fix: harden DGR-002 protocol bounds

packages/node/meshnet_node/activation_compression.py

@@ -99,7 +99,12 @@ def compress_activation(body: bytes, policy: CompressionPolicy) -> CompressionRe
     return CompressionResult(candidate, "zstd", len(body), len(candidate), time.monotonic() - started, "compressed")
 
 
-def decompress_activation(body: bytes, encoding: str | None) -> CompressionResult:
+def decompress_activation(
+    body: bytes,
+    encoding: str | None,
+    *,
+    max_output_bytes: int | None = None,
+) -> CompressionResult:
     """Decode a modern zstd body or preserve a legacy raw body with metrics."""
     started = time.monotonic()
     if not encoding:
@@ -110,8 +115,23 @@ def decompress_activation(body: bytes, encoding: str | None) -> CompressionResul
         import zstandard as zstd
     except ImportError as exc:
         raise ValueError("zstd support is unavailable") from exc
+    if max_output_bytes is not None and max_output_bytes < 0:
+        raise ValueError("max_output_bytes must be non-negative")
     try:
-        raw = zstd.ZstdDecompressor().decompress(body)
+        if max_output_bytes is None:
+            raw = zstd.ZstdDecompressor().decompress(body)
+        else:
+            # Cap both decoder window allocation and bytes read. zstandard's
+            # max_window_size unit is KiB.
+            max_window_kib = max(1024, (max_output_bytes + 1023) // 1024)
+            decompressor = zstd.ZstdDecompressor(max_window_size=max_window_kib)
+            # `decompress(max_output_size=...)` may trust a frame's advertised
+            # content size. A bounded stream read enforces the limit regardless
+            # of frame metadata and detects trailing expansion with one byte.
+            with decompressor.stream_reader(body) as reader:
+                raw = reader.read(max_output_bytes + 1)
+            if len(raw) > max_output_bytes:
+                raise ValueError("zstd activation body exceeds its output limit")
     except zstd.ZstdError as exc:
         raise ValueError("invalid zstd activation body") from exc
     return CompressionResult(raw, "zstd", len(body), len(raw), time.monotonic() - started, "decompressed")