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gh-144125: email: verify headers are sound in BytesGenerator #144126

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gh-144125: email: verify headers are sound in BytesGenerator #144126

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1132e45
gh-144125: email: verify headers are sound in BytesGenerator
beledouxdenis Oct 27, 2025
4a5845c
Fix ref in NEWS
sethmlarson Jan 21, 2026
697163b
Convert test docstrings to comments
sethmlarson Jan 22, 2026
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12 changes: 11 additions & 1 deletion Lib/email/generator.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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:
Expand Down Expand Up @@ -429,7 +430,16 @@ 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}')
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)

Expand Down
4 changes: 3 additions & 1 deletion Lib/test/test_email/test_generator.py
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Original file line number Diff line number Diff line change
Expand Up @@ -313,7 +313,7 @@ def test_flatten_unicode_linesep(self):
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):
Expand All @@ -334,6 +334,8 @@ 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):
Expand Down
6 changes: 5 additions & 1 deletion Lib/test/test_email/test_policy.py
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Original file line number Diff line number Diff line change
Expand Up @@ -296,7 +296,7 @@ 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',
Expand All @@ -319,6 +319,10 @@ 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
Expand Down
4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst
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@@ -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`).
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