Reserve contest - rvierdiiev's results

Lines of code

https://github.com/reserve-protocol/protocol/blob/df7ecadc2bae74244ace5e8b39e94bc992903158/contracts/p1/RToken.sol#L344-L370 https://github.com/reserve-protocol/protocol/blob/df7ecadc2bae74244ace5e8b39e94bc992903158/contracts/p1/RToken.sol#L406-L418

Vulnerability details

Impact

Minting of RTokens can be blocked using cancel function.

Proof of Concept

Any user can mint RTokens using issue function. This function is allowed to mint no more than some amount of tokens per block. According to amount you want to mint it calculates vestingEnd variable. In case if you want to mint more tokens than it's allowed in 1 block then your request will be put into the queue and later when vesting block passes you can vest your tokens. What is important here is that when this vestingEnd time is calculated using whenFinished function then it's saved inside allVestAt variable. That means that next one who wants to mint tokens will be able to do that after allVestAt block is passed.

Also, any time when user has orders in the queue he can cancel it and return money. That means that no RTokens will be minted for him. Cancel function will just call refundSpan function which will return users funds. Now we need to remember our allVestAt variable which is not reset in cancel function.

This allows attacker to block minting of RTokens. 1.Attacker calls issue with big amount that can't be minted in same block(he can get a loan). For example it will take week to mint all that tokens. allVestAt is set to 1 week in future. 2.Attacker cancels his order in same tx and returns funds(now he can return them to loaner). 3.Because allVestAt is set to 1 week in future, users can't mint tokens at the moment, they need wait when 1 week will pass.

As you can see attacker can block users from issuing RTokens with no costs for him.

Tools Used

VsCode

Recommended Mitigation Steps

You can add cooldown period, which user should wait after new order was created to avoid flashloans. However this doesn't solve all issue.