Reject empty header names on the outbound path

The inbound pipeline runs _reject_empty_header_names before
_reject_pseudo_header_fields, so an empty name is caught before the
`header[0][0]` lookup. The outbound pipeline has no such guard, so sending
a header block with an empty name raises IndexError from utilities.py:335
instead of ProtocolError.

Add the guard to validate_outbound_headers in the same position. The
message differs by direction, so the body is shared by
_validate_nonempty_header_names, mirroring how _check_host_authority_header
and _check_sent_host_authority_header already share
_validate_host_authority_header.

Author: avalyset

src/h2/utilities.py

@@ -265,21 +265,36 @@ def _reject_illegal_characters(headers: Iterable[Header],
         yield header
 
 
-def _reject_empty_header_names(headers: Iterable[Header],
-                               hdr_validation_flags: HeaderValidationFlags) -> Generator[Header, None, None]:
+def _validate_nonempty_header_names(headers: Iterable[Header], msg: str) -> Generator[Header, None, None]:
     """
-    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.
+    Raises a ProtocolError with the given message if any header name is 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:
-            msg = "Received header name with zero length."
             raise ProtocolError(msg)
         yield header
 
 
+def _reject_empty_header_names(headers: Iterable[Header],
+                               hdr_validation_flags: HeaderValidationFlags) -> Generator[Header, None, None]:
+    """
+    Raises a ProtocolError if a header block arrives with an empty header name.
+    """
+    return _validate_nonempty_header_names(headers, "Received header name with zero length.")
+
+
+def _reject_sent_empty_header_names(headers: Iterable[Header],
+                                    hdr_validation_flags: HeaderValidationFlags) -> Generator[Header, None, None]:
+    """
+    Raises a ProtocolError if we try to send a header block with an empty
+    header name.
+    """
+    return _validate_nonempty_header_names(headers, "Sent header name with zero length.")
+
+
 def _reject_te(headers: Iterable[Header], hdr_validation_flags: HeaderValidationFlags) -> Generator[Header, None, None]:
     """
     Raises a ProtocolError if the TE header is present in a header block and
@@ -690,6 +705,9 @@ def validate_outbound_headers(headers: Iterable[Header],
     :param headers: The HTTP header set.
     :param hdr_validation_flags: An instance of HeaderValidationFlags.
     """
+    headers = _reject_sent_empty_header_names(
+        headers, hdr_validation_flags,
+    )
     headers = _reject_te(
         headers, hdr_validation_flags,
     )

tests/test_invalid_headers.py

@@ -672,6 +672,37 @@ def test_inbound_header_name_length(self, hdr_validation_flags) -> None:
         with pytest.raises(h2.exceptions.ProtocolError):
             list(h2.utilities.validate_headers([(b"", b"foobar")], hdr_validation_flags))
 
+    @pytest.mark.parametrize("hdr_validation_flags", hdr_validation_combos)
+    def test_outbound_header_name_length(self, hdr_validation_flags) -> None:
+        # An empty outbound header name must raise ProtocolError, not IndexError
+        # from the `header[0][0]` lookup in _reject_pseudo_header_fields.
+        with pytest.raises(h2.exceptions.ProtocolError):
+            list(h2.utilities.validate_outbound_headers([(b"", b"foobar")], hdr_validation_flags))
+
+    def test_outbound_header_name_length_send_headers(self, frame_factory) -> None:
+        c = h2.connection.H2Connection()
+        c.initiate_connection()
+        c.clear_outbound_data_buffer()
+
+        headers = [
+            (b":authority", b"example.com"),
+            (b":path", b"/"),
+            (b":scheme", b"https"),
+            (b":method", b"GET"),
+            (b"", b"foobar"),
+        ]
+        with pytest.raises(h2.exceptions.ProtocolError, match=r"Sent header name with zero length\."):
+            c.send_headers(1, headers)
+
+    @pytest.mark.parametrize("hdr_validation_flags", [
+        flags for flags in hdr_validation_combos
+        if flags.is_trailer and not flags.is_response_header
+    ])
+    def test_valid_header_name_accepted_both_directions(self, hdr_validation_flags) -> None:
+        headers = [(b"x-custom-header", b"foobar")]
+        assert list(h2.utilities.validate_headers(list(headers), hdr_validation_flags))
+        assert list(h2.utilities.validate_outbound_headers(list(headers), hdr_validation_flags))
+
     def test_inbound_header_name_length_full_frame_decode(self, frame_factory) -> None:
         f = frame_factory.build_headers_frame([])
         f.data = b"\x00\x00\x01\x04"