gh-121650: Encode newlines in headers, and verify headers are sound (GH-122233)gentoo-3.10.9_p1

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.

This should fail for custom fold() implementations that aren't careful
about newlines.

(cherry picked from commit 097633981879b3c9de9a1dd120d3aa585ecc2384)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Bas Bloemsaat <bas@bloemsaat.org>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Closes: https://github.com/python/cpython/pull/122609

9 files changed, 150 insertions, 4 deletions

diff --git a/Doc/library/email.errors.rst b/Doc/library/email.errors.rst
index 194a98696f4..f737f0282c5 100644
--- a/Doc/library/email.errors.rst
+++ b/Doc/library/email.errors.rst
@@ -59,6 +59,12 @@ The following exception classes are defined in the :mod:`email.errors` module:
    :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
 where the problem was found, so for example, if a message nested inside a
diff --git a/Doc/library/email.policy.rst b/Doc/library/email.policy.rst
index bf53b9520fc..eba43b5169d 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:: 3.10.15
+
+
    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 @@ TOKEN_ENDS = TSPECIALS | WSP
 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 @@ import random
 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 @@ class Generator:
 
     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.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
 
 
@@ -216,6 +217,44 @@ class TestGeneratorBase:
         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 @@ class PolicyAPITests(unittest.TestCase):
                 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).
diff --git a/Misc/NEWS.d/next/Library/2024-07-27-16-10-41.gh-issue-121650.nf6oc9.rst b/Misc/NEWS.d/next/Library/2024-07-27-16-10-41.gh-issue-121650.nf6oc9.rst
new file mode 100644
index 00000000000..83dd28d4ac5
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-07-27-16-10-41.gh-issue-121650.nf6oc9.rst
@@ -0,0 +1,5 @@
+:mod:`email` headers with embedded newlines are now quoted on output. The
+:mod:`~email.generator` 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`.)