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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Petr Viktorin <encukou@gmail.com>
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Date: Wed, 31 Jul 2024 00:19:48 +0200
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Subject: [PATCH] 00435: gh-121650: Encode newlines in headers, and verify
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headers are sound (GH-122233)
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Per RFC 2047:
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> [...] these encoding schemes allow the
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> encoding of arbitrary octet values, mail readers that implement this
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> decoding should also ensure that display of the decoded data on the
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> recipient's terminal will not cause unwanted side-effects
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It seems that the "quoted-word" scheme is a valid way to include
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a newline character in a header value, just like we already allow
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undecodable bytes or control characters.
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They do need to be properly quoted when serialized to text, though.
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This should fail for custom fold() implementations that aren't careful
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about newlines.
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(cherry picked from commit 097633981879b3c9de9a1dd120d3aa585ecc2384)
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Co-authored-by: Petr Viktorin <encukou@gmail.com>
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Co-authored-by: Bas Bloemsaat <bas@bloemsaat.org>
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Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
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---
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Doc/library/email.errors.rst | 6 ++
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Doc/library/email.policy.rst | 18 ++++++
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Doc/whatsnew/3.9.rst | 12 ++++
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Lib/email/_header_value_parser.py | 12 +++-
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Lib/email/_policybase.py | 8 +++
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Lib/email/errors.py | 4 ++
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Lib/email/generator.py | 13 +++-
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Lib/test/test_email/test_generator.py | 62 +++++++++++++++++++
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Lib/test/test_email/test_policy.py | 26 ++++++++
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...-07-27-16-10-41.gh-issue-121650.nf6oc9.rst | 5 ++
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10 files changed, 162 insertions(+), 4 deletions(-)
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create mode 100644 Misc/NEWS.d/next/Library/2024-07-27-16-10-41.gh-issue-121650.nf6oc9.rst
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diff --git a/Doc/library/email.errors.rst b/Doc/library/email.errors.rst
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index f4b9f52509..878c09bb04 100644
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--- a/Doc/library/email.errors.rst
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+++ b/Doc/library/email.errors.rst
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@@ -59,6 +59,12 @@ The following exception classes are defined in the :mod:`email.errors` module:
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:class:`~email.mime.image.MIMEImage`).
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+.. exception:: HeaderWriteError()
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+
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+ Raised when an error occurs when the :mod:`~email.generator` outputs
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+ headers.
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+
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+
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Here is the list of the defects that the :class:`~email.parser.FeedParser`
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can find while parsing messages. Note that the defects are added to the message
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where the problem was found, so for example, if a message nested inside a
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diff --git a/Doc/library/email.policy.rst b/Doc/library/email.policy.rst
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index bf53b9520f..57a75ce452 100644
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--- a/Doc/library/email.policy.rst
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+++ b/Doc/library/email.policy.rst
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@@ -229,6 +229,24 @@ added matters. To illustrate::
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.. versionadded:: 3.6
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+
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+ .. attribute:: verify_generated_headers
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+
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+ If ``True`` (the default), the generator will raise
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+ :exc:`~email.errors.HeaderWriteError` instead of writing a header
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+ that is improperly folded or delimited, such that it would
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+ be parsed as multiple headers or joined with adjacent data.
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+ Such headers can be generated by custom header classes or bugs
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+ in the ``email`` module.
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+
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+ As it's a security feature, this defaults to ``True`` even in the
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+ :class:`~email.policy.Compat32` policy.
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+ For backwards compatible, but unsafe, behavior, it must be set to
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+ ``False`` explicitly.
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+
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+ .. versionadded:: 3.9.20
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+
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+
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The following :class:`Policy` method is intended to be called by code using
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the email library to create policy instances with custom settings:
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diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
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index 1756a37338..eeda4e6955 100644
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--- a/Doc/whatsnew/3.9.rst
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+++ b/Doc/whatsnew/3.9.rst
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@@ -1625,3 +1625,15 @@ ipaddress
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* Fixed ``is_global`` and ``is_private`` behavior in ``IPv4Address``,
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``IPv6Address``, ``IPv4Network`` and ``IPv6Network``.
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+
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+email
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+-----
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+
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+* Headers with embedded newlines are now quoted on output.
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+
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+ The :mod:`~email.generator` will now refuse to serialize (write) headers
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+ that are improperly folded or delimited, such that they would be parsed as
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+ multiple headers or joined with adjacent data.
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+ If you need to turn this safety feature off,
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+ set :attr:`~email.policy.Policy.verify_generated_headers`.
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+ (Contributed by Bas Bloemsaat and Petr Viktorin in :gh:`121650`.)
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diff --git a/Lib/email/_header_value_parser.py b/Lib/email/_header_value_parser.py
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index 8a8fb8bc42..e394cfd2e1 100644
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--- a/Lib/email/_header_value_parser.py
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+++ b/Lib/email/_header_value_parser.py
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@@ -92,6 +92,8 @@ TOKEN_ENDS = TSPECIALS | WSP
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ASPECIALS = TSPECIALS | set("*'%")
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ATTRIBUTE_ENDS = ASPECIALS | WSP
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EXTENDED_ATTRIBUTE_ENDS = ATTRIBUTE_ENDS - set('%')
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+NLSET = {'\n', '\r'}
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+SPECIALSNL = SPECIALS | NLSET
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def quote_string(value):
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return '"'+str(value).replace('\\', '\\\\').replace('"', r'\"')+'"'
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@@ -2778,9 +2780,13 @@ def _refold_parse_tree(parse_tree, *, policy):
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wrap_as_ew_blocked -= 1
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continue
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tstr = str(part)
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- if part.token_type == 'ptext' and set(tstr) & SPECIALS:
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- # Encode if tstr contains special characters.
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- want_encoding = True
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+ if not want_encoding:
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+ if part.token_type == 'ptext':
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+ # Encode if tstr contains special characters.
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+ want_encoding = not SPECIALSNL.isdisjoint(tstr)
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+ else:
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+ # Encode if tstr contains newlines.
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+ want_encoding = not NLSET.isdisjoint(tstr)
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try:
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tstr.encode(encoding)
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charset = encoding
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diff --git a/Lib/email/_policybase.py b/Lib/email/_policybase.py
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index c9cbadd2a8..d1f48211f9 100644
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--- a/Lib/email/_policybase.py
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+++ b/Lib/email/_policybase.py
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@@ -157,6 +157,13 @@ class Policy(_PolicyBase, metaclass=abc.ABCMeta):
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message_factory -- the class to use to create new message objects.
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If the value is None, the default is Message.
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+ verify_generated_headers
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+ -- if true, the generator verifies that each header
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+ they are properly folded, so that a parser won't
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+ treat it as multiple headers, start-of-body, or
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+ part of another header.
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+ This is a check against custom Header & fold()
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+ implementations.
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"""
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raise_on_defect = False
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@@ -165,6 +172,7 @@ class Policy(_PolicyBase, metaclass=abc.ABCMeta):
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max_line_length = 78
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mangle_from_ = False
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message_factory = None
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+ verify_generated_headers = True
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def handle_defect(self, obj, defect):
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"""Based on policy, either raise defect or call register_defect.
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diff --git a/Lib/email/errors.py b/Lib/email/errors.py
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index d28a680010..1a0d5c63e6 100644
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--- a/Lib/email/errors.py
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+++ b/Lib/email/errors.py
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@@ -29,6 +29,10 @@ class CharsetError(MessageError):
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"""An illegal charset was given."""
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+class HeaderWriteError(MessageError):
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+ """Error while writing headers."""
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+
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+
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# These are parsing defects which the parser was able to work around.
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class MessageDefect(ValueError):
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"""Base class for a message defect."""
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diff --git a/Lib/email/generator.py b/Lib/email/generator.py
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index c9b121624e..89224ae41c 100644
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--- a/Lib/email/generator.py
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+++ b/Lib/email/generator.py
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@@ -14,12 +14,14 @@ import random
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from copy import deepcopy
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from io import StringIO, BytesIO
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from email.utils import _has_surrogates
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+from email.errors import HeaderWriteError
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UNDERSCORE = '_'
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NL = '\n' # XXX: no longer used by the code below.
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NLCRE = re.compile(r'\r\n|\r|\n')
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fcre = re.compile(r'^From ', re.MULTILINE)
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+NEWLINE_WITHOUT_FWSP = re.compile(r'\r\n[^ \t]|\r[^ \n\t]|\n[^ \t]')
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@@ -223,7 +225,16 @@ class Generator:
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def _write_headers(self, msg):
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for h, v in msg.raw_items():
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- self.write(self.policy.fold(h, v))
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+ folded = self.policy.fold(h, v)
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+ if self.policy.verify_generated_headers:
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+ linesep = self.policy.linesep
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+ if not folded.endswith(self.policy.linesep):
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+ raise HeaderWriteError(
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+ f'folded header does not end with {linesep!r}: {folded!r}')
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+ if NEWLINE_WITHOUT_FWSP.search(folded.removesuffix(linesep)):
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+ raise HeaderWriteError(
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+ f'folded header contains newline: {folded!r}')
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+ self.write(folded)
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# A blank line always separates headers from body
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self.write(self._NL)
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diff --git a/Lib/test/test_email/test_generator.py b/Lib/test/test_email/test_generator.py
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index 89e7edeb63..d29400f0ed 100644
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--- a/Lib/test/test_email/test_generator.py
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+++ b/Lib/test/test_email/test_generator.py
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@@ -6,6 +6,7 @@ from email.message import EmailMessage
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from email.generator import Generator, BytesGenerator
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from email.headerregistry import Address
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from email import policy
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+import email.errors
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from test.test_email import TestEmailBase, parameterize
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@@ -216,6 +217,44 @@ class TestGeneratorBase:
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g.flatten(msg)
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self.assertEqual(s.getvalue(), self.typ(expected))
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+ def test_keep_encoded_newlines(self):
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+ msg = self.msgmaker(self.typ(textwrap.dedent("""\
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+ To: nobody
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+ Subject: Bad subject=?UTF-8?Q?=0A?=Bcc: injection@example.com
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+
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+ None
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+ """)))
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+ expected = textwrap.dedent("""\
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+ To: nobody
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+ Subject: Bad subject=?UTF-8?Q?=0A?=Bcc: injection@example.com
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+
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+ None
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+ """)
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+ s = self.ioclass()
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+ g = self.genclass(s, policy=self.policy.clone(max_line_length=80))
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+ g.flatten(msg)
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+ self.assertEqual(s.getvalue(), self.typ(expected))
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+
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+ def test_keep_long_encoded_newlines(self):
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+ msg = self.msgmaker(self.typ(textwrap.dedent("""\
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+ To: nobody
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+ Subject: Bad subject=?UTF-8?Q?=0A?=Bcc: injection@example.com
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+
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+ None
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+ """)))
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+ expected = textwrap.dedent("""\
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+ To: nobody
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+ Subject: Bad subject
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+ =?utf-8?q?=0A?=Bcc:
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+ injection@example.com
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+
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+ None
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+ """)
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+ s = self.ioclass()
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+ g = self.genclass(s, policy=self.policy.clone(max_line_length=30))
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+ g.flatten(msg)
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+ self.assertEqual(s.getvalue(), self.typ(expected))
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+
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class TestGenerator(TestGeneratorBase, TestEmailBase):
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@@ -224,6 +263,29 @@ class TestGenerator(TestGeneratorBase, TestEmailBase):
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ioclass = io.StringIO
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typ = str
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+ def test_verify_generated_headers(self):
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+ """gh-121650: by default the generator prevents header injection"""
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+ class LiteralHeader(str):
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+ name = 'Header'
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+ def fold(self, **kwargs):
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+ return self
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+
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+ for text in (
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+ 'Value\r\nBad Injection\r\n',
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+ 'NoNewLine'
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+ ):
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+ with self.subTest(text=text):
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+ message = message_from_string(
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+ "Header: Value\r\n\r\nBody",
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+ policy=self.policy,
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+ )
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+
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+ del message['Header']
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+ message['Header'] = LiteralHeader(text)
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+
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+ with self.assertRaises(email.errors.HeaderWriteError):
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+ message.as_string()
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+
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class TestBytesGenerator(TestGeneratorBase, TestEmailBase):
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diff --git a/Lib/test/test_email/test_policy.py b/Lib/test/test_email/test_policy.py
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index e87c275549..ff1ddf7d7a 100644
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--- a/Lib/test/test_email/test_policy.py
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+++ b/Lib/test/test_email/test_policy.py
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@@ -26,6 +26,7 @@ class PolicyAPITests(unittest.TestCase):
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'raise_on_defect': False,
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'mangle_from_': True,
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'message_factory': None,
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+ 'verify_generated_headers': True,
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}
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# These default values are the ones set on email.policy.default.
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# If any of these defaults change, the docs must be updated.
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@@ -277,6 +278,31 @@ class PolicyAPITests(unittest.TestCase):
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with self.assertRaises(email.errors.HeaderParseError):
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policy.fold("Subject", subject)
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+ def test_verify_generated_headers(self):
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+ """Turning protection off allows header injection"""
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+ policy = email.policy.default.clone(verify_generated_headers=False)
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+ for text in (
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+ 'Header: Value\r\nBad: Injection\r\n',
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+ 'Header: NoNewLine'
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+ ):
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+ with self.subTest(text=text):
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+ message = email.message_from_string(
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+ "Header: Value\r\n\r\nBody",
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+ policy=policy,
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+ )
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+ class LiteralHeader(str):
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+ name = 'Header'
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+ def fold(self, **kwargs):
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+ return self
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+
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+ del message['Header']
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+ message['Header'] = LiteralHeader(text)
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+
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+ self.assertEqual(
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+ message.as_string(),
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+ f"{text}\nBody",
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+ )
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+
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# XXX: Need subclassing tests.
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# For adding subclassed objects, make sure the usual rules apply (subclass
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# wins), but that the order still works (right overrides left).
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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
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new file mode 100644
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index 0000000000..83dd28d4ac
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--- /dev/null
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+++ b/Misc/NEWS.d/next/Library/2024-07-27-16-10-41.gh-issue-121650.nf6oc9.rst
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@@ -0,0 +1,5 @@
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+:mod:`email` headers with embedded newlines are now quoted on output. The
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+:mod:`~email.generator` will now refuse to serialize (write) headers that
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+are unsafely folded or delimited; see
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+:attr:`~email.policy.Policy.verify_generated_headers`. (Contributed by Bas
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+Bloemsaat and Petr Viktorin in :gh:`121650`.)
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