reject empty header names

fixes #1257

src/h2/utilities.py

@@ -206,6 +206,9 @@ def validate_headers(headers, hdr_validation_flags):
     # For example, we avoid tuple unpacking in loops because it represents a
     # fixed cost that we don't want to spend, instead indexing into the header
     # tuples.
+    headers = _reject_empty_header_names(
+        headers, hdr_validation_flags
+    )
     headers = _reject_uppercase_header_fields(
         headers, hdr_validation_flags
     )
@@ -229,6 +232,19 @@ def validate_headers(headers, hdr_validation_flags):
     return headers
 
 
+def _reject_empty_header_names(headers, hdr_validation_flags):
+    """
+    Raises a ProtocolError if any header names are empty (length 0).
+    While hpack decodes such headers without errors, they are semantically
+    forbidden in HTTP, see RFC 7230, stating that they must be at least one
+    character long.
+    """
+    for header in headers:
+        if len(header[0]) == 0:
+            raise ProtocolError("Received header name with zero length.")
+        yield header
+
+
 def _reject_uppercase_header_fields(headers, hdr_validation_flags):
     """
     Raises a ProtocolError if any uppercase character is found in a header

test/test_invalid_headers.py

@@ -688,6 +688,24 @@ def test_inbound_resp_header_extra_pseudo_headers(self,
         with pytest.raises(h2.exceptions.ProtocolError):
             list(h2.utilities.validate_headers(headers, hdr_validation_flags))
 
+    @pytest.mark.parametrize('hdr_validation_flags', hdr_validation_combos)
+    def test_inbound_header_name_length(self, hdr_validation_flags):
+        with pytest.raises(h2.exceptions.ProtocolError):
+            list(h2.utilities.validate_headers([(b'', b'foobar')], hdr_validation_flags))
+
+    def test_inbound_header_name_length_full_frame_decode(self, frame_factory):
+        f = frame_factory.build_headers_frame([])
+        f.data = b"\x00\x00\x05\x00\x00\x00\x00\x04"
+        data = f.serialize()
+
+        c = h2.connection.H2Connection(config=h2.config.H2Configuration(client_side=False))
+        c.initiate_connection()
+        c.receive_data(frame_factory.preamble())
+        c.clear_outbound_data_buffer()
+
+        with pytest.raises(h2.exceptions.ProtocolError, match="Received header name with zero length."):
+            c.receive_data(data)
+
 
 class TestOversizedHeaders(object):
     """