From b3776e92f45ff1c9cb61b7f2a3261e47eb3c7066 Mon Sep 17 00:00:00 2001
From: Slyghtning <hieblmi@protonmail.com>
Date: Tue, 11 Aug 2026 11:36:27 +0200
Subject: [PATCH] instantout: validate MuSig2 response dimensions

Check nonce, signature, session, and transaction input counts before
indexing signing vectors, returning clear errors for incomplete data.
---
 instantout/instantout.go      | 38 ++++++++++++++++++++++++++++
 instantout/instantout_test.go | 47 +++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+)
 create mode 100644 instantout/instantout_test.go

diff --git a/instantout/instantout.go b/instantout/instantout.go
index f8c89eb0..40cbf12e 100644
--- a/instantout/instantout.go
+++ b/instantout/instantout.go
@@ -263,12 +263,31 @@ func (i *InstantOut) signMusig2Tx(ctx context.Context,
 	if err != nil {
 		return nil, err
 	}
+	if tx == nil {
+		return nil, errors.New("transaction is nil")
+	}
+	if len(tx.TxIn) != len(inputs) {
+		return nil, fmt.Errorf("invalid number of transaction inputs: "+
+			"expected %d, got %d", len(inputs), len(tx.TxIn))
+	}
+	if len(musig2sessions) != len(inputs) {
+		return nil, fmt.Errorf("invalid number of MuSig2 sessions: "+
+			"expected %d, got %d", len(inputs), len(musig2sessions))
+	}
+	if len(counterPartyNonces) != len(inputs) {
+		return nil, fmt.Errorf("invalid number of server nonces: "+
+			"expected %d, got %d", len(inputs), len(counterPartyNonces))
+	}
 
 	prevOutFetcher := inputs.GetPrevoutFetcher()
 	sigHashes := txscript.NewTxSigHashes(tx, prevOutFetcher)
 	sigs := make([][]byte, len(inputs))
 
 	for idx, reservation := range inputs {
+		if musig2sessions[idx] == nil {
+			return nil, fmt.Errorf("MuSig2 session %d is nil", idx)
+		}
+
 		if !reflect.DeepEqual(tx.TxIn[idx].PreviousOutPoint,
 			reservation.Outpoint) {
 
@@ -329,8 +348,27 @@ func (i *InstantOut) finalizeMusig2Transaction(ctx context.Context,
 	if err != nil {
 		return nil, err
 	}
+	if tx == nil {
+		return nil, errors.New("transaction is nil")
+	}
+	if len(tx.TxIn) != len(inputs) {
+		return nil, fmt.Errorf("invalid number of transaction inputs: "+
+			"expected %d, got %d", len(inputs), len(tx.TxIn))
+	}
+	if len(musig2Sessions) != len(inputs) {
+		return nil, fmt.Errorf("invalid number of MuSig2 sessions: "+
+			"expected %d, got %d", len(inputs), len(musig2Sessions))
+	}
+	if len(serverSigs) != len(inputs) {
+		return nil, fmt.Errorf("invalid number of server signatures: "+
+			"expected %d, got %d", len(inputs), len(serverSigs))
+	}
 
 	for idx := range inputs {
+		if musig2Sessions[idx] == nil {
+			return nil, fmt.Errorf("MuSig2 session %d is nil", idx)
+		}
+
 		haveAllSigs, finalSig, err := signer.MuSig2CombineSig(
 			ctx, musig2Sessions[idx].SessionID,
 			[][]byte{serverSigs[idx]},
diff --git a/instantout/instantout_test.go b/instantout/instantout_test.go
new file mode 100644
index 00000000..6b0e7cb5
--- /dev/null
+++ b/instantout/instantout_test.go
@@ -0,0 +1,47 @@
+package instantout
+
+import (
+	"context"
+	"testing"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightninglabs/loop/instantout/reservation"
+	"github.com/lightningnetwork/lnd/input"
+	"github.com/stretchr/testify/require"
+)
+
+// TestMuSig2VectorLengthValidation verifies that malformed server-controlled
+// vectors are rejected before they can be indexed.
+func TestMuSig2VectorLengthValidation(t *testing.T) {
+	_, pubKey := btcec.PrivKeyFromBytes([]byte{1})
+	instantOut := &InstantOut{
+		Reservations: []*reservation.Reservation{
+			{
+				ClientPubkey: pubKey,
+				ServerPubkey: pubKey,
+				Value:        btcutil.Amount(100_000),
+				Expiry:       200,
+				Outpoint:     &wire.OutPoint{},
+			},
+		},
+	}
+	tx := wire.NewMsgTx(2)
+	tx.AddTxIn(&wire.TxIn{})
+	sessions := []*input.MuSig2SessionInfo{{}}
+
+	require.NotPanics(t, func() {
+		_, err := instantOut.signMusig2Tx(
+			context.Background(), nil, tx, sessions, nil,
+		)
+		require.ErrorContains(t, err, "server nonces")
+	})
+
+	require.NotPanics(t, func() {
+		_, err := instantOut.finalizeMusig2Transaction(
+			context.Background(), nil, sessions, tx, nil,
+		)
+		require.ErrorContains(t, err, "server signatures")
+	})
+}