Validate chunk size in Chunked Encoding are HEXDIG

RFC7230 states that a chunk-size should be 1*HEXDIG, this is now
validated before passing the resulting string to int() which would also
parse other formats for hex, such as: `0x01` as `1` and `+0x01` as `1`.
This would lead to a potential for a frontend proxy server and waitress
to disagree on where a chunk started and ended, thereby potentially
leading to request smuggling.

With the increased validation if the size is not just hex digits,
Waitress now returns a Bad Request and stops processing the request.

src/waitress/receiver.py

@@ -150,12 +150,21 @@ def received(self, s):
                                 self.all_chunks_received = True
 
                                 break
+
                             line = line[:semi]
-                        try:
-                            sz = int(line.strip(), 16)  # hexadecimal
-                        except ValueError:  # garbage in input
-                            self.error = BadRequest("garbage in chunked encoding input")
-                            sz = 0
+
+                        # Remove any whitespace
+                        line = line.strip()
+
+                        if not ONLY_HEXDIG_RE.match(line):
+                            self.error = BadRequest("Invalid chunk size")
+                            self.all_chunks_received = True
+
+                            break
+
+                        # Can not fail due to matching against the regular
+                        # expression above
+                        sz = int(line.strip(), 16)  # hexadecimal
 
                         if sz > 0:
                             # Start a new chunk.

tests/test_functional.py

@@ -364,6 +364,28 @@ def test_broken_chunked_encoding(self):
             self.send_check_error(to_send)
             self.assertRaises(ConnectionClosed, read_http, fp)
 
+    def test_broken_chunked_encoding_invalid_hex(self):
+        control_line = b"0x20\r\n"  # 20 hex = 32 dec
+        s = b"This string has 32 characters.\r\n"
+        to_send = b"GET / HTTP/1.1\r\nTransfer-Encoding: chunked\r\n\r\n"
+        to_send += control_line + s + b"\r\n"
+        self.connect()
+        self.sock.send(to_send)
+        with self.sock.makefile("rb", 0) as fp:
+            line, headers, response_body = read_http(fp)
+            self.assertline(line, "400", "Bad Request", "HTTP/1.1")
+            cl = int(headers["content-length"])
+            self.assertEqual(cl, len(response_body))
+            self.assertIn(b"Invalid chunk size", response_body)
+            self.assertEqual(
+                sorted(headers.keys()),
+                ["connection", "content-length", "content-type", "date", "server"],
+            )
+            self.assertEqual(headers["content-type"], "text/plain")
+            # connection has been closed
+            self.send_check_error(to_send)
+            self.assertRaises(ConnectionClosed, read_http, fp)
+
     def test_broken_chunked_encoding_invalid_extension(self):
         control_line = b"20;invalid=\r\n"  # 20 hex = 32 dec
         s = b"This string has 32 characters.\r\n"

tests/test_receiver.py

@@ -262,6 +262,18 @@ def test_received_valid_extensions(self, valid_extension):
         assert result == len(data)
         assert inst.error == None
 
+    @pytest.mark.parametrize("invalid_size", [b"0x04", b"+0x04", b"x04", b"+04"])
+    def test_received_invalid_size(self, invalid_size):
+        from waitress.utilities import BadRequest
+
+        buf = DummyBuffer()
+        inst = self._makeOne(buf)
+        data = invalid_size + b"\r\ntest\r\n"
+        result = inst.received(data)
+        assert result == len(data)
+        assert inst.error.__class__ == BadRequest
+        assert inst.error.body == "Invalid chunk size"
+
 
 class DummyBuffer:
     def __init__(self, data=None):