diff --git a/core/blockchain_impl.go b/core/blockchain_impl.go
index f0e7e237ff..4757d3e5cc 100644
--- a/core/blockchain_impl.go
+++ b/core/blockchain_impl.go
@@ -2708,7 +2708,14 @@ func (bc *BlockChainImpl) CXMerkleProof(toShardID uint32, block *block.Header) (
 
 func (bc *BlockChainImpl) WriteCXReceiptsProofSpent(db rawdb.DatabaseWriter, cxps []*types.CXReceiptsProof) error {
 	for _, cxp := range cxps {
-		if cxp.Header != nil && bc.Config().IsCXMerkleProofReplayFixEpoch(cxp.Header.Epoch()) {
+		// Key the spent-marker off the signed Header, not the unauthenticated
+		// MerkleProof.ShardID/BlockNum: those fields are only bound to the
+		// Header by ValidateCXReceiptsProof from IsCXMerkleProofReplayFixEpoch
+		// onward, so a proof claiming an earlier epoch can carry a mutated
+		// MerkleProof while keeping a genuine Header/signature. Deriving the
+		// key from MerkleProof would let such a mutated copy of an already
+		//-applied receipt look unspent and be replayed for a fresh credit.
+		if cxp.Header != nil {
 			if err := rawdb.WriteCXReceiptsProofSpentWithKey(
 				db, cxp.Header.ShardID(), cxp.Header.Number().Uint64(),
 			); err != nil {
@@ -2726,7 +2733,10 @@ func (bc *BlockChainImpl) WriteCXReceiptsProofSpent(db rawdb.DatabaseWriter, cxp
 func (bc *BlockChainImpl) IsSpent(cxp *types.CXReceiptsProof) bool {
 	shardID := cxp.MerkleProof.ShardID
 	blockNum := cxp.MerkleProof.BlockNum.Uint64()
-	if cxp.Header != nil && bc.Config().IsCXMerkleProofReplayFixEpoch(cxp.Header.Epoch()) {
+	// See WriteCXReceiptsProofSpent: always resolve the spent-marker key from
+	// the signed Header so the check can't be bypassed by mutating the
+	// unauthenticated MerkleProof fields on a genuine, previously-applied proof.
+	if cxp.Header != nil {
 		shardID = cxp.Header.ShardID()
 		blockNum = cxp.Header.Number().Uint64()
 	}
diff --git a/core/blockchain_impl_test.go b/core/blockchain_impl_test.go
index ce3113f26d..1e23ee3bd1 100644
--- a/core/blockchain_impl_test.go
+++ b/core/blockchain_impl_test.go
@@ -11,6 +11,51 @@ import (
 	staking "github.com/harmony-one/harmony/staking/types"
 )
 
+// TestIsSpentIgnoresMutatedMerkleProofIdentity guards against replaying a
+// genuine, already-applied CXReceiptsProof by mutating the unauthenticated
+// MerkleProof.ShardID/BlockNum while keeping the same signed Header: the
+// spent-marker must be keyed off the Header, which cannot be altered without
+// invalidating the commit signature, not off MerkleProof fields that
+// ValidateCXReceiptsProof only binds to the Header from
+// IsCXMerkleProofReplayFixEpoch onward.
+func TestIsSpentIgnoresMutatedMerkleProofIdentity(t *testing.T) {
+	key, _ := crypto.GenerateKey()
+	chain, _, header, database := getTestEnvironment(*key)
+
+	header = header.With().ShardID(1).Number(big.NewInt(42)).Header()
+
+	original := &types.CXReceiptsProof{
+		Header: header,
+		MerkleProof: &types.CXMerkleProof{
+			ShardID:  1,
+			BlockNum: big.NewInt(42),
+		},
+	}
+
+	batch := database.NewBatch()
+	if err := chain.WriteCXReceiptsProofSpent(batch, []*types.CXReceiptsProof{original}); err != nil {
+		t.Fatalf("WriteCXReceiptsProofSpent failed: %v", err)
+	}
+	if err := batch.Write(); err != nil {
+		t.Fatalf("batch.Write failed: %v", err)
+	}
+
+	if !chain.IsSpent(original) {
+		t.Fatal("expected original proof to be marked spent")
+	}
+
+	replay := &types.CXReceiptsProof{
+		Header: header, // same genuine, signed header
+		MerkleProof: &types.CXMerkleProof{
+			ShardID:  99,               // mutated, unauthenticated
+			BlockNum: big.NewInt(9999), // mutated, unauthenticated
+		},
+	}
+	if !chain.IsSpent(replay) {
+		t.Fatal("expected replay with mutated MerkleProof identity to be detected as already spent")
+	}
+}
+
 func TestPrepareStakingMetadata(t *testing.T) {
 	key, _ := crypto.GenerateKey()
 	chain, db, header, _ := getTestEnvironment(*key)