PoolTogether
Findings & Analysis Report

2021-09-16

Overview

About C4

Code 432n4 (C4) is an open organization consisting of security researchers, auditors, developers, and individuals with domain expertise in smart contracts.

A C4 code contest is an event in which community participants, referred to as Wardens, review, audit, or analyze smart contract logic in exchange for a bounty provided by sponsoring projects.

During the code contest outlined in this document, C4 conducted an analysis of PoolTogether smart contract system written in Solidity. The code contest took place between June 16—June 23.

Wardens

12 Wardens contributed reports to the PoolTogether code contest:

This contest was judged by LSDan (ElasticDAO).

Final report assembled by moneylegobatman and ninek.

Summary

The C4 analysis yielded an aggregated total of 30 unique vulnerabilities. All of the issues presented here are linked back to their original finding

Of these vulnerabilities, 5 received a risk rating in the category of HIGH severity, 7 received a risk rating in the category of MEDIUM severity, and 18 received a risk rating in the category of LOW severity.

C4 analysis also identified 6 non-critical recommendations and 34 gas optimizations.

Scope

The code under review can be found within the C4 PoolTogether code contest repository is comprised of 10 smart contracts written in the Solidity programming language and includes 1,395 lines of Solidity code.

Severity Criteria

C4 assesses the severity of disclosed vulnerabilities according to a methodology based on OWASP standards.

Vulnerabilities are divided into three primary risk categories: high, medium, and low.

High-level considerations for vulnerabilities span the following key areas when conducting assessments:

  • Malicious Input Handling
  • Escalation of privileges
  • Arithmetic
  • Gas use

Further information regarding the severity criteria referenced throughout the submission review process, please refer to the documentation provided on the C4 website.

High Risk Findings (5)

[H-01] User could lose underlying tokens when redeeming from the IdleYieldSource

Submitted by shw

The redeemToken function in IdleYieldSource uses redeemedShare instead of redeemAmount as the input parameter when calling redeemIdleToken of the Idle yield source. As a result, users could get fewer underlying tokens than they should.

When burning users’ shares, it is correct to use redeemedShare (line 130). However, when redeeming underlying tokens from Idle Finance, redeemAmount should be used instead of redeemedShare (line 131). Usually, the tokenPriceWithFee() is greater than ONE_IDLE_TOKEN, and thus redeemedShare is less than redeemAmount, causing users to get fewer underlying tokens than expected.

Recommend changing redeemedShare to redeemAmount at line L131.

PierrickGT (PoolTogether) confirmed and patched:

PR: https://github.com/pooltogether/idle-yield-source/pull/4

[H-02] YearnV2YieldSource wrong subtraction in withdraw

Submitted by cmichel, also found by 0xRajeev

When withdrawing from the vault, one redeems yTokens for tokens, thus the token balance of the contract should increase after withdrawal. But the contract subtracts the currentBalance from the previousBalance:

uint256 yShares = _tokenToYShares(amount);
uint256 previousBalance = token.balanceOf(address(this));
// we accept losses to avoid being locked in the Vault (if losses happened for some reason)
if(maxLosses != 0) {
    vault.withdraw(yShares, address(this), maxLosses);
} else {
    vault.withdraw(yShares);
}
uint256 currentBalance = token.balanceOf(address(this));
// @audit-issue this seems wrong
return previousBalance.sub(currentBalance);

All vault withdrawals fail due to the integer underflow as the previousBalance is less than currentBalance. Users won’t be able to get back their investment.

Recommend that It should return currentBalance > previousBalance ? currentBalance - previousBalance : 0

kamescg (PoolTogether) confirmed and patched:

[H-03] BadgerYieldSource balanceOfToken share calculation seems wrong

Submitted by cmichel

When suppling to the BadgerYieldSource, some amount of badger is deposited to badgerSett and one receives badgerSett share tokens in return which are stored in the balances mapping of the user. So far this is correct.

The balanceOfToken function should then return the redeemable balance in badger for the user’s badgerSett balance. It computes it as the pro-rata share of the user balance (compared to the total-supply of badgerSett) on the badger in the vault:

balances[addr].mul(
  badger.balanceOf(address(badgerSett))
).div(
  badgerSett.totalSupply()
)

However, badger.balanceOf(address(badgerSett)) is only a small amount of badger that is deployed in the vault (“Sett”) due to most of the capital being deployed to the strategies. Therefore, it under-reports the actual balance:

Typically, a Sett will keep a small portion of deposited funds in reserve to handle small withdrawals cheaply. Badger Docs

Any contract or user calling the balanceOf function will receive a value that is far lower than the actual balance. Using this value as a basis for computations will lead to further errors in the integrations.

Recommend using badgerSett.balance() instead of badger.balanceOf(address(badgerSett)) to also account for “the balance in the Sett, the Controller, and the Strategy”.

asselstine (PoolTogether) confirmed

[H-04] withdraw timelock can be circumvented

Submitted by cmichel

One can withdraw the entire PrizePool deposit by circumventing the timelock. Assume the user has no credits for ease of computation:

  • user calls withdrawWithTimelockFrom(user, amount=userBalance) with their entire balance. This “mints” an equivalent amount of timelock and resets _unlockTimestamps[user] = timestamp = blockTime + lockDuration.
  • user calls withdrawWithTimelockFrom(user, amount=0) again but this time withdrawing 0 amount. This will return a lockDuration of 0 and thus unlockTimestamp = blockTime. The inner _mintTimelock now resets _unlockTimestamps[user] = unlockTimestamp
  • As if (timestamp <= _currentTime()) is true, the full users amount is now transferred out to the user in the _sweepTimelockBalances call.

Users don’t need to wait for their deposit to contribute their fair share to the prize pool. They can join before the awards and leave right after without a penalty which leads to significant issues for the protocol. It’s the superior strategy but it leads to no investments in the strategy to earn the actual interest.

Recommend that the unlock timestamp should be increased by duration each time, instead of being reset to the duration.

asselstine (PoolTogether) confirmed:

Mitigation:

If a user’s timelock balance is non-zero, the prize strategy rejects the ticket burn.

[H-05] IdleYieldSource doesn’t use mantissa calculations

Submitted by tensors

Because mantissa calculations are not used in this case to account for decimals, the arithmetic can zero out the number of shares or tokens that should be given.

For example, say I deposit 1 token, expecting 1 share in return. On L95, if the totalUnderlyingAssets is increased to be larger than the number of total shares, then the division would output 0 and I wouldn’t get any shares.

Recommend implementing mantissa calculations like in the contract for the AAVE yield.

PierrickGT (PoolTogether) confirmed and patched:

PR: https://github.com/pooltogether/idle-yield-source/pull/5

Medium Risk Findings (7)

[M-01] safeApprove() for Yearn Vault may revert preventing deposits causing DoS

Submitted by 0xRajeev, also found by pauliax

The _depositInVault() function for Yearn yield source uses ERC20 safeApprove() from OpenZeppelin’s SafeERC20 library to give maximum allowance to the Yearn Vault address if the current allowance is less than contract’s token balance.

However, the safeApprove function prevents changing an allowance between non-zero values to mitigate a possible front-running attack. It reverts if that is the case. Instead, the safeIncreaseAllowance and safeDecreaseAllowance functions should be used. Comment from the OZ library for this function:

“// safeApprove should only be called when setting an initial allowance, // or when resetting it to zero. To increase and decrease it, use // ‘safeIncreaseAllowance’ and ‘safeDecreaseAllowance’”

If the existing allowance is non-zero (say, for e.g., previously the entire balance was not deposited due to vault balance limit resulting in the allowance being reduced but not made 0), then safeApprove() will revert causing the user’s token deposits to fail leading to denial-of-service. The condition predicate indicates that this scenario is possible. See similar Medium-severity finding M03.

Recommend using safeIncreaseAllowance() function instead of safeApprove().

kamescg (PoolTogether) confirmed and patched:

[M-02] Return values of ERC20 transfer and transferFrom are unchecked

Submitted by shw, gpersoon, JMukesh, also found by adelamo and cmichel_

In the contracts BadgerYieldSource and SushiYieldSource, the return values of ERC20 transfer and transferFrom are not checked to be true, which could be false if the transferred tokens are not ERC20-compliant (e.g., BADGER). In that case, the transfer fails without being noticed by the calling contract.

If warden’s understanding of the BadgerYieldSource is correct, the badger variable should be the BADGER token at address 0x3472a5a71965499acd81997a54bba8d852c6e53d. However, this implementation of BADGER is not ERC20-compliant, which returns false when the sender does not have enough token to transfer (both for transfer and transferFrom). See the source code on Etherscan (at line 226) for more details.

Recommend using the SafeERC20 library implementation from Openzeppelin and call safeTransfer or safeTransferFrom when transferring ERC20 tokens.

kamescg (PoolTogether) confirmed and patched:

Sushi

Badger

dmvt (judge) commented:

Sponsor has repeatedly stated in duplicate issues that: “It’s more of a 1 (Low Risk) because the subsequent deposit calls will fail. There is no advantage to be gained; the logic is simply poor.”

I disagree with this assessment. The function(s) in question do not immediately call deposit or another function that would cause a revert. In fact the balances are updated:

        balances[msg.sender] = balances[msg.sender].sub(requiredSharesBalance);
        badger.transfer(msg.sender, badgerBalanceDiff);
        return (badgerBalanceDiff);

The impact that this would have on the rest of the system is substantial, including causing incorrect balances to be returned and potentially lost funds.

That said, I do not think this is very likely and so high severity seems excessive here. Im adjusting all of these reports to Medium Risk given that lower likelihood.

[M-03] SafeMath not completely used in yield source contracts

Submitted by shw, also found by cmichel

SafeMath is not completely used at the following lines of yield source contracts, which could potentially cause arithmetic underflow and overflow:

  1. line 78 in SushiYieldSource
  2. line 67 in BadgerYieldSource
  3. line 91 and 98 in IdleYieldSource

Recommend using the SafeMath library functions in the above lines.

asselstine (PoolTogether) confirmed and disagreed with severity:

While the arithmetic ceiling is quite high, if an overflow occurred this would significantly disrupt the yield sources. I’d qualify this issue higher as 2 (Med Risk).

dmvt (judge) commented:

I agree with the sponsor’s risk evaluation. Increasing to medium.

[M-04] The assumption that operator == to (user) may not hold leading to failed timelock deposits

Submitted by 0xRajeev

The contract uses _msgSender() to denote an operator who is operating on behalf of the user. This is typically used for meta-transactions where the operator is an intermediary/relayer who may facilitate gas-less transactions on behalf of the user. They may be the same address but it is safer to assume that they may not be.

While the code handles this separation of role in most cases, it misses doing so in timelockDepositTo() function where it accounts the _timelockBalances to the operator address instead of the user specified to address. It assumes they are the same. The corresponding usage in _mintTimelock() which is called from withdrawWithTimelockFrom() uses the user specified ‘from’ address and not the _msgSender(). Therefore the corresponding usage in timelockDepositTo() should be the same.

In the scenario where the operator address != user specified from/to addresses, i.e. meta-transactions, the timelock deposits and withdrawals are made to/from different addresses and so the deposits of timelocked tokens will fail because the operator’s address does not have the required amount of _timelockBalances.

Recommend changing operator to from on L281 of timelockDepositTo() and specifying the scenarios where the role of the operator is applicable and document/implement those accordingly.

asselstine (PoolTogether) disputed:

In the function timelockDepositTo() the msg.sender is using their timelocked funds to re-enter the pool. They can only spend their own funds; they should not be able to spend other user’s funds.

The warden is saying the timelockDepositTo should be callable by anyone and allow them to transfer other user’s funds from the timelock back into tickets. This actually introduces an attack vector.

dmvt (judge) commented:

I think sponsor is misunderstanding warden’s concern here. The issue is not that msg.sender is being checked, but that _msgSender is being checked. Happy to discuss this more if sponsor still disagrees, but I think the concern raised is valid.

[M-05] Actual yield source check on address will succeed for non-existent contract

Submitted by 0xRajeev

Low-level calls call/delegatecall/staticcall return true even if the account called is non-existent (per EVM design). Solidity documentation warns:

“The low-level functions call, delegatecall and staticcall return true as their first return value if the account called is non-existent, as part of the design of the EVM. Account existence must be checked prior to calling if needed.”

The staticcall here will return True even if the _yieldSource contract doesn’t exist at any incorrect-but-not-zero address, e.g. EOA address, used during initialization by accident.

The hack, as commented, to check if it’s an actual yield source contract, will fail if the address is indeed a contract account which doesn’t implement the depositToken function. However, if the address is that of an EOA account, the check will pass here but will revert in all future calls to the yield source forcing contract redeployment after the pool is active. Users will not be able to interact with the pool and abandon it.

Recommend that a contract existence check should be performed on _yieldSource prior to the depositToken function existence hack for determining yield source contract.

asselstine (PoolTogether) confirmed

[M-06] YieldSourcePrizePool_canAwardExternal does not work

Submitted by cmichel

The idea of YieldSourcePrizePool_canAwardExternal seems to be to disallow awarding the interest-bearing token of the yield source, like aTokens, cTokens, yTokens.

“@dev Different yield sources will hold the deposits as another kind of token: such a Compound’s cToken. The prize strategy should not be allowed to move those tokens.”

However, the code checks _externalToken != address(yieldSource) where yieldSource is the actual yield strategy contract and not the strategy’s interest-bearing token. Note that the yieldSource is usually not even a token contract except for ATokenYieldSource and YearnV2YieldSource.

The _canAwardExternal does not work as expected. It might be possible to award the interest-bearing token which would lead to errors and loss of funds when trying to redeem underlying.

There doesn’t seem to be a function to return the interest-bearing token. It needs to be added, similar to depositToken() which retrieves the underlying token.

asselstine (PoolTogether) acknowledged:

This is an interesting one:

  • the yield source interface does not require the deposit be tokenized; the implementation is entirely up to the yield source.
  • the _canAwardExternal is a legacy of older code. Since it had to be included it was set to assume the yield source was tokenized.

Since yield sources are audited and analyzed, I think this is a pretty low risk. Additionally, not all of the yield sources are tokenized (Badger and Sushi are not), so it isn’t a risk for them.

We could have canAwardExternal on the yield source itself, but it would add gas overhead.

aodhgan (PoolTogether) commented:

Could we add an check - function _canAwardExternal(address _externalToken) internal override view returns (bool) { return _externalToken != address(yieldSource) && _externalToken != address(yieldSource.depositToken()) }

asselstine (PoolTogether) commented:

We could add another check, but it’s still arbitrary. The point is that the yield source knows what token the prize pool may or may not hold, so without asking the yield source it’s just a guess.

Let’s leave it as-is

[M-07] Using transferFrom on ERC721 tokens

Submitted by shw

In the function awardExternalERC721 of contract PrizePool, when awarding external ERC721 tokens to the winners, the transferFrom keyword is used instead of safeTransferFrom. If any winner is a contract and is not aware of incoming ERC721 tokens, the sent tokens could be locked.

Recommend consider changing transferFrom to safeTransferFrom at line 602. However, it could introduce a DoS attack vector if any winner maliciously rejects the received ERC721 tokens to make the others unable to get their awards. Possible mitigations are to use a try/catch statement to handle error cases separately or provide a function for the pool owner to remove malicious winners manually if this happens.

asselstine (PoolTogether) confirmed and disagreed with severity:

This issue poses no risk to the Prize Pool, so it’s more of a 1 (Low Risk IMO.

This is just about triggering a callback on the ERC721 recipient. We omitted it originally because we didn’t want a revert on the callback to DoS the prize pool.

However, to respect the interface it makes sense to implement it fully. That being said, if it does throw we must ignore it to prevent DoS attacks.

dmvt (judge) commented:

I agree with the medium risk rating provided by the warden.

Low Risk Findings (18)

[L-01] no check for _stakeToken!=0

Submitted by gpersoon

The initializeYieldSourcePrizePool function of YieldSourcePrizePool.sol has a check to make sure _yieldSource !=0. However, the initialize function of the comparable StakePrizePool.sol doesn’t do this check.

Although unlikely this will introduce problems, it is more consistent to check for 0.

YieldSourcePrizePool.sol L24

 function initializeYieldSourcePrizePool (... IYieldSource _yieldSource) ... {
..
    require(address(_yieldSource) != address(0), "YieldSourcePrizePool/yield-source-zero");
    PrizePool.initialize(

StakePrizePool.sol L20

function initialize ( ..  IERC20Upgradeable _stakeToken)...  {
    PrizePool.initialize(

Recommend adding something like the following in the initialize function of StakePrizePool.sol:

  require(address(_stakeToken) != address(0), "StakePrizePool/stakeToken-zero");

[L-02] Lack of nonReentrant modifier in yield source contracts

_Submitted by shw, also found by gpersoon, 0xRajeev and pauliax__

The YearnV2YieldSource contract prevents the supplyTokenTo, redeemToken, and sponsor functions from being reentered by applying a nonReentrant modifier. Since these contracts share a similar logic, adding a nonReentrant modifier to these functions in all of the yield source contracts is reasonable. However, the same protection is not seen in other yield source contracts.

A nonReentrant modifier in the following functions is missing:

  1. The sponsor function of ATokenYieldSource
  2. The supplyTokenTo and redeemToken function of BadgerYieldSource
  3. The sponsor function of IdleYieldSource
  4. The supplyTokenTo and redeemToken function of SushiYieldSource

Recommend adding a nonReentrant modifier to these functions. For BadgerYieldSource and SushiYieldSource contracts, make them inherit from Openzeppelin’s ReentrancyGuardUpgradeable to use the nonReentrant modifier.

kamescg (PoolTogether) confirmed and patched:

ATokenYieldSource: https://github.com/pooltogether/aave-yield-source/tree/fix/119 SushiYieldSource: https://github.com/pooltogether/sushi-pooltogether/pull/new/fix/119 BadgerYieldSource: https://github.com/pooltogether/badger-yield-source/pull/new/fix/119 IdleYieldSource: https://github.com/pooltogether/idle-yield-source/pull/new/fix/119

[L-03] What is default duration when creditRateMantissa is not set

Submitted by gpersoon

In PrizePool.sol, if the value of _tokenCreditPlans[_controlledToken].creditRateMantissa isn’t set (yet), then the function _estimateCreditAccrualTime returns 0. This means the TimelockDuration is 0 and funds can be withdrawn immediately, defeating the entire timelock mechanism.

Recommend perhaps a different default would be useful.

PrizePool.sol L783

 function _estimateCreditAccrualTime( address _controlledToken,uint256 _principal,uint256 _interest ) internal view returns (uint256 durationSeconds)  {
    uint256 accruedPerSecond = FixedPoint.multiplyUintByMantissa(_principal, _tokenCreditPlans[_controlledToken].creditRateMantissa);
    if (accruedPerSecond == 0) {
      return 0;
    }
    return _interest.div(accruedPerSecond);
  }

PrizePool.sol L710

function _calculateTimelockDuration( address from, address controlledToken, uint256 amount) internal returns (uint256 durationSeconds, uint256 burnedCredit )  {
...
    uint256 duration = _estimateCreditAccrualTime(controlledToken, amount, exitFee);
    if (duration > maxTimelockDuration) {
      duration = maxTimelockDuration;
    }
    return (duration, _burnedCredit);
  }

Recommend considering the default duration for the case _tokenCreditPlans[_controlledToken].creditRateMantissa isn’t set.

[L-04] staticCall to yieldSource.depositToken doesn’t provide any security guarantees

Submitted by GalloDaSballo

The assumption that a yield source is valid, just because it has the method depositToken, is not a security guarantee. I could create any random contract with that function but that is not a guarantee that the contract will behave as intended.

I believe a better solution would be to have a registry, controlled by governance, that accepts the valid yield sources. A valid registry ensures the the yield sources are properly maintained.

In summary: There is no security difference between having the check and not having the check, because the check can be sidelined without any effort and doesn’t truly provide any guarantee of the contract being valid. Having no checks would save you gas. While having a governance registry would guarantee that the yield sources usable are exclusively the community vetted ones.

asselstine (PoolTogether) acknowledged:

It’s possible for a malicious developer to fork our code and create a pool with a rugging yield source. That can’t really be helped either way.

We decide which pools to display on https://app.pooltogether.com, so we can vet pools already.

[L-05] Switch modifier order to consistently place the non-reentrant modifier as the first one

Submitted by 0xRajeev

If a function has multiple modifiers they are executed in the order specified. If checks or logic of modifiers depend on other modifiers this has to be considered in their ordering. PrizePool has functions with multiple modifiers with one of them being non-reentrant which prevents reentrancy on the functions. This should ideally be the first one to prevent even the execution of other modifiers in case of re-entrancies.

While there is no obvious vulnerability currently with non-reentrant being the last modifier in the list, it is safer to place it in the first. This is of slight concern with the deposit functions which have the canAddLiquidity() modifier (before non-reentrant) that makes external calls to get totalSupply of controlled tokens.

For reference, see similar finding in Consensys’s audit of Balancer.

Recommend switching modifier order to consistently place the non-reentrant modifier as the first one to run so that all other modifiers are executed only if the call is non-reentrant.

kamescg (PoolTogether) confirmed and patched:

[L-06] Missing modifier onlyControlledToken may result in undefined/exceptional behavior

Submitted by 0xRajeev

The modifier onlyControlledToken is used for functions that allow the controlledToken address as a parameter to ensure that only whitelisted tokens (ticket and sponsorship) are provided. This is used in all functions except calculateEarlyExitFee().

The use of a non-whitelisted controlledToken will result in calls to potentially malicious token contract and cause undefined behavior for the from user address specified in the call.

Recommend adding missing modifier onlyControlledToken to calculateEarlyExitFee().

kamescg (PoolTogether) confirmed:

This would likely break assumptions made by other contracts when used to get the early exit fee.

For example in Pods to calculate the exit fee. Plus this is called statically from JS frontends to get the fee.

/**
     * @notice Calculate the cost of withdrawing from the Pod if the
     * @param amount Amount of tokens to withdraw when calculating early exit fee.
     * @dev Based of the Pod's total token/ticket balance and totalSupply it calculates the pricePerShare.
     */
    function getEarlyExitFee(uint256 amount) external returns (uint256) {
        uint256 tokenBalance = _podTokenBalance();
        if (amount <= tokenBalance) {
            return 0;
        } else {
            // Calculate Early Exit Fee
            (uint256 exitFee, ) =
                _prizePool.calculateEarlyExitFee(
                    address(this),
                    address(ticket),
                    amount.sub(tokenBalance)
                );

            // Early Exit Fee
            return exitFee;
        }
    }

asselstine (PoolTogether) commented:

@kamescg Rajeev is suggesting to add the modifier onlyControlledToken to calculateEarlyExitFee()

That means it would revert on invalid controlled tokens. It would still be a static call!

That being said this isn’t a deal breaker. We can skip this one and it wouldn’t hurt.

[L-07] Missing calls to init functions of inherited contracts

Submitted by 0xRajeev, also found by shw

Most contracts use the delegateCall proxy pattern and hence their implementations require the use of initialize() functions instead of constructors. This requires derived contracts to call the corresponding init functions of their inherited base contracts. This is done in most places except a few.

The inherited base classes do not get initialized which may lead to undefined behavior.

  • Missing call to __ReentrancyGuard_init in ATokenYieldSource.sol L99-L102 and L59-L61
  • Missing call to __ERC20_init in ATokenYieldSource.sol L59-L61 and L83-L86

Recommend adding missing calls to init functions of inherited contracts.

PierrickGT (PoolTogether) confirmed and patched:

ATokenYieldSource PR: https://github.com/pooltogether/aave-yield-source/pull/18

PierrickGT (PoolTogether) commented:

Has been fixed already for IdleYieldSource: https://github.com/pooltogether/idle-yield-source/blob/master/contracts/IdleYieldSource.sol#L60-#62

PierrickGT (PoolTogether) commented:

YearnV2YieldSource: https://github.com/pooltogether/pooltogether-yearnv2-yield-source/pull/8

[L-08] Unlocked pragma used in multiple contracts

Submitted by shw, also found by 0xRajeev and JMukesh

Some contracts (e.g., PrizePool) use an unlocked pragma (e.g., pragma solidity >=0.6.0 <0.7.0;) which is not fixed to a specific Solidity version. Locking the pragma helps ensure that contracts do not accidentally get deployed using a different compiler version with which they have been tested the most.

Please use grep -R pragma . to find the unlocked pragma statements.

Recommend locking pragmas to a specific Solidity version. Consider the compiler bugs in the following lists and ensure the contracts are not affected by them. It is also recommended to use the latest version of Solidity when deploying contracts (see Solidity docs).

Solidity compiler bugs: Solidity repo - known bugs Solidity repo - bugs by version

kamescg (PoolTogether) confirmed:

PrizePool

Remaining Contracts

[L-09] Missing zero-address checks

Submitted by 0xRajeev

Checking addresses against zero-address during initialization or during setting is a security best-practice. However, such checks are missing in all address variable initializations.

Allowing zero-addresses will lead to contract reverts and force redeployments if there are no setters for such address variables.

Recommend adding zero-address checks for all initializations of address state variables.

kamescg (PoolTogether) commented:

Core

Aave

Sushi

Idle

Badger

[L-10] Overly permissive threshold check allows high yield loss

Submitted by0xRajeev

The Yearn yield source defines maxLosses as: “Max % of losses that the Yield Source will accept from the Vault in BPS” and uses a setter setMaxLosses() to allow owner to set this value. However, the threshold check implemented only checks if this value is less than 10_000 or 100%, which is a good sanity check but allows loss of even 100%. The buffer for the loss is to avoid funds being locked in the Yearn vault in any emergency situation.

If the losses are really high for some reason, it will impact the interest and the prizes.

Perform a tighter upper threshold check to allow a more acceptable max loss value in setMaxLosses()

asselstine (PoolTogether) acknowledged:

Yield sources are controlled by governance, so this isn’t a concern

[L-11] Ignored return values may lead to undefined behavior

Submitted by 0xRajeev

The _depositInVault() returns the value returned from its call to the Yearn vault’s deposit() function. However, the return value is ignored at both call sites in supplyTokenTo() and sponsor().

It is unclear what the intended usage is and how, if any, the return value should be checked. This should perhaps check how much of the full balance was indeed deposited/rejected in the vault by comparing the return value of issued vault shares as commented: “The actual amount of shares that were received for the deposited tokens” because “if deposit limit is reached, tokens will remain in the Yield Source and they will be queued for retries in subsequent deposits.”

Recommend checking return value appropriately or if not, document why this is not necessary.

asselstine (PoolTogether) disputed:

Regardless of how much of the deposit made it into the underlying vault, the depositor will hold the correct number shares. It doesn’t matter if only a portion of the funds made it into the vault.

dmvt (judge) commented:

I think this is a reasonable finding by the warden. If the return value isn’t needed, it should be removed or at least documented that it’s there for no reason. If there is a reason to have the return value, the return value should be considered by the calling functions.

[L-12] Using memory[] parameter without checking its length

Submitted by JMukesh

Using memory array parameters (e.g.uint[] memory) as function parameters can be tricky in Solidity, because an attack is possible with a very large array which will overlap with other parts of the memory.

This an example to show the exploit based on Exploit.sol:

pragma solidity ^0.4.24; // only works with low solidity version

contract test{
    struct Overlap {
        uint field0;
    }
    event log(uint);

  function mint(uint[] memory amounts) public  returns (uint) {   // this can be in any solidity version
       Overlap memory v;
       v.field0 = 1234;
       emit log(amounts[0]); // would expect to be 0 however is 1234
       return 1;
     }

  function go() public { // this part requires the low solidity version
      uint x=0x800000000000000000000000000000000000000000000000000000000000000; // 2^251
      bytes memory payload = abi.encodeWithSelector(this.mint.selector, 0x20, x);
      bool success=address(this).call(payload);
  }
}

Recommend checking the array length before using it

asselstine (PoolTogether) confirmed:

We’re going to remove the timelock functions. The initializer I’m not concerned about.

[L-13] Uneven use of events

Submitted by JMukesh

To track off-chain data it is necessary to use events

In ATokenYieldSource.sol, IdleYieldSource.sol, and yearnV2yieldsource, events are emmitted in supplyTokenTo(), redeemToken(), and sponsor(), but not in BadgerYieldsource.sol and shushiyieldsource.sol

Recommend using events.

asselstine (PoolTogether) confirmed

[L-14] Missing parameter validation

Submitted by cmichel

Some parameters of functions are not checked for invalid values:

  • StakePrizePool.initialize: address _stakeToken not checked for non-zero or contract
  • ControlledToken.initialize: address controller not checked for non-zero or contract
  • PrizePool.withdrawReserve: address to not checked for non-zero, funds will be lost when sending to zero address
  • ATokenYieldSource.initialize: address _aToken, _lendingPoolAddressesProviderRegistry not checked for non-zero or contract
  • BadgerYieldSource.initialize: address badgerSettAddr, badgerAddr not checked for non-zero or contract
  • SushiYieldSource.constructor: address _sushiBar, _sushiAddr not checked for non-zero or contract

Wrong user input or wallets defaulting to the zero addresses for a missing input can lead to the contract needing to redeploy or wasted gas.

Recommend validating the parameters.

PierrickGT (PoolTogether) confirmed and patched:

ATokenYieldSource PR: https://github.com/pooltogether/aave-yield-source/pull/19

PierrickGT (PoolTogether) commented:

BadgerYieldSource PR: https://github.com/pooltogether/badger-yield-source/pull/6

PierrickGT (PoolTogether) commented:

SushiYieldSource PR: https://github.com/pooltogether/sushi-pooltogether/pull/16

PierrickGT (PoolTogether) commented:

ControlledToken PR: https://github.com/pooltogether/pooltogether-pool-contracts/pull/306

PierrickGT (PoolTogether) commented:

StakePrizePool PR: https://github.com/pooltogether/pooltogether-pool-contracts/pull/314

PierrickGT (PoolTogether) commented:

@asselstine (PoolTogether) I’m not sure we want to check for non zero address in the PrizePool withdrawReserve function since this function is only callable by the Reserve and the owner of the Reserve contract. https://github.com/pooltogether/pooltogether-pool-contracts/blob/192429c808ad9714e9e05821386eb926150a009f/contracts/reserve/Reserve.sol#L32 https://github.com/pooltogether/pooltogether-pool-contracts/blob/4449bb2e4216511b7187b1ab420118c30af39eb7/contracts/prize-pool/PrizePool.sol#L473

asselstine (PoolTogether) commented:

Yeah @PierrickGT (PoolTogether) I don’t think the withdrawReserve needs to do the check. Many tokens reject on transfer to zero anyway.

kamescg (PoolTogether) commented:

LGTM

[L-15] ATokenYieldSource mixes aTokens and underlying when redeeming

Submitted by cmichel

The ATokenYieldSource.redeemToken function burns aTokens and sends out underlying; however, it’s used in a reverse way in the code: The balanceDiff is used as the depositToken that is transferred out but it’s computed on the aTokens that were burned instead of on the depositToken received.

It should not directly lead to issues as aTokens are 1-to-1 with their underlying but we still recommend doing it correctly to make the code more robust against any possible rounding issues.

Recommend computing balanceDiff on the underyling balance (depositToken), not on the aToken. Then, subtract the actual burned aTokens from the user shares.

PierrickGT (PoolTogether) confirmed:

I agree that we should compute balanceDiff on the underlying balance. Regarding the burn of the user’s shares, we should keep it as is to verify first that the user has enough shares. This way if he doesn’t, the code execution will revert before funds are withdrawn from Aave.

PierrickGT (PoolTogether) commented:

PR: https://github.com/pooltogether/aave-yield-source/pull/17

[L-16] BadgerYieldSource and SushiYieldSource are not upgradeable

Submitted by shw

The contracts BadgerYieldSource and SushiYieldSource are not upgradeable since they do not inherit from any Openzeppelin’s upgradeable contract (e.g., ERC20Upgradeable) as the other yield source contracts.

Recommend making BadgerYieldSource and SushiYieldSource upgradable.

asselstine (PoolTogether) disputed:

We don’t want them to be upgradeable! It’s a feature not a bug.

[L-17] onERC721Received not implemented in PrizePool

Submitted by shw

The PrizePool contract does not implement the onERC721Received function, which is considered a best practice to transfer ERC721 tokens from contracts to contracts. The absence of this function could prevent PrizePool from receiving ERC721 tokens from other contracts via safeTransferFrom.

Consider adding an implementation of the onERC721Received function in PrizePool.

kamescg (PoolTogether) confirmed:

https://github.com/pooltogether/pooltogether-pool-contracts/pull/new/fix/118

https://github.com/pooltogether/pooltogether-pool-contracts/pull/300

[L-18] Lack of event emission after critical initialize() functions

Submitted by 0xRajeev

Most contracts use initialize() functions instead of constructor given the delegatecall proxy pattern. While most of them emit an event in the critical initialize() functions to record the init parameters for off-chain monitoring and transparency reasons, Ticket.sol nor its base class ControlledToken.sol emit such an event in their initialize() functions.

These contracts are initialized but their critical init parameters (name, symbol, decimals and controller address) are not logged for any off-chain monitoring.

See similar Medium-severity Finding M01 in OpenZeppelin’s audit of UMA protocol.

Recommend emitting an initialized event in Ticket.sol and ControlledToken.sol logging their init parameters.

asselstine (PoolTogether) confirmed but disagree with severity:

This is just event emission; it’s severity is 0 (Non-critical).

kamescg (PoolTogether) patched:

dmvt (judge) commented:

I’m going to split the difference here. Events are important for various reasons. In this case, due to the proxy pattern, the creation of the contract in the initialize function happen at the same time, making it trivial for a user to go back and look at the initialization parameters in the creation transaction.

Non-Critical findings (6)

Gas Optimizations (34)

Disclosures

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C4 Contests incentivize the discovery of exploits, vulnerabilities, and bugs in smart contracts. Security researchers are rewarded at an increasing rate for finding higher-risk issues. Contest submissions are judged by a knowledgeable security researcher and solidity developer and disclosed to sponsoring developers. C4 does not conduct formal verification regarding the provided code but instead provides final verification.

C4 does not provide any guarantee or warranty regarding the security of this project. All smart contract software should be used at the sole risk and responsibility of users.