KittenSwap Voter — deactivating a gauge deletes claimable rewards

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 58013-h-02-loss-of-claimable-rewards-upon-gauge-deactivation-pasho. Standalone Foundry PoC and full write-up: 58013-h-02-loss-of-claimable-rewards-upon-gauge-deactivation-pasho_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/logic/reward-calculation · vuln/logic/state-update · vuln/dos/frozen-funds

Key info#

TL;DR#

A gauge with the full voting weight is notified 1,000 KITTEN. updateFor writes that 1,000 into claimable. killGauge then sets claimable to 0 and marks the gauge dead. The staker's claim returns 0. reviveGauge turns the gauge back on and does not restore the deleted amount. The 1,000 remains in the Voter with no claim path.

The control pays that claimable balance to the staker as part of deactivation. The staker receives 1,000 and the Voter balance goes to 0.

The vulnerable code#

function killGauge(address _gauge) external {
    require(msg.sender == emergencyCouncil, "not emergency council");
    require(isAlive[_gauge], "gauge already dead");
    isAlive[_gauge] = false;
    // @> VULN: accrued claimable rewards are deleted.
    claimable[_gauge] = 0;
}

Accrual while the gauge is alive is the usual index share:

uint _delta = _index - _supplyIndex;
if (_delta > 0) {
    uint _share = (uint(_supplied) * _delta) / 1e18;
    if (isAlive[_gauge]) {
        claimable[_gauge] += _share;
    }
}

reviveGauge only sets isAlive back to true. The warden's fix is to keep claimable recoverable, or to pay it out when the gauge is killed.

Root cause#

claimable is the only record of rewards already earned and not yet sent to the gauge. Zeroing it on deactivation throws that record away. The tokens were already transferred into the Voter by notifyRewardAmount, and nothing else is allowed to withdraw them. Revival does not replay the deleted accrual.

Preconditions#

The emergency council calls killGauge on a gauge that has a non-zero claimable. Here the gauge holds the entire totalWeight, so one notification of 1,000 accrues 1,000 before the kill. No epoch boundary and no time warp are required. The staker is not the caller of the kill.

Attack walkthrough#

1. The council notifies 1,000 KITTEN. index increases by amount * 1e18 / totalWeight.
2. updateFor sees a live gauge and sets claimable to 1,000.
3. killGauge passes the council check, sets isAlive false, and stores claimable = 0.
4. claim reads 0 and transfers nothing. The staker's balance stays 0.
5. reviveGauge sets isAlive true. claimable is still 0.
6. The Voter's token balance is still 1,000.

The control runs the same accrual, then on killGauge transfers the 1,000 to the staker before clearing the accounting. The staker's balance is 1,000 and the Voter's balance is 0.

Diagrams#

Impact#

Stakers of a deactivated gauge lose every reward that had accrued and not yet been claimed. In this distribution that is the full 1,000. Reviving the gauge does not give it back. The tokens sit in the Voter with claimable at 0, so a later claim still pays 0.

Sources#

• AuditVault finding: https://github.com/Auditware/AuditVault/blob/main/findings/58013-h-02-loss-of-claimable-rewards-upon-gauge-deactivation-pasho.md
• Report: https://github.com/pashov/audits/blob/master/team/md/KittenSwap-security-review_2025-05-07.md
• Reviewed tree: https://github.com/Kittenswap/contracts/commit/b191141c1efa9eda997c75460dab383db878e2b1 (Voter.killGauge)

Sources & further analysis#

Reproductions & code

• Standalone PoC + full trace: 58013-h-02-loss-of-claimable-rewards-upon-gauge-deactivation-pasho_exp (evm-hack-registry mirror).
• AuditVault finding: 58013-h-02-loss-of-claimable-rewards-upon-gauge-deactivation-pasho.

Alerts & third-party analyses

• Web3Sec X hacked database: search.
• Rekt leaderboard: search.
• Solodit incident search: search.

These dashboards index community alerts tweets, post-mortems, and independent write-ups. Reach them through the protocol name above to cross-check this reproduction against other analyses.