Jan 23, 2026 · Jan 26, 2026
diff --git a/Lib/email/generator.py b/Lib/email/generator.py
 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:
        # 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)

diff --git a/Lib/test/test_email/test_generator.py b/Lib/test/test_email/test_generator.py
 self.assertEqual(s.getvalue(),self.typ(expected))

 deftest_verify_generated_headers(self):
 """gh-121650: by default the generator prevents header injection"""
 #gh-121650: by default the generator prevents header injection
 classLiteralHeader(str):
 name='Header'
 deffold(self,**kwargs):

 withself.assertRaises(email.errors.HeaderWriteError):
 message.as_string()
 withself.assertRaises(email.errors.HeaderWriteError):
 message.as_bytes()


 classTestBytesGenerator(TestGeneratorBase,TestEmailBase):
diff --git a/Lib/test/test_email/test_policy.py b/Lib/test/test_email/test_policy.py
 policy.fold("Subject",subject)

 deftest_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)
 fortextin (
 'Header: Value\r\nBad: Injection\r\n',
 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
diff --git a/Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst b/Misc/NEWS.d/next/Security/2026-01-21-12-34-05.gh-issue-144125.TAz5uo.rst
 :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`).
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 DownExpand 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
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))

		deftest_verify_generated_headers(self):
		"""gh-121650: by default the generator prevents header injection"""
		#gh-121650: by default the generator prevents header injection
		classLiteralHeader(str):
		name='Header'
		deffold(self,**kwargs):
Expand All		@@ -334,6 +334,8 @@ def fold(self, **kwargs):

		withself.assertRaises(email.errors.HeaderWriteError):
		message.as_string()
		withself.assertRaises(email.errors.HeaderWriteError):
		message.as_bytes()


		classTestBytesGenerator(TestGeneratorBase,TestEmailBase):
Expand Down
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)

		deftest_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)
		fortextin (
		'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
Original file line number	Diff line number	Diff line change
		@@ -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`).