fixup! Bound decoder work to prevent a pointer fan-out denial of service

Author: oschwald

maxminddb/decoder.py

@@ -179,6 +179,13 @@ def _decode_pointer(
         if self._pointer_test:
             return pointer, new_offset
 
+        # The pointer itself was charged by its containing array or map. Charge
+        # its target separately because it is decoded separately each time the
+        # pointer is followed.
+        remaining = budget[0] - 1
+        if remaining < 0:
+            raise InvalidDatabaseError(_TOO_MANY_VALUES)
+        budget[0] = remaining
         budget[1] += 1
         if budget[1] > _MAX_DEPTH:
             raise InvalidDatabaseError(_TOO_DEEP)

tests/decoder_test.py

@@ -255,6 +255,19 @@ def test_pointer_fan_out_is_bounded(self) -> None:
         with self.assertRaises(InvalidDatabaseError):
             Decoder(bytes(buf), pointer_base=0).decode(prev)
 
+    def test_flat_scalar_pointer_fan_out_is_bounded(self) -> None:
+        # The array's 32,769 pointer fields and their separately decoded scalar
+        # targets exceed the 65,536-value limit even without nested containers.
+        # 0x1e: extended type with size code 30; 0x04: array; 0x7ee4:
+        # 32,769 - 285.
+        data = bytes([0xA0, 0x1E, 0x04, 0x7E, 0xE4]) + self._pointer(0) * 32_769
+
+        with self.assertRaisesRegex(
+            InvalidDatabaseError,
+            "^The MaxMind DB file's data section exceeds the maximum number of values$",
+        ):
+            Decoder(data, pointer_base=0).decode(1)
+
     def test_cyclic_pointer_raises(self) -> None:
         # A pointer to itself must hit the decoder's own depth limit even when
         # Python's process-wide recursion limit is much higher.