Security fix: defend against zip bombs.

docs/changelog.rst

@@ -8,6 +8,14 @@ Changelog
 
 *In development*
 
+.. note::
+
+    **Version 5.0 fixes a security issue introduced in version 4.0.**
+
+    websockets 4.0 was vulnerable to denial of service by memory exhaustion
+    because it didn't enforce ``max_size`` when decompressing compressed
+    messages.
+
 .. warning::
 
     **Version 5.0 adds a** ``user_info`` **field to the return value of**

websockets/extensions/base.py

@@ -72,7 +72,7 @@ class Extension:
     """
     name = ...
 
-    def decode(self, frame):
+    def decode(self, frame, *, max_size=None):
         """
         Decode an incoming frame.
 

websockets/extensions/permessage_deflate.py

@@ -8,7 +8,7 @@
 
 from ..exceptions import (
     DuplicateParameter, InvalidParameterName, InvalidParameterValue,
-    NegotiationError
+    NegotiationError, PayloadTooBig
 )
 from ..framing import CTRL_OPCODES, OP_CONT
 
@@ -463,7 +463,7 @@ def __repr__(self):
                 self.local_max_window_bits),
         ]))
 
-    def decode(self, frame):
+    def decode(self, frame, *, max_size=None):
         """
         Decode an incoming frame.
 
@@ -495,11 +495,18 @@ def decode(self, frame):
                 self.decoder = zlib.decompressobj(
                     wbits=-self.remote_max_window_bits)
 
-        # Uncompress compressed frames.
+        # Uncompress compressed frames. Protect against zip bombs by
+        # preventing zlib from decompressing more than max_length bytes
+        # (except when the limit is disabled with max_size = None).
         data = frame.data
         if frame.fin:
             data += _EMPTY_UNCOMPRESSED_BLOCK
-        data = self.decoder.decompress(data)
+        max_length = 0 if max_size is None else max_size
+        data = self.decoder.decompress(data, max_length)
+        if self.decoder.unconsumed_tail:
+            raise PayloadTooBig(
+                "Uncompressed payload length exceeds size limit (? > {} bytes)"
+                .format(max_size))
 
         # Allow garbage collection of the decoder if it won't be reused.
         if frame.fin and self.remote_no_context_takeover:

websockets/extensions/test_permessage_deflate.py

@@ -3,7 +3,7 @@
 
 from ..exceptions import (
     DuplicateParameter, InvalidParameterName, InvalidParameterValue,
-    NegotiationError
+    NegotiationError, PayloadTooBig
 )
 from ..framing import (
     OP_BINARY, OP_CLOSE, OP_CONT, OP_PING, OP_PONG, OP_TEXT, Frame,
@@ -835,3 +835,15 @@ def test_compress_settings(self):
             rsv1=True,
             data=b'\x00\x05\x00\xfa\xffcaf\xc3\xa9\x00',    # not compressed
         ))
+
+    # Frames aren't decoded beyond max_length.
+
+    def test_decompress_max_size(self):
+        frame = Frame(True, OP_TEXT, ('a' * 20).encode('utf-8'))
+
+        enc_frame = self.extension.encode(frame)
+
+        self.assertEqual(enc_frame.data, b'JL\xc4\x04\x00\x00')
+
+        with self.assertRaises(PayloadTooBig):
+            self.extension.decode(enc_frame, max_size=10)

websockets/framing.py

@@ -119,7 +119,7 @@ def read(cls, reader, *, mask, max_size=None, extensions=None):
             length, = struct.unpack('!Q', data)
         if max_size is not None and length > max_size:
             raise PayloadTooBig(
-                "Payload length exceeds limit: {} > {} bytes"
+                "Payload length exceeds size limit ({} > {} bytes)"
                 .format(length, max_size))
         if mask:
             mask_bits = yield from reader(4)
@@ -134,7 +134,7 @@ def read(cls, reader, *, mask, max_size=None, extensions=None):
         if extensions is None:
             extensions = []
         for extension in reversed(extensions):
-            frame = extension.decode(frame)
+            frame = extension.decode(frame, max_size=max_size)
 
         frame.check()
 

websockets/test_client_server.py

@@ -204,7 +204,7 @@ class NoOpExtension:
     def __repr__(self):
         return 'NoOpExtension()'
 
-    def decode(self, frame):
+    def decode(self, frame, *, max_size=None):
         return frame
 
     def encode(self, frame):

websockets/test_framing.py

@@ -217,7 +217,9 @@ def encode(frame):
                 return frame._replace(data=data)
 
             # This extensions is symmetrical.
-            decode = encode
+            @staticmethod
+            def decode(frame, *, max_size=None):
+                return Rot13.encode(frame)
 
         self.round_trip(
             b'\x81\x05uryyb',