BattleChain Confidence Pools
Cyfrin
FoundrySolidityLayer 2
7.25 ETH
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Submission Details
Impact: medium
Likelihood: medium

Bonus sweep can redirect value during the moderator reflag window

Author Revealed upon completion

Root + Impact

Description

flagOutcome() allows the moderator to correct a previously flagged outcome until a claim starts. The design document describes claimsStarted as the value-movement finality latch: once value has left under an outcome, later correction can break accounting.

sweepUnclaimedBonus() is intended to recover bonus or excess tokens that are not owed to stakers. In the no-risk-window case, _bonusShare() returns zero, so accounted bonus can become sweepable to recoveryAddress.

Issue: sweepUnclaimedBonus() can transfer accounted totalBonus to recoveryAddress and decrement totalBonus without setting claimsStarted. This allows value to leave during the moderator reflag window while still allowing a later reflag that snapshots depleted bonus accounting.

When an outcome is first flagged as SURVIVED and later corrected to good-faith CORRUPTED, a prior sweepUnclaimedBonus() can reduce the named attacker's bounty from snapshotTotalStaked + original snapshotTotalBonus to only the remaining post-sweep balance.

function flagOutcome(PoolStates.Outcome newOutcome, bool goodFaith_, address attacker_) external onlyModerator {
// Re-flag allowed pre-claim so the moderator can fix a typo'd outcome/attacker before
// any participant locks in the wrong distribution.
if (outcome != PoolStates.Outcome.UNRESOLVED && claimsStarted) revert OutcomeAlreadySet();
​
...
​
snapshotTotalStaked = totalEligibleStake;
snapshotTotalBonus = totalBonus;
corruptedReserve = newOutcome == PoolStates.Outcome.CORRUPTED ? snapshotTotalStaked + snapshotTotalBonus : 0;
bountyEntitlement = willBeGoodFaithCorrupted ? snapshotTotalStaked + snapshotTotalBonus : 0;
}
​
function sweepUnclaimedBonus() external nonReentrant {
if (outcome != PoolStates.Outcome.SURVIVED && outcome != PoolStates.Outcome.EXPIRED) {
revert OutcomeNotEligibleForSweep();
}
​
...
​
uint256 amount = freeBalance > reserved ? freeBalance - reserved : 0;
if (amount == 0) revert NothingToSweep();
​
if (totalEligibleStake == 0 || riskWindowStart == 0) {
// @> Accounted bonus can be decremented and swept while the outcome is still reflaggable.
totalBonus -= amount <= totalBonus ? amount : totalBonus;
}
​
// @> Value leaves the contract, but claimsStarted is intentionally not set.
stakeToken.safeTransfer(recoveryAddress, amount);
}

Risk

Likelihood:

  • A moderator flags SURVIVED while the registry is terminal CORRUPTED, then later corrects the outcome to good-faith CORRUPTED.

  • A no-risk-window pool has riskWindowStart == 0, causing _bonusShare() to return zero and making accounted bonus sweepable before the moderator correction occurs.

  • Any caller can execute sweepUnclaimedBonus() between the first flag and the moderator's corrective reflag.

Impact:

  • The named good-faith attacker receives a smaller bounty than the pool accounting promised for CORRUPTED resolution.

  • Accounted bonus is redirected to recoveryAddress before the final corrected outcome is established.

  • The reflag window no longer preserves value-distribution correctness, despite claimsStarted being treated as the finality latch.

Proof of Concept

// Initial state:
// - totalEligibleStake = 100
// - totalBonus = 50
// - stakeToken.balanceOf(pool) = 150
// - riskWindowStart = 0
// - registry state = CORRUPTED
// - outcome = UNRESOLVED
​
// 1. Moderator initially flags SURVIVED.
pool.flagOutcome(PoolStates.Outcome.SURVIVED, false, address(0));
​
// 2. Because riskWindowStart == 0, no staker is owed bonus.
// sweepUnclaimedBonus() treats the 50 bonus as free balance.
pool.sweepUnclaimedBonus();
​
// State after sweep:
// - recoveryAddress received 50
// - stakeToken.balanceOf(pool) = 100
// - totalBonus = 0
// - claimsStarted = false
​
// 3. Moderator corrects to good-faith CORRUPTED.
pool.flagOutcome(PoolStates.Outcome.CORRUPTED, true, attacker);
​
// 4. Bounty is snapshotted from depleted accounting.
// bountyEntitlement = snapshotTotalStaked + snapshotTotalBonus
// = 100 + 0
// = 100
​
// Expected without the intermediate sweep:
// bountyEntitlement = 100 + 50 = 150
​
// Result:
// - attacker receives only 100 instead of 150
// - 50 was redirected to recoveryAddress during the reflag window
​

Recommended Mitigation

function sweepUnclaimedBonus() external nonReentrant {
+ if (outcome != PoolStates.Outcome.UNRESOLVED && !claimsStarted) {
+ revert OutcomeStillReflaggable();
+ }
+
if (outcome != PoolStates.Outcome.SURVIVED && outcome != PoolStates.Outcome.EXPIRED) {
revert OutcomeNotEligibleForSweep();
}
​

An alternative mitigation is to keep sweepUnclaimedBonus() callable before claimsStarted, but reserve accounted totalBonus while the outcome remains reflaggable and allow only unaccounted donations or dust to be swept.

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