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Prevent 256-bytes compression windows, unsupported by zlib. #1768

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Merged
aaugustin merged 1 commit into from
Sep 19, 2026
+61 −10
Merged

Prevent 256-bytes compression windows, unsupported by zlib. #1768

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2 changes: 2 additions & 0 deletions docs/project/changelog.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,8 @@ Improvements
* :func:`~asyncio.client.connect` now closes connections with close code 1011
(internal error) when exiting the context manager with an exception.

* Improved error handling when a compression window of size 8 is requested.

Bug fixes
.........

Expand Down
30 changes: 22 additions & 8 deletions src/websockets/extensions/permessage_deflate.py
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Original file line number Diff line number Diff line change
Expand Up @@ -56,7 +56,10 @@ def __init__(
assert remote_no_context_takeover in [False, True]
assert local_no_context_takeover in [False, True]
assert 8 <= remote_max_window_bits <= 15
assert 8 <= local_max_window_bits <= 15
# Due to https://github.com/madler/zlib/issues/171, zlib.compressobj
# rejects wbits=-8 with ValueError: Invalid initialization option.
# This makes it impossible to support local_max_window_bits = 8.
assert 9 <= local_max_window_bits <= 15
assert "wbits" not in compress_settings

self.remote_no_context_takeover = remote_no_context_takeover
Expand Down Expand Up @@ -314,11 +317,13 @@ class ClientPerMessageDeflateFactory(ClientExtensionFactory):
server_max_window_bits: Maximum size of the server's LZ77 sliding window
in bits, between 8 and 15.
client_max_window_bits: Maximum size of the client's LZ77 sliding window
in bits, between 8 and 15, or :obj:`True` to indicate support without
setting a limit.
in bits, between 9 and 15, or :obj:`True` to indicate support without
setting a limit. 8 isn't supported due to a `limitation of zlib`_.
compress_settings: Additional keyword arguments for :func:`zlib.compressobj`,
excluding ``wbits``.

.. _limitation of zlib: https://github.com/madler/zlib/issues/171

"""

name = ExtensionName("permessage-deflate")
Expand All @@ -340,9 +345,9 @@ def __init__(
if not (
client_max_window_bits is None
or client_max_window_bits is True
or 8 <= client_max_window_bits <= 15
or 9 <= client_max_window_bits <= 15
):
raise ValueError("client_max_window_bits must be between 8 and 15")
raise ValueError("client_max_window_bits must be between 9 and 15")
if compress_settings is not None and "wbits" in compress_settings:
raise ValueError(
"compress_settings must not include wbits, "
Expand Down Expand Up @@ -464,6 +469,9 @@ def process_response_params(
elif client_max_window_bits > self.client_max_window_bits:
raise NegotiationError("unsupported client_max_window_bits")

if client_max_window_bits == 8:
raise NegotiationError("unsupported client_max_window_bits = 8, use 9")

return PerMessageDeflate(
server_no_context_takeover, # remote_no_context_takeover
client_no_context_takeover, # local_no_context_takeover
Expand Down Expand Up @@ -512,7 +520,8 @@ class ServerPerMessageDeflateFactory(ServerExtensionFactory):
server_no_context_takeover: Prevent server from using context takeover.
client_no_context_takeover: Prevent client from using context takeover.
server_max_window_bits: Maximum size of the server's LZ77 sliding window
in bits, between 8 and 15.
in bits, between 9 and 15. 8 isn't supported due to a `limitation of
zlib`_.
client_max_window_bits: Maximum size of the client's LZ77 sliding window
in bits, between 8 and 15.
compress_settings: Additional keyword arguments for :func:`zlib.compressobj`,
Expand All @@ -522,6 +531,8 @@ class ServerPerMessageDeflateFactory(ServerExtensionFactory):
the default behavior is to enable compression without enforcing
``client_max_window_bits``.

.. _limitation of zlib: https://github.com/madler/zlib/issues/171

"""

name = ExtensionName("permessage-deflate")
Expand All @@ -539,8 +550,8 @@ def __init__(
Configure the Per-Message Deflate extension factory.

"""
if not (server_max_window_bits is None or 8 <= server_max_window_bits <= 15):
raise ValueError("server_max_window_bits must be between 8 and 15")
if not (server_max_window_bits is None or 9 <= server_max_window_bits <= 15):
raise ValueError("server_max_window_bits must be between 9 and 15")
if not (client_max_window_bits is None or 8 <= client_max_window_bits <= 15):
raise ValueError("client_max_window_bits must be between 8 and 15")
if compress_settings is not None and "wbits" in compress_settings:
Expand Down Expand Up @@ -633,6 +644,9 @@ def process_request_params(
elif server_max_window_bits > self.server_max_window_bits:
server_max_window_bits = self.server_max_window_bits

if server_max_window_bits == 8:
raise NegotiationError("unsupported server_max_window_bits = 8, use 9")

# client_max_window_bits

# Config Req. Resp.
Expand Down
39 changes: 37 additions & 2 deletions tests/extensions/test_permessage_deflate.py
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Original file line number Diff line number Diff line change
Expand Up @@ -324,7 +324,7 @@ def test_init(self):
for config in [
(False, False, 8, None), # server_max_window_bits ≥ 8
(False, True, 15, None), # server_max_window_bits ≤ 15
(True, False, None, 8), # client_max_window_bits ≥ 8
(True, False, None, 9), # client_max_window_bits ≥ 9
(True, True, None, 15), # client_max_window_bits ≤ 15
(False, False, None, True), # client_max_window_bits
(False, False, None, None, {"memLevel": 4}),
Expand All @@ -337,6 +337,7 @@ def test_init_error(self):
for config in [
(False, False, 7, 8), # server_max_window_bits < 8
(False, True, 8, 7), # client_max_window_bits < 8
(False, True, 8, 8), # client_max_window_bits = 8
(True, False, 16, 15), # server_max_window_bits > 15
(True, True, 15, 16), # client_max_window_bits > 15
(False, False, True, None), # server_max_window_bits
Expand Down Expand Up @@ -463,11 +464,21 @@ def test_process_response_params(self):
[("server_max_window_bits", "7")],
NegotiationError,
),
(
(False, False, None, None),
[("server_max_window_bits", "8")],
(False, False, 8, 15),
),
(
(False, False, None, None),
[("server_max_window_bits", "10")],
(False, False, 10, 15),
),
(
(False, False, None, None),
[("server_max_window_bits", "15")],
(False, False, 15, 15),
),
(
(False, False, None, None),
[("server_max_window_bits", "16")],
Expand Down Expand Up @@ -519,6 +530,11 @@ def test_process_response_params(self):
[("client_max_window_bits", "7")],
NegotiationError,
),
(
(False, False, None, True),
[("client_max_window_bits", "8")],
NegotiationError,
),
Comment thread
aaugustin marked this conversation as resolved.
(
(False, False, None, True),
[("client_max_window_bits", "10")],
Expand Down Expand Up @@ -662,7 +678,7 @@ def test_name(self):

def test_init(self):
for config in [
(False, False, 8, None), # server_max_window_bits ≥ 8
(False, False, 9, None), # server_max_window_bits ≥ 9
(False, True, 15, None), # server_max_window_bits ≤ 15
(True, False, None, 8), # client_max_window_bits ≥ 8
(True, True, None, 15), # client_max_window_bits ≤ 15
Expand All @@ -677,6 +693,7 @@ def test_init_error(self):
for config in [
(False, False, 7, 8), # server_max_window_bits < 8
(False, True, 8, 7), # client_max_window_bits < 8
(False, False, 8, 8), # server_max_window_bits = 8
(True, False, 16, 15), # server_max_window_bits > 15
(True, True, 15, 16), # client_max_window_bits > 15
(False, False, None, True), # client_max_window_bits
Expand Down Expand Up @@ -774,6 +791,12 @@ def test_process_request_params(self):
None,
NegotiationError,
),
(
(False, False, None, None),
[("server_max_window_bits", "8")],
None,
NegotiationError,
),
Comment thread
aaugustin marked this conversation as resolved.
(
(False, False, None, None),
[("server_max_window_bits", "10")],
Expand Down Expand Up @@ -835,12 +858,24 @@ def test_process_request_params(self):
None,
InvalidParameterValue,
),
(
(False, False, None, None),
[("client_max_window_bits", "8")],
[("client_max_window_bits", "8")], # doesn't matter
(False, False, 8, 15),
),
(
(False, False, None, None),
[("client_max_window_bits", "10")],
[("client_max_window_bits", "10")], # doesn't matter
(False, False, 10, 15),
),
(
(False, False, None, None),
[("client_max_window_bits", "15")],
[("client_max_window_bits", "15")], # doesn't matter
(False, False, 15, 15),
),
(
(False, False, None, None),
[("client_max_window_bits", "16")],
Expand Down
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