CVE-2026-54754

Summary

When a marketplace order is settled (MarketBuy / BuyItNow, and auction Claim), the buyer's payment is split three ways — referral, royalties, and the seller (market-order owner) remainder:

marketOwnerAmount = CurrentBid − referralAmount − royaltiesAmount

Referral and royalties are paid out unconditionally, but the seller remainder is only paid when positive (computeMarketOwnerAmount returns Ok and pays nothing when the amount is <= 0). When referral% + royalty% exceeds 100% of the bid, marketOwnerAmount goes negative and is silently skipped — so the marketplace pays out more KLV / sale currency than the buyer paid in, minting the difference out of thin air.

The combined ceiling royalty% + referral% <= 100% is checked once, at listing time (Sell). But the two percentages are sourced asymmetrically at settlement:

• referral % is snapshotted into the order at Sell (MarketOrderData.ReferralPercentage);
• royalty % is never snapshotted — it is read live from the asset at buy time (asset.Royalties.MarketPercentage).

So the listing-time invariant is a time-of-check/time-of-use guarantee only. After a valid listing, the asset owner raises the royalty MarketPercentage via AssetTrigger → UpdateRoyalties; at the next buy the live royalty plus the snapshotted referral exceed 100%, and the settlement mints the overflow. The minted funds land in attacker-controlled referral / royalty addresses.

This was actively exploited on mainnet (see Evidence), minting tens of millions of KLV before the emergency guard was deployed.

Affected component

• Repository: klever-io/klever-go (node).
• Settlement / mint site: core/kapp/market/market.go — executeBuyMarket (L575+), computeReferralAmount (L361+), computeRoyaltiesAmount (L490+), computeRoyaltiesFixedDeposit (L443+), computeMarketOwnerAmount (L540+).
• TOCTOU sources: Sell combined check (market.go:908), order snapshot of referral but not royalty (market.go:997), live royalty mutation via core/kapp/kda/trigger.go — handleUpdateRoyaltiesNFTandSFT (L613+, sets asset.Royalties.MarketPercentage at L670).
• Reachable from both Buy (BuyItNow, market.go:204+) and auction Claim (market.go:705, market.go:731).
• Pre-fix: not gated by any fork flag — exploitable on mainnet. The fix is gated behind the new FixMarketBuyOverflow activation-epoch flag.

---

Root cause

1. Settlement pays referral + royalty unconditionally, seller remainder only if positive

core/kapp/market/market.go — executeBuyMarket (L575+):

referralAmount, _  := tools.ComputePercentageI64(marketOrder.CurrentBid,
                          int64(marketOrder.ReferralPercentage), ...)        // L583: SNAPSHOT referral %
royaltiesAmount, _ := tools.ComputePercentageI64(marketOrder.CurrentBid,
                          int64(asset.Royalties.MarketPercentage), ...)      // L587: LIVE royalty %
marketOwnerAmount := marketOrder.CurrentBid - referralAmount - royaltiesAmount  // L591: can go negative

// ---- FIX (FixMarketBuyOverflow), added by the patch ----
if m.forkController.FixMarketBuyOverflow() && marketOwnerAmount < 0 {         // L593-596
    ctx.Receipts().AddError(ctx.ContractID(), common.ErrFieldInvalidRoyalties, common.ErrInvalidValue.Error())
    return transaction.Transaction_AmountInvalid, common.ErrInvalidValue
}

m.computeReferralAmount(ctx, marketOrder, referralAmount, currencyID)   // pays referral in full
m.computeRoyaltiesFixedDeposit(ctx, marketOrder, asset)                 // pays fixed royalty (KLV)
m.computeRoyaltiesAmount(ctx, marketOrder, asset, currencyID, royaltiesAmount) // pays % royalty in full
m.computeMarketOwnerAmount(ctx, marketOrder, currencyID, marketOwnerAmount)    // <-- skips when <= 0

computeMarketOwnerAmount (L540-542) — the silent skip:

func (m *marketKapp) computeMarketOwnerAmount(... marketOwnerAmount int64) (... , error) {
	if marketOwnerAmount <= 0 {
		return transaction.Transaction_Ok, nil   // negative seller share dropped, NO error
	}
	// ... AddToBalance(marketOwnerAmount) ...
}

Meanwhile computeReferralAmount (L376) and computeRoyaltiesAmount (L515) each AddToBalance(...) the full computed amount with no matching debit from the buyer beyond the single bidderAcc.SubFromBalance(amount) taken in Buy (market.go:301).

Conservation breaks: buyer is debited bid once; recipients are credited referralAmount + royaltiesAmount. When that sum > bid, the surplus (referralAmount + royaltiesAmount − bid) is minted.

2. The combined ≤100% invariant is enforced only at listing time

Sell (market.go:908) correctly rejects a listing whose combined cut exceeds 100%:

if asset.Royalties.MarketPercentage + marketplace.ReferralPercentage > core.HundredPercent {
	return transaction.Transaction_ParameterInvalid, common.ErrInvalidValue
}

…and snapshots referral into the order, but not royalty (market.go:997-998):

marketOrder := &kapps.MarketOrderData{
	// ...
	ReferralPercentage:    marketplace.ReferralPercentage, // snapshotted
	RoyaltiesFixedDeposit: asset.Royalties.MarketFixed,    // snapshotted
	// NOTE: asset.Royalties.MarketPercentage is NOT snapshotted -> read live at buy
}

MarketOrderData has no field for the royalty percentage (kapps/market.pb.go), so settlement always re-reads it live from the (mutable) asset.

3. Royalty % is mutable after listing

core/kapp/kda/trigger.go — handleUpdateRoyaltiesNFTandSFT (L613+) lets the asset owner overwrite asset.Royalties.MarketPercentage (L670) with only a per-field <= 100% check (CheckValid100Params, L651) — it has no knowledge of any outstanding marketplace listing's snapshotted referral. So the owner can list at, e.g., referral 100% / royalty 0% (sum 100%, passes Sell), then raise royalty to 100%, making the buy-time sum 200%.

The shipped emergency-guard source documents this exact vector: "The royalty percentage is read live at buy time, so a listing made now can be weaponised later via UpdateRoyalties." (common/emergencyGuard.go)

Net effect: referralAmount + royaltiesAmount = bid + bid = 2·bid; marketOwnerAmount = −bid (skipped); bid KLV minted per settlement, paid to attacker-controlled addresses.

---

Proof of Concept

A. Committed regression test (deterministic, runnable today)

core/kapp/market/market_test.go — TestMarketKApp_ExecuteBuyMarket_RoyaltyReferralInflation. It builds an order with ReferralPercentage = 100% and an asset with MarketPercentage = 100% (the attacker is both the referral and the royalty address), then settles a bid of 25,600,000 KLV (25600000000000 base units):

go test ./core/kapp/market/ -run TestMarketKApp_ExecuteBuyMarket_RoyaltyReferralInflation -v

• FixDisabled_MintsKLVFromThinAir: settlement returns Ok; the attacker address ends with 2·bid credited while only bid was paid in — i.e. bid KLV minted.
• FixEnabled_RejectsInflation: with FixMarketBuyOverflow on, settlement returns Transaction_AmountInvalid and the attacker balance stays 0 — no payout runs.

B. End-to-end on a local node (the real attack path)

A single-node local network is sufficient. The exploit is four transactions from one ordinary funded account; nothing privileged is required.

1. Create an NFT collection you own, with royalties.marketPercentage = 0 and a royalties address you control.
2. Create a marketplace with referralPercentage = 10000 (100%) and a referral address you control (CreateMarketplace).
3. List one NFT for sale (Sell) on that marketplace. The Sell check passes because 0 (royalty) + 10000 (referral) = 10000 = HundredPercent. The order snapshots ReferralPercentage = 10000.
4. Raise the royalty on the asset to 100% (`AssetTrigger / UpdateRoyal