From 61793762daa355dc7a2b5edd104ef38efb3efb2a Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=C5=81ukasz=20Langa?= Date: Wed, 4 Sep 2024 17:37:28 +0200 Subject: [3.11] gh-121650: Encode newlines in headers, and verify headers are sound (GH-122233) (#122608) Per RFC 2047: > [...] these encoding schemes allow the > encoding of arbitrary octet values, mail readers that implement this > decoding should also ensure that display of the decoded data on the > recipient's terminal will not cause unwanted side-effects It seems that the "quoted-word" scheme is a valid way to include a newline character in a header value, just like we already allow undecodable bytes or control characters. They do need to be properly quoted when serialized to text, though. Verify that email headers are well-formed. This should fail for custom fold() implementations that aren't careful about newlines. (cherry picked from commit 097633981879b3c9de9a1dd120d3aa585ecc2384) Co-authored-by: Petr Viktorin Co-authored-by: Bas Bloemsaat Co-authored-by: Serhiy Storchaka --- Doc/library/email.errors.rst | 5 +++ Doc/library/email.policy.rst | 18 ++++++++ Lib/email/_header_value_parser.py | 12 ++++-- Lib/email/_policybase.py | 8 ++++ Lib/email/errors.py | 4 ++ Lib/email/generator.py | 13 +++++- Lib/test/test_email/test_generator.py | 62 +++++++++++++++++++++++++++ Lib/test/test_email/test_policy.py | 26 +++++++++++ 8 files changed, 144 insertions(+), 4 deletions(-) diff --git a/Doc/library/email.errors.rst b/Doc/library/email.errors.rst index 194a98696f4..a98a2777310 100644 --- a/Doc/library/email.errors.rst +++ b/Doc/library/email.errors.rst @@ -58,6 +58,11 @@ The following exception classes are defined in the :mod:`email.errors` module: :class:`~email.mime.nonmultipart.MIMENonMultipart` (e.g. :class:`~email.mime.image.MIMEImage`). +.. exception:: HeaderWriteError() + + Raised when an error occurs when the :mod:`~email.generator` outputs + headers. + Here is the list of the defects that the :class:`~email.parser.FeedParser` can find while parsing messages. Note that the defects are added to the message diff --git a/Doc/library/email.policy.rst b/Doc/library/email.policy.rst index bf53b9520fc..f5ed3d20b0e 100644 --- a/Doc/library/email.policy.rst +++ b/Doc/library/email.policy.rst @@ -229,6 +229,24 @@ added matters. To illustrate:: .. versionadded:: 3.6 + + .. attribute:: verify_generated_headers + + If ``True`` (the default), the generator will raise + :exc:`~email.errors.HeaderWriteError` instead of writing a header + that is improperly folded or delimited, such that it would + be parsed as multiple headers or joined with adjacent data. + Such headers can be generated by custom header classes or bugs + in the ``email`` module. + + As it's a security feature, this defaults to ``True`` even in the + :class:`~email.policy.Compat32` policy. + For backwards compatible, but unsafe, behavior, it must be set to + ``False`` explicitly. + + .. versionadded:: 3.11.2-6+deb12u5 + + The following :class:`Policy` method is intended to be called by code using the email library to create policy instances with custom settings: diff --git a/Lib/email/_header_value_parser.py b/Lib/email/_header_value_parser.py index e637e6df066..e1b99d5b417 100644 --- a/Lib/email/_header_value_parser.py +++ b/Lib/email/_header_value_parser.py @@ -92,6 +92,8 @@ ASPECIALS = TSPECIALS | set("*'%") ATTRIBUTE_ENDS = ASPECIALS | WSP EXTENDED_ATTRIBUTE_ENDS = ATTRIBUTE_ENDS - set('%') +NLSET = {'\n', '\r'} +SPECIALSNL = SPECIALS | NLSET def quote_string(value): return '"'+str(value).replace('\\', '\\\\').replace('"', r'\"')+'"' @@ -2778,9 +2780,13 @@ def _refold_parse_tree(parse_tree, *, policy): wrap_as_ew_blocked -= 1 continue tstr = str(part) - if part.token_type == 'ptext' and set(tstr) & SPECIALS: - # Encode if tstr contains special characters. - want_encoding = True + if not want_encoding: + if part.token_type == 'ptext': + # Encode if tstr contains special characters. + want_encoding = not SPECIALSNL.isdisjoint(tstr) + else: + # Encode if tstr contains newlines. + want_encoding = not NLSET.isdisjoint(tstr) try: tstr.encode(encoding) charset = encoding diff --git a/Lib/email/_policybase.py b/Lib/email/_policybase.py index c9cbadd2a80..d1f48211f90 100644 --- a/Lib/email/_policybase.py +++ b/Lib/email/_policybase.py @@ -157,6 +157,13 @@ class Policy(_PolicyBase, metaclass=abc.ABCMeta): message_factory -- the class to use to create new message objects. If the value is None, the default is Message. + verify_generated_headers + -- if true, the generator verifies that each header + they are properly folded, so that a parser won't + treat it as multiple headers, start-of-body, or + part of another header. + This is a check against custom Header & fold() + implementations. """ raise_on_defect = False @@ -165,6 +172,7 @@ class Policy(_PolicyBase, metaclass=abc.ABCMeta): max_line_length = 78 mangle_from_ = False message_factory = None + verify_generated_headers = True def handle_defect(self, obj, defect): """Based on policy, either raise defect or call register_defect. diff --git a/Lib/email/errors.py b/Lib/email/errors.py index 3ad00565549..02aa5eced6a 100644 --- a/Lib/email/errors.py +++ b/Lib/email/errors.py @@ -29,6 +29,10 @@ class CharsetError(MessageError): """An illegal charset was given.""" +class HeaderWriteError(MessageError): + """Error while writing headers.""" + + # These are parsing defects which the parser was able to work around. class MessageDefect(ValueError): """Base class for a message defect.""" diff --git a/Lib/email/generator.py b/Lib/email/generator.py index c9b121624e0..89224ae41cb 100644 --- a/Lib/email/generator.py +++ b/Lib/email/generator.py @@ -14,12 +14,14 @@ from copy import deepcopy from io import StringIO, BytesIO from email.utils import _has_surrogates +from email.errors import HeaderWriteError UNDERSCORE = '_' NL = '\n' # XXX: no longer used by the code below. 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]') @@ -223,7 +225,16 @@ def _dispatch(self, msg): def _write_headers(self, msg): for h, v in msg.raw_items(): - self.write(self.policy.fold(h, v)) + folded = self.policy.fold(h, v) + if self.policy.verify_generated_headers: + linesep = self.policy.linesep + if not folded.endswith(self.policy.linesep): + raise HeaderWriteError( + f'folded header does not end with {linesep!r}: {folded!r}') + if NEWLINE_WITHOUT_FWSP.search(folded.removesuffix(linesep)): + raise HeaderWriteError( + f'folded header contains newline: {folded!r}') + self.write(folded) # A blank line always separates headers from body self.write(self._NL) diff --git a/Lib/test/test_email/test_generator.py b/Lib/test/test_email/test_generator.py index 89e7edeb63a..d29400f0ed1 100644 --- a/Lib/test/test_email/test_generator.py +++ b/Lib/test/test_email/test_generator.py @@ -6,6 +6,7 @@ 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 @@ -216,6 +217,44 @@ def test_rfc2231_wrapping_switches_to_default_len_if_too_narrow(self): g.flatten(msg) self.assertEqual(s.getvalue(), self.typ(expected)) + def test_keep_encoded_newlines(self): + msg = self.msgmaker(self.typ(textwrap.dedent("""\ + To: nobody + Subject: Bad subject=?UTF-8?Q?=0A?=Bcc: injection@example.com + + None + """))) + expected = textwrap.dedent("""\ + To: nobody + Subject: Bad subject=?UTF-8?Q?=0A?=Bcc: injection@example.com + + None + """) + s = self.ioclass() + g = self.genclass(s, policy=self.policy.clone(max_line_length=80)) + g.flatten(msg) + self.assertEqual(s.getvalue(), self.typ(expected)) + + def test_keep_long_encoded_newlines(self): + msg = self.msgmaker(self.typ(textwrap.dedent("""\ + To: nobody + Subject: Bad subject=?UTF-8?Q?=0A?=Bcc: injection@example.com + + None + """))) + expected = textwrap.dedent("""\ + To: nobody + Subject: Bad subject + =?utf-8?q?=0A?=Bcc: + injection@example.com + + None + """) + s = self.ioclass() + g = self.genclass(s, policy=self.policy.clone(max_line_length=30)) + g.flatten(msg) + self.assertEqual(s.getvalue(), self.typ(expected)) + class TestGenerator(TestGeneratorBase, TestEmailBase): @@ -224,6 +263,29 @@ class TestGenerator(TestGeneratorBase, TestEmailBase): ioclass = io.StringIO typ = str + def test_verify_generated_headers(self): + """gh-121650: by default the generator prevents header injection""" + class LiteralHeader(str): + name = 'Header' + def fold(self, **kwargs): + return self + + for text in ( + 'Value\r\nBad Injection\r\n', + 'NoNewLine' + ): + with self.subTest(text=text): + message = message_from_string( + "Header: Value\r\n\r\nBody", + policy=self.policy, + ) + + del message['Header'] + message['Header'] = LiteralHeader(text) + + with self.assertRaises(email.errors.HeaderWriteError): + message.as_string() + class TestBytesGenerator(TestGeneratorBase, TestEmailBase): diff --git a/Lib/test/test_email/test_policy.py b/Lib/test/test_email/test_policy.py index e87c2755494..ff1ddf7d7a8 100644 --- a/Lib/test/test_email/test_policy.py +++ b/Lib/test/test_email/test_policy.py @@ -26,6 +26,7 @@ class PolicyAPITests(unittest.TestCase): 'raise_on_defect': False, 'mangle_from_': True, 'message_factory': None, + 'verify_generated_headers': True, } # These default values are the ones set on email.policy.default. # If any of these defaults change, the docs must be updated. @@ -277,6 +278,31 @@ def test_short_maxlen_error(self): with self.assertRaises(email.errors.HeaderParseError): policy.fold("Subject", subject) + def test_verify_generated_headers(self): + """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', + 'Header: NoNewLine' + ): + with self.subTest(text=text): + message = email.message_from_string( + "Header: Value\r\n\r\nBody", + policy=policy, + ) + class LiteralHeader(str): + name = 'Header' + def fold(self, **kwargs): + return self + + del message['Header'] + message['Header'] = LiteralHeader(text) + + self.assertEqual( + message.as_string(), + f"{text}\nBody", + ) + # XXX: Need subclassing tests. # For adding subclassed objects, make sure the usual rules apply (subclass # wins), but that the order still works (right overrides left). -- 2.30.2