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[3.9] gh-121227: Disallow setting an empty list for NPN (GH-137161)

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Stan Ulbrych 2025-10-07 13:09:33 +01:00 • committed by GitHub
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2
Lib/ssl.py
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@ -520,6 +520,8 @@ def wrap_bio(self, incoming, outgoing, server_side=False,
def set_npn_protocols(self, npn_protocols):
protos = bytearray()
if not npn_protocols:
raise SSLError('NPN protocols must not be empty')
for protocol in npn_protocols:
b = bytes(protocol, 'ascii')
if len(b) == 0 or len(b) > 255:

6
Lib/test/test_ssl.py
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@ -4219,6 +4219,12 @@ def test_npn_protocols(self):
if len(stats['server_npn_protocols']) else 'nothing'
self.assertEqual(server_result, expected, msg % (server_result, "server"))
def test_empty_npn_protocols(self):
"""npn_protocols cannot be empty, see CVE-2024-5642 & gh-121227"""
client_context, server_context, hostname = testing_context()
with self.assertRaises(ssl.SSLError):
server_context.set_npn_protocols([])
def sni_contexts(self):
server_context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
server_context.load_cert_chain(SIGNED_CERTFILE)

2
Misc/NEWS.d/next/Security/2025-07-28-10-35-59.gh-issue-121227.Orp1wf.rst Normal file
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@ -0,0 +1,2 @@
Raise an :exc:`SSL.SSLError` if an empty *protocols* argument is passed to
:meth:`ssl.SSLContext.set_npn_protocols` to fix ``CVE-2024-5642``.