psbt: harden non-witness UTXO validation

src/psbt.rs

@@ -471,14 +471,17 @@ where
             }
         }
 
+        let funding_utxo =
+            funding_utxo(input, txin, i)?.ok_or(Error::MissingInputFundingUtxo { index: i })?;
+
         // Keep validation aligned with bdk_wallet's default signing policy. The
-        // signer checks every non-finalized input, including inputs that don't
-        // belong to this wallet, and identifies Taproot inputs by their PSBT
-        // metadata rather than by inspecting the funding output.
+        // Taproot fields are untrusted and may be attached to a non-Taproot
+        // input, so only exempt them when the funding output is actually P2TR.
+        let has_taproot_metadata =
+            input.tap_internal_key.is_some() || input.tap_merkle_root.is_some();
         if input.final_script_witness.is_none()
             && input.final_script_sig.is_none()
-            && input.tap_internal_key.is_none()
-            && input.tap_merkle_root.is_none()
+            && !(funding_utxo.script_pubkey.is_p2tr() && has_taproot_metadata)
             && input.non_witness_utxo.is_none()
         {
             return Err(Error::MissingNonWitnessUtxo { index: i });
@@ -508,9 +511,6 @@ where
             continue;
         }
 
-        let funding_utxo =
-            funding_utxo(input, txin, i)?.ok_or(Error::MissingInputFundingUtxo { index: i })?;
-
         if funding_utxo.script_pubkey.is_p2tr() {
             // Only single-sig P2TR supported for now.
             if input.tap_key_origins.len() != 1 {

tests/spend_test.rs

@@ -256,7 +256,7 @@ mod psbt_security_tests {
     // non-taproot inputs require non_witness_utxo
 
     #[test]
-    fn psbt_rejects_p2wpkh_without_non_witness_utxo() {
+    fn psbt_rejects_p2wpkh_with_spurious_taproot_metadata() {
         let secp = test_secp();
         let master = test_master_key();
         let (pk, path, fp) = derive_test_key(&secp, "m/84'/1'/0'/0/0");
@@ -271,6 +271,7 @@ mod psbt_security_tests {
                 value: Amount::from_sat(100_000),
                 script_pubkey: address.script_pubkey(),
             }),
+            tap_merkle_root: Some(TapNodeHash::all_zeros()),
             ..Default::default()
         };
         input.bip32_derivation.insert(pk, (fp, path));