Omit padding when the PADDED flag is unset

CHANGELOG.rst

@@ -19,7 +19,8 @@ dev
 
 **Bugfixes**
 
--
+- Omit configured padding from DATA, HEADERS, and PUSH_PROMISE frames when the
+  PADDED flag is unset.
 
 6.1.0 (2025-01-22)
 ------------------

src/hyperframe/frame.py

@@ -301,7 +301,8 @@ def __init__(self, stream_id: int, data: bytes = b"", **kwargs: Any) -> None:
 
     def serialize_body(self) -> bytes:
         padding_data = self.serialize_padding_data()
-        padding = b"\0" * self.pad_length
+        # 2026-10-11: Padding is payload unless the PADDED flag describes it.
+        padding = b"\0" * self.pad_length if "PADDED" in self.flags else b""
         if isinstance(self.data, memoryview):
             self.data = self.data.tobytes()
         return b"".join([padding_data, self.data, padding])
@@ -514,7 +515,8 @@ def _body_repr(self) -> str:
 
     def serialize_body(self) -> bytes:
         padding_data = self.serialize_padding_data()
-        padding = b"\0" * self.pad_length
+        # 2026-10-11: Padding is payload unless the PADDED flag describes it.
+        padding = b"\0" * self.pad_length if "PADDED" in self.flags else b""
         data = _STRUCT_L.pack(self.promised_stream_id)
         return b"".join([padding_data, data, self.data, padding])
 
@@ -741,7 +743,8 @@ def _body_repr(self) -> str:
 
     def serialize_body(self) -> bytes:
         padding_data = self.serialize_padding_data()
-        padding = b"\0" * self.pad_length
+        # 2026-10-11: Padding is payload unless the PADDED flag describes it.
+        padding = b"\0" * self.pad_length if "PADDED" in self.flags else b""
 
         if "PRIORITY" in self.flags:
             priority_data = self.serialize_priority_data()

tests/test_frames.py

@@ -112,6 +112,37 @@ def test_cannot_parse_invalid_frame_header(self):
         with pytest.raises(InvalidFrameError):
             Frame.parse_frame_header(b'\x00\x00\x08\x00\x01\x00\x00\x00')
 
+    @pytest.mark.parametrize('frame_type', [DataFrame, HeadersFrame, PushPromiseFrame])
+    @pytest.mark.parametrize('pad_length', [0, 1, 255])
+    @pytest.mark.parametrize('remove_flag', [False, True])
+    def test_unpadded_frames_omit_configured_padding(self, frame_type, pad_length, remove_flag):
+        # 2026-10-11: A cleared PADDED flag must not turn padding into payload.
+        f = frame_type(1, data=b'hello', pad_length=pad_length)
+        fixed_length = 0
+        if frame_type is HeadersFrame:
+            f.flags.add('PRIORITY')
+            fixed_length = 5
+        elif frame_type is PushPromiseFrame:
+            f.promised_stream_id = 2
+            fixed_length = 4
+        if remove_flag:
+            f.flags.add('PADDED')
+            assert decode_frame(f.serialize()).data == b'hello'
+            f.flags.discard('PADDED')
+
+        wire = f.serialize()
+        decoded = decode_frame(wire)
+        assert decoded.data == b'hello'
+        assert len(wire) - 9 == fixed_length + len(b'hello')
+        assert decoded.pad_length == 0
+        if frame_type is DataFrame:
+            assert f.flow_controlled_length == len(wire) - 9
+
+        f.flags.add('PADDED')
+        decoded = decode_frame(f.serialize())
+        assert decoded.data == b'hello'
+        assert decoded.pad_length == pad_length
+
 
 class TestDataFrame:
     payload = b'\x00\x00\x08\x00\x01\x00\x00\x00\x01testdata'