core/bitcoin: make change check more robust against short paths

core/src/apps/bitcoin/sign_tx/bitcoin.py

@@ -17,7 +17,7 @@ from apps.common import coininfo, seed
 from apps.common.writers import write_bitcoin_varint
 
 from .. import addresses, common, multisig, scripts, writers
-from ..common import SIGHASH_ALL, ecdsa_sign
+from ..common import BIP32_WALLET_DEPTH, SIGHASH_ALL, ecdsa_sign
 from ..ownership import verify_nonownership
 from ..verification import SignatureVerifier
 from . import approvers, helpers, progress
@@ -541,6 +541,7 @@ class Bitcoin:
             return False
         return (
             self.wallet_path.output_matches(txo)
+            and len(txo.address_n) >= BIP32_WALLET_DEPTH
             and txo.address_n[-2] <= _BIP32_CHANGE_CHAIN
             and txo.address_n[-1] <= _BIP32_MAX_LAST_ELEMENT
             and txo.amount > 0

core/src/apps/bitcoin/sign_tx/matchcheck.py

@@ -79,7 +79,7 @@ class MatchChecker:
 
 class WalletPathChecker(MatchChecker):
     def attribute_from_tx(self, txio: Union[TxInputType, TxOutputType]) -> Any:
-        if not txio.address_n:
+        if len(txio.address_n) < BIP32_WALLET_DEPTH:
             return None
         return txio.address_n[:-BIP32_WALLET_DEPTH]