Bind pack signature verification to actual content (closes #9)

packages/volo-packs/src/volo_packs/signing.py

@@ -17,7 +17,7 @@
 import json
 from pathlib import Path
 
-from volo_packs.pack import Pack, PackSignature
+from volo_packs.pack import Pack, PackSignature, content_checksum
 
 HMAC_SHA256 = "hmac-sha256"
 
@@ -40,10 +40,18 @@ def sign_pack(pack: Pack, *, publisher: str, secret: str) -> Pack:
 
 
 def verify_pack_signature(pack: Pack, keyring: Keyring) -> bool:
-    """True if the pack carries a signature from a keyring publisher over its current content."""
+    """True if the pack carries a valid signature from a keyring publisher over its **actual** content.
+
+    The signed message binds the manifest checksum, so verification MUST first confirm the manifest
+    checksum still matches the real items — otherwise an attacker could swap ``pack.items`` while
+    leaving ``manifest.checksum`` (and thus the signature) untouched and still verify as valid.
+    """
     sig = pack.manifest.signature
     if sig is None or sig.algorithm != HMAC_SHA256:
         return False
+    # Re-bind to real content: a stale/forged manifest checksum invalidates the signature.
+    if content_checksum(pack.items) != pack.manifest.checksum:
+        return False
     secret = keyring.get(sig.publisher)
     if secret is None:
         return False

packages/volo-packs/tests/test_signing_content_binding.py

@@ -0,0 +1,33 @@
+"""Regression: a signature must not verify once the pack's actual content is tampered."""
+
+from __future__ import annotations
+
+from volo_packs import build_pack, sign_pack, starter_items, verify_pack_signature
+from volo_packs.pack import Pack
+
+
+def _signed() -> Pack:
+    pack = build_pack(name="p", version="1.0.0", kind="attacks", items=starter_items("attacks"))
+    sign_pack(pack, publisher="acme", secret="s3cret")
+    return pack
+
+
+def test_untampered_pack_verifies() -> None:
+    assert verify_pack_signature(_signed(), {"acme": "s3cret"}) is True
+
+
+def test_content_tamper_invalidates_signature() -> None:
+    pack = _signed()
+    # swap the payload but leave manifest.checksum (and thus the HMAC) untouched
+    pack.items.append(
+        {"id": "evil", "category": "x", "technique": "pwn", "payload": "rm -rf /",
+         "expected_behavior": "refuse"}
+    )
+    assert verify_pack_signature(pack, {"acme": "s3cret"}) is False
+
+
+def test_manifest_checksum_forgery_invalidates_signature() -> None:
+    pack = _signed()
+    pack.items.clear()
+    pack.manifest.checksum = "deadbeef"  # forged to match nothing real
+    assert verify_pack_signature(pack, {"acme": "s3cret"}) is False