From 1132e45e8b588cd89bd168583afa37ba7c9f0afa Mon Sep 17 00:00:00 2001 From: Denis Ledoux Date: Mon, 27 Oct 2025 17:47:59 +0100 Subject: [PATCH 1/3] gh-144125: email: verify headers are sound in BytesGenerator GH-122233 added an implementation to `Generator` to refuse to serialize (write) headers that are unsafely folded or delimited. This revision adds the same implementation to `BytesGenerator`, so it gets the same safety protections for unsafely folded or delimited headers 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> --- Lib/email/generator.py | 12 +++++++++- Lib/test/test_email/test_generator.py | 4 ++- Lib/test/test_email/test_policy.py | 6 ++++- Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst | 4 +++ 4 files changed, 23 insertions(+), 3 deletions(-) create mode 100644 Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst Index: Python-3.15.0a3/Lib/email/generator.py =================================================================== --- Python-3.15.0a3.orig/Lib/email/generator.py 2026-01-27 17:26:06.289940190 +0100 +++ Python-3.15.0a3/Lib/email/generator.py 2026-01-27 17:26:14.361081031 +0100 @@ -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]') class Generator: @@ -429,7 +430,16 @@ # 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}') + if NEWLINE_WITHOUT_FWSP_BYTES.search(folded.removesuffix(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) Index: Python-3.15.0a3/Lib/test/test_email/test_generator.py =================================================================== --- Python-3.15.0a3.orig/Lib/test/test_email/test_generator.py 2026-01-27 17:26:08.009520772 +0100 +++ Python-3.15.0a3/Lib/test/test_email/test_generator.py 2026-01-27 17:26:14.363103139 +0100 @@ -313,7 +313,7 @@ self.assertEqual(s.getvalue(), self.typ(expected)) 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): @@ -334,6 +334,8 @@ with self.assertRaises(email.errors.HeaderWriteError): message.as_string() + with self.assertRaises(email.errors.HeaderWriteError): + message.as_bytes() class TestBytesGenerator(TestGeneratorBase, TestEmailBase): Index: Python-3.15.0a3/Lib/test/test_email/test_policy.py =================================================================== --- Python-3.15.0a3.orig/Lib/test/test_email/test_policy.py 2026-01-27 17:26:08.021649854 +0100 +++ Python-3.15.0a3/Lib/test/test_email/test_policy.py 2026-01-27 17:26:14.363238384 +0100 @@ -296,7 +296,7 @@ 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', @@ -319,6 +319,10 @@ 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 Index: Python-3.15.0a3/Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst =================================================================== --- /dev/null 1970-01-01 00:00:00.000000000 +0000 +++ Python-3.15.0a3/Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst 2026-01-27 17:26:14.362392400 +0100 @@ -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`).