00476: CVE-2026-1299

pythongh-144125: email: verify headers are sound in BytesGenerator

(cherry picked from commit 8cdf620)

Co-authored-by: Seth Michael Larson <seth@python.org>
Co-authored-by: Denis Ledoux <dle@odoo.com>
Co-authored-by: Denis Ledoux <5822488+beledouxdenis@users.noreply.github.com>
Co-authored-by: Petr Viktorin <302922+encukou@users.noreply.github.com>
Co-authored-by: Bas Bloemsaat <1586868+basbloemsaat@users.noreply.github.com>

The fix for the CVE uncovered a known issue in handling
policy.linesep lengths fixed by:

bpo-34424: Handle different policy.linesep lengths correctly. (python#8803)

(cherry-picked from commit 45b2f88)

Co-authored-by: Jens Troeger <jenstroeger@users.noreply.github.com>

Author: 2 people

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]):

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)
 

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()

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

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.

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`).