Lib/wsgiref/headers.py
@@ -9,6 +9,7 @@
 # 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 @@ def __init__(self, headers=None):
     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/Library/2018-08-18-14-47-00.bpo-34424.wAlRuS.rst b/Misc/NEWS.d/next/Library/2018-08-18-14-47-00.bpo-34424.wAlRuS.rst
new file mode 100644
index 00000000000000..2b384cd5513fca
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-08-18-14-47-00.bpo-34424.wAlRuS.rst
@@ -0,0 +1,2 @@
+Fix serialization of messages containing encoded strings when the
+policy.linesep is set to a multi-character string. Patch by Jens Troeger.
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 00000000000000..44bd0b27059f94
--- /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.
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.
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.
diff --git a/Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst b/Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst
new file mode 100644
index 00000000000000..e6333e724972c5
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst
@@ -0,0 +1,4 @@
+:mod:`~email.generator.BytesGenerator` will now refuse to serialize (write) headers
+that are unsafely folded or delimited; see
+:attr:`~email.policy.Policy.verify_generated_headers`. (Contributed by Bas
+Bloemsaat and Petr Viktorin in :gh:`121650`).