Correct padded HEADERS length and reject padding over fixed fields

CHANGELOG.rst

@@ -19,7 +19,8 @@ dev
 
 **Bugfixes**
 
--
+- Preserve the full padded HEADERS body length and reject padding that overlaps
+  fixed fields in HEADERS and PUSH_PROMISE frames.
 
 6.1.0 (2025-01-22)
 ------------------

src/hyperframe/frame.py

@@ -538,7 +538,7 @@ def parse_body(self, data: memoryview) -> None:
             msg = f"Invalid PUSH_PROMISE promised stream id: {self.promised_stream_id}"
             raise InvalidDataError(msg)
 
-        if self.pad_length and self.pad_length >= self.body_len:
+        if self.pad_length > self.body_len - padding_data_length - 4:
             msg = "Padding is too long."
             raise InvalidPaddingError(msg)
 
@@ -751,6 +751,7 @@ def serialize_body(self) -> bytes:
         return b"".join([padding_data, priority_data, self.data, padding])
 
     def parse_body(self, data: memoryview) -> None:
+        self.body_len = len(data)
         padding_data_length = self.parse_padding_data(data)
         data = data[padding_data_length:]
 
@@ -759,12 +760,11 @@ def parse_body(self, data: memoryview) -> None:
         else:
             priority_data_length = 0
 
-        self.body_len = len(data)
         self.data = (
             data[priority_data_length:len(data)-self.pad_length].tobytes()
         )
 
-        if self.pad_length and self.pad_length >= self.body_len:
+        if self.pad_length > len(data) - priority_data_length:
             msg = "Padding is too long."
             raise InvalidPaddingError(msg)
 

tests/test_frames.py

@@ -995,3 +995,32 @@ class TestExtensionFrame:
     def test_repr(self):
         f = ExtensionFrame(0xFF, 1, 42, b'hello')
         assert repr(f).endswith("type=255, flag_byte=42, body=<hex:68656c6c6f>")
+
+
+@pytest.mark.parametrize('pad_length', [0, 1, 5])
+@pytest.mark.parametrize('payload', [b'', b'hello'])
+@pytest.mark.parametrize('priority', [False, True])
+def test_padded_headers_body_length(pad_length, payload, priority):
+    flags = ['PADDED', 'END_HEADERS']
+    if priority:
+        flags.append('PRIORITY')
+    frame = HeadersFrame(1, flags=flags, pad_length=pad_length, data=payload)
+    wire = frame.serialize()
+    decoded = decode_frame(wire)
+    assert decoded.data == payload
+    assert decoded.body_len == len(wire) - 9
+    assert decoded.serialize() == wire
+
+
+@pytest.mark.parametrize('frame_type', [HeadersFrame, PushPromiseFrame])
+def test_padding_cannot_overlap_fixed_fields(frame_type):
+    # The pad-length byte claims one padding byte, but all remaining bytes
+    # belong to the mandatory priority or promised-stream-ID field.
+    if frame_type is HeadersFrame:
+        frame = HeadersFrame(1, flags=['PADDED', 'PRIORITY'])
+        body = b'\x01\x00\x00\x00\x00\x00'
+    else:
+        frame = PushPromiseFrame(1, flags=['PADDED'])
+        body = b'\x01\x00\x00\x00\x02'
+    with pytest.raises(InvalidPaddingError):
+        frame.parse_body(memoryview(body))