Enhance fee transaction sanity checks.

internal/webapi/payfee.go

@@ -1,4 +1,4 @@
-// Copyright (c) 2021-2024 The Decred developers
+// Copyright (c) 2021-2026 The Decred developers
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
@@ -143,6 +143,29 @@ func (w *WebAPI) payFee(c *gin.Context) {
 		return
 	}
 
+	// Confirm all inputs of the fee transaction exist.
+	for _, input := range feeTx.TxIn {
+		prevOut := input.PreviousOutPoint
+		txOut, err := dcrdClient.GetTxOut(prevOut.Hash, prevOut.Index, prevOut.Tree)
+		if err != nil {
+			w.log.Errorf("%s: dcrd.GetTxOut for fee tx input failed "+
+				"(ticketHash=%s, output=%s:%d:%d, clientIP=%s): %v",
+				funcName, ticket.Hash, prevOut.Hash, prevOut.Index, prevOut.Tree,
+				c.ClientIP(), err)
+			w.sendError(types.ErrInternalError, c)
+			return
+		}
+		if txOut == nil {
+			w.log.Warnf("%s: Fee tx contains non-existent input "+
+				"(ticketHash=%s, output=%s:%d:%d, clientIP=%s)",
+				funcName, ticket.Hash, prevOut.Hash, prevOut.Index, prevOut.Tree,
+				c.ClientIP())
+			w.sendErrorWithMsg("fee tx includes non-existent input",
+				types.ErrInvalidFeeTx, c)
+			return
+		}
+	}
+
 	// Decode fee address to get its payment script details.
 	feeAddr, err := stdaddr.DecodeAddress(ticket.FeeAddress, w.cfg.Network)
 	if err != nil {

rpc/dcrd.go

@@ -1,4 +1,4 @@
-// Copyright (c) 2021-2025 The Decred developers
+// Copyright (c) 2021-2026 The Decred developers
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
@@ -306,6 +306,20 @@ func (c *DcrdRPC) GetBlockHash(height int64) (string, error) {
 	return resp, nil
 }
 
+// GetTxOut returns the transaction output info if it's unspent and nil
+// otherwise.
+func (c *DcrdRPC) GetTxOut(hash chainhash.Hash, index uint32,
+	tree int8) (*dcrdtypes.GetTxOutResult, error) {
+	var resp *dcrdtypes.GetTxOutResult
+	const includeMempool = true
+	err := c.Call(context.TODO(), "gettxout", &resp, hash.String(), index, tree,
+		includeMempool)
+	if err != nil {
+		return nil, err
+	}
+	return resp, nil
+}
+
 // GetCFilterV2 retrieves the GCS filter for the provided block header,
 // optionally verifies the inclusion proof, then returns the filter along with
 // its key.