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Vault portion calculation in PrizePool::getVaultPortion() is incorrect as _startDrawIdInclusive has been erased

Crit/HighPoolTogether Prize Layer·Sherlock · Raffle · 16th May 2024CodebaseH-1

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Summary

The vault portion calculation in PrizePool::getVaultPortion() is incorrect because it fetches a _startDrawIdInclusive that has been overwritten in the total accumulator but not in the vaults accumulator or the donation accumulator.

Vulnerability Detail

To illustrate the issue, consider the grand prize, where odds are 1 / g = 0.00273972602e18 with g == 365 days. The estimated prize frequency is 1e18 / 0.00273972602e18 == 365.000000985 rounded up to the nearest integer, 366. If lastAwardedDrawId_ is 366, then startDrawIdInclusive is 366 - 366 + 1 == 1. This means that it will fetch the total, vault and donation accumulators since the opening of drawId == 1.

However, when lastAwardedDrawId_ == 366, the current open drawId is 367, which is written to index 0 in the circular buffer, overwriting drawId == 1. But, in the donation and vault cases, the accumulator still has drawId == 1, as the buffer will likely not have been overwritten (draw periods take 1 day, it is highly likely the individual accumulators are not written to every day, but the total accumulator is, or users may exploit this on purpose). As the buffer is circular, this will keep happening every time after the buffer has been filled and indexes start being overwritten.

Thus, due to this, the vault portion will be bigger than it should, as the total accumulator is calculated between 365 days (from 2 to 366, as 1 has been erased), but the vault and donation accumulators for 366 days (between 1 and 366). Users will have bigger than expected chances of winning prizes and in the worst case, they may even not be able to claim prizes at all, in case the erased drawId had a significant contribution to the prizes through a donation, and then it underflows when doing totalSupply = totalContributed - totalDonated; in PrizePool::getVaultShares().

Add the following test to PrizePool.t.sol and log the vault and total portions in PrizePool::getVaultShares(). The contribution of the vault will be bigger than the total.

solidity
function testAccountedBalance_POC() public {
  for (uint i = 0; i < 366; i++) {
    contribute(100e18);
    awardDraw(1);
    mockTwab(address(this), msg.sender, 0);
  }
  address otherVault = makeAddr("otherVault");
  vm.prank(otherVault);
  prizePool.contributePrizeTokens(otherVault, 0);
  uint256 prize = claimPrize(msg.sender, 0, 0);
}

Impact

Users get better prize chances than supposed and/or it can lead to permanent reverts when trying to claim prizes.

Code Snippet

https://github.com/sherlock-audit/2024-05-pooltogether/blob/main/pt-v5-prize-pool/src/PrizePool.sol#L1014

Tool used

Manual Review

Vscode

Recommendation

When calculating PrizePool::computeRangeStartDrawIdInclusive(), the rangeSize should be capped to 365 to ensure the right startDrawIdInclusive is fetched, not one that has already been erased. In the scenario above, if the range is capped at 365, when lastAwardedDrawId_ == 366, startDrawIdInclusive would be 366 - 365 + 1 == 2, which has not been erased as the current drawId is 367, having only overwritten drawId == 1, but 2 still has the information.