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Sherlock contest
Findings & Analysis Report

2022-05-17

Table of contents

Overview

About C4

Code4rena (C4) is an open organization consisting of security researchers, auditors, developers, and individuals with domain expertise in smart contracts.

A C4 audit 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 audit contest outlined in this document, C4 conducted an analysis of the Sherlock smart contract system written in Solidity. The audit contest took place between January 20—January 26 2022.

Wardens

40 Wardens contributed reports to the Sherlock contest:

  1. GreyArt (hickuphh3 and itsmeSTYJ)
  2. OriDabush
  3. egjlmn1
  4. kirk-baird
  5. pauliax
  6. hyh
  7. static
  8. sirhashalot
  9. ye0lde
  10. danb
  11. hack3r-0m
  12. cccz
  13. harleythedog
  14. kenzo
  15. Dravee
  16. 0x0x0x
  17. Jujic
  18. 0x1f8b
  19. RamiRond
  20. gzeon
  21. Tomio
  22. bobi
  23. Fitraldys
  24. haku
  25. robee
  26. Funen
  27. Afanasyevich
  28. p4st13r4 (0x69e8 and 0xb4bb4)
  29. wuwe1
  30. defsec
  31. Ruhum
  32. pedroais
  33. ych18
  34. byterocket (pseudorandom and pmerkleplant)
  35. saian
  36. ACai
  37. GeekyLumberjack

This contest was judged by Jack the Pug.

Final report assembled by liveactionllama.

Summary

The C4 analysis yielded an aggregated total of 14 unique vulnerabilities and 71 total findings. All of the issues presented here are linked back to their original finding.

Of these vulnerabilities, 1 received a risk rating in the category of HIGH severity, 4 received a risk rating in the category of MEDIUM severity, and 9 received a risk rating in the category of LOW severity.

C4 analysis also identified 33 non-critical recommendations and 24 gas optimizations.

Scope

The code under review can be found within the C4 Sherlock contest repository, and is composed of 8 smart contracts written in the Solidity programming language and includes 612 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 (1)

[H-01] first user can steal everyone else’s tokens

Submitted by egjlmn1, also found by OriDabush

A user who joins the systems first (stakes first) can steal everybody’s tokens by sending tokens to the system externally. This attack is possible because you enable staking a small amount of tokens.

Proof of Concept

See the following attack:

  1. the first user (user A) who enters the system stake 1 token
  2. another user (user B) is about to stake X tokens
  3. user A frontrun and transfer X tokens to the system via ERC20.transfer
  4. user B stakes X tokens, and the shares he receives is:

shares = (_amount * totalStakeShares_) / (totalTokenBalanceStakers() - _amount); shares = (X * 1) / (X + 1 + X - X) = X/(X+1) = 0 meaning all the tokens he staked got him no shares, and those tokens are now a part of the single share that user A holds 5. user A can now redeem his shares and get the 1 token he staked, the X tokens user B staked, and the X tokens he ERC20.transfer to the system because all the money in the system is in a single share that user A holds.

In general, since there is only a single share, for any user who is going to stake X tokens, if the system has X+1 tokens in its balance, the user won’t get any shares and all the money will go to the attacker.

Force users to stake at least some amount in the system (Uniswap forces users to pay at least 1e18) That way the amount the attacker will need to ERC20.transfer to the system will be at least X*1e18 instead of X which is unrealistic

Evert0x (Sherlock) confirmed and commented:

Thanks. I agree it’s an issue that could theoretically affect all deposits.

Evert0x (Sherlock) resolved

Medium Risk Findings (4)

[M-01] SherlockClaimManager: Incorrect amounts needed and paid for escalated claims

Submitted by GreyArt, also found by static

When escalating claims, the documentation states that the protocol agent is required to pay and stake a certain amount for the process. If the covered protocol is proven correct, then the amount specified by the claim will be paid out. They will also receive the stake amount back in full. If the covered protocol’s escalation is not successful, then the amount specified by the claim is not paid out and the stake amount is not returned.

The protocol agent is reasonably expected to pay the following:

  • The stake (BOND) and
  • UMA’s final fee

In reality, the protocol agent will end up paying more, as we shall see in the proof of concept.

Proof of Concept

Let us assume the following:

  • BOND = 9600 as defined in SherlockClaimManager
  • umaFee = 400 (at the time of writing, this value has been updated to 1500 USDC: see [Store.computeFinalFee(usdc)](https://etherscan.io/address/0x54f44eA3D2e7aA0ac089c4d8F7C93C27844057BF#readContract)).

On invoking escalate(), the following amounts are required:

  1. BOND + umaFee = 9600 + 400 will be transferred to UMA when invoking requestAndProposePriceFor()
// Taken from https://github.com/UMAprotocol/protocol/blob/master/packages/core/contracts/oracle/implementation/SkinnyOptimisticOracle.sol
// Only relevant lines are referenced
uint256 finalFee = _getStore().computeFinalFee(address(currency)).rawValue;
request.finalFee = finalFee;
totalBond = request.bond.add(request.finalFee);
if (totalBond > 0) currency.safeTransferFrom(msg.sender, address(this), totalBond);
  1. Another BOND + umaFee = 9600 + 400 will be transfered when invoking disputePriceFor()

totalBond = request.bond.add(request.finalFee);
if (totalBond > 0) request.currency.safeTransferFrom(msg.sender, address(this), totalBond);

However, what’s important to note is that UMA will “burn” half of the BOND collected + final fee. This will go against the claim that the protocol agent will be able to reclaim his stake in full.

StoreInterface store = _getStore();

// Avoids stack too deep compilation error.
{
    // Along with the final fee, "burn" part of the loser's bond to ensure that a larger bond always makes it
    // proportionally more expensive to delay the resolution even if the proposer and disputer are the same
    // party.
    uint256 burnedBond = _computeBurnedBond(disputedRequest);

    // The total fee is the burned bond and the final fee added together.
    uint256 totalFee = request.finalFee.add(burnedBond);

    if (totalFee > 0) {
        request.currency.safeIncreaseAllowance(address(store), totalFee);
        _getStore().payOracleFeesErc20(address(request.currency), FixedPoint.Unsigned(totalFee));
    }
}

function _computeBurnedBond(Request memory request) private pure returns (uint256) {
  // burnedBond = floor(bond / 2)
  return request.bond.div(2);
}

We finally note that on settlement, the eventual payout is

// Winner gets:
// - Their bond back.
// - The unburned portion of the loser's bond: proposal bond (not including final fee) - burned bond.
// - Their final fee back.
// - The request reward (if not already refunded -- if refunded, it will be set to 0).
payout = request.bond.add(request.bond.sub(_computeBurnedBond(settledRequest))).add(request.finalFee).add(
    request.reward
);
request.currency.safeTransfer(disputeSuccess ? request.disputer : request.proposer, payout);

Hence, in reality, the protocol agent will only receive 9600 * 2 - 4800 + 400 = 14800 should the dispute be successful. We note that the burnt amount of 4800 / 2 + 400 = 5200 has been taken by UMA.

One can further verify this behaviour by looking at a past resolution of another protocol:

https://dashboard.tenderly.co/tx/main/0x0f03f73a2093e385146791e8f2739dbc04b39145476d6940776680243460100f/debugger?trace=0.6.1

The above example has a bond is 0.0075 ETH, with UMA’s final fee being 0.2 ETH. We see that UMA takes 0.2 + 0.5 * 0.0075 = 0.02375 ETH.

Thus, we see that the protocol agent will be charged disproportionally to what is expected.

We suggest changing the parameters of requestAndProposePriceFor() to

UMA.requestAndProposePriceFor(
  UMA_IDENTIFIER, // Sherlock ID so UMA knows the request came from Sherlock
  claim.timestamp, // Timestamp to identify the request
  claim.ancillaryData, // Ancillary data such as the coverage agreement
  TOKEN, // USDC
  BOND, // While sherlock handles rewards on its own, we use the BOND as the reward
  // because using it as UMA's bond would result in 0.5 * BOND charged by UMA excluding final fee
  1, // Ideally 0, but 0 = final fee used. Hence, we set it to the next lowest 
  // possible value
  LIVENESS, // Proposal liveness
  address(sherlockCore), // Sherlock core address
  0 // price
);

where BOND becomes the reward and the actual bond for UMA is 1. Ideally, it should be set to 0, but if set as such, UMA interprets it to use the final fee as the bond amount instead.

[request.bond = bond != 0 ? bond : finalFee;](https://github.com/UMAprotocol/protocol/blob/master/packages/core/contracts/oracle/implementation/SkinnyOptimisticOracle.sol#L321)

This way, the actual amount required from the protocol agent is the BOND + 2 * (USDC wei + umaFee) for the process. He will additionally be returned his BOND + umaFee if his dispute is successful.

Evert0x (Sherlock) disagreed with High severity and commented:

Non critical as documentation is incorrect about this.

Jack the Pug (judge) decreased severity to Medium and commented:

Downgrading to Med as it’s mostly because the documentation is incorrect.

rcstanciu (Sherlock) commented:

@jack-the-pug The docs have been updated to explain the correct burned amount.

Jack the Pug (judge) commented:

Thank you @rcstanciu

While I agreed that the issue only impacts a small sum of users and the impact is not significant. And the root cause of this issue may not be a wrong implementation but actual wrong documentation.

However, I still tend to make this a Med rather than a Low for the following reasons:

  1. A wrong documentation is arguably indistinguishable from a wrong implementation that actually violates the intention of the design, especial from an outsider’s pov;
  2. This write-up indicates a dedicated and in-depth effort of the warden by digging into the documentation and trying to understand the intention and cross-compare with the actual implementation to find any differences. As a judge, part of my job is to make sure that the wardens’ findings are being rewarded in a just and fair manner.

Therefore, I’m keeping this as a Med and I encourage the future wardens to continue finding the inconsistency between the documentation and the implementation.

Keep up the good work! GreyArt is a great name btw.

[M-02] tokenBalanceOfAddress of nftOwner becomes permanently incorrect after arbRestake

Submitted by hyh, also found by GreyArt and hack3r-0m

Successful arbRestake performs _redeemShares for arbRewardShares amount to extract the arbitrager reward. This effectively reduces shares accounted for an NFT, but leaves untouched the addressShares of an nftOwner.

As a result the tokenBalanceOfAddress function will report an old balance that existed before arbitrager reward was slashed away. This will persist if the owner will transfer the NFT to someone else as its new reduced shares value will be subtracted from addressShares in _beforeTokenTransfer, leaving the arbitrage removed shares permanently in addressShares of the NFT owner, essentially making all further reporting of his balance incorrectly inflated by the cumulative arbitrage reward shares from all arbRestakes happened to the owner’s NFTs.

Proof of Concept

arbRestake redeems arbRewardShares, which are a part of total shares of an NFT:

Sherlock.sol#L673

This will effectively reduce the stakeShares:

Sherlock.sol#L491

But there is no mechanics in place to reduce addressShares of the owner apart from mint/burn/transfer, so addressShares will still correspond to NFT shares before arbitrage. This discrepancy will be accumulated further with arbitrage restakes.

Add a flag to _redeemShares indicating that it was called for a partial shares decrease, say isPartialRedeem, and do addressShares[nftOwner] -= _stakeShares when isPartialRedeem == true.

Another option is to do bigger refactoring, making stakeShares and addressShares always change simultaneously.

Evert0x (Sherlock) confirmed and commented:

This is a legit issue and needs to be addressed. I think we choose to delete this functionality all together.

The function has some potential future benefit but it might be too little benefit to make these relatively complex changes that make the code harder to understand.

Evert0x (Sherlock) resolved

[M-03] updateYieldStrategy will freeze some funds with the old Strategy if yieldStrategy fails to withdraw all the funds because of liquidity issues

Submitted by hyh, also found by harleythedog, GreyArt, and pauliax

Part of the funds held with the strategy can be frozen if the current strategy has tight liquidity when updateYieldStrategy is run as this function makes an attempt to withdraw all the funds and then unconditionally removes the strategy.

The Sherlock to YieldStrategy link will be broken as a result: Sherlock points to the new Strategy, while old Strategy still allows only this Sherlock contract to withdraw.

This way back and forth switches will be required in the future to return the funds: withdraw all from new strategy and switch to old, withdraw all from old and point to new one again, reinvest there.

Proof of Concept

In peer-to-peer lending protocols it is not always possible for the token supplier to withdraw all what’s due. This happens on high utilization of the market (when it has a kind of liquidity crunch).

This way yieldStrategy.withdrawAll is not guaranteed to obtain all the funds held with the strategy:

Sherlock.sol#L286

The worst case scenario here seems to be the remainder funds to be left frozen within the strategy.

For example, AaveV2Strategy withdraw and withdrawAll have onlySherlockCore modifier:

AaveV2Strategy.sol#L78-100

While Sherlock core is immutable for the Strategy by default:

Manager.sol#L26-41

Consider implementing a new method that fails whenever a strategy cannot withdraw all what’s due now, and rename current implementation to, for example, forceUpdateYieldStrategy, to have a degree of flexibility around liquidity issues.

Also, to avoid back and forth switching, a strategy argument can be introduced to yieldStrategyWithdrawAll to allow withdrawals from any (not only current) yieldStrategy:

Sherlock.sol#L322

Now:

function yieldStrategyWithdrawAll() external override onlyOwner {

To be (if _yieldStrategy is zero then utilize current):

function yieldStrategyWithdrawAll(IStrategyManager _yieldStrategy) external override onlyOwner {

Evert0x (Sherlock) disagreed with Medium severity and commented:

I think this should be low risk but it’s an interesting feature to add

Jack the Pug (judge) commented:

I think this worths a Med, the scenario is not impossible to happen, and the handling in the current implementation is quite rough.

[M-04] Reenterancy in _sendSherRewardsToOwner()

Submitted by kirk-baird

This is a reentrancy vulnerability that would allow the attacker to drain the entire SHER balance of the contract.

Note: this attack requires gaining control of execution sher.transfer() which will depend on the implementation of the SHER token. Control may be gained by the attacker if the contract implements ERC777 or otherwise makes external calls during transfer().

Proof of Concept

See _sendSherRewards

  function _sendSherRewardsToOwner(uint256 _id, address _nftOwner) internal {
    uint256 sherReward = sherRewards_[_id];
    if (sherReward == 0) return;

    // Transfers the SHER tokens associated with this NFT ID to the address of the NFT owner
    sher.safeTransfer(_nftOwner, sherReward);
    // Deletes the SHER reward mapping for this NFT ID
    delete sherRewards_[_id];
  }

Here sherRewards are deleted after the potential external call is made in sher.safeTransfer(). As a result if an attacker reenters this function sherRewards_ they will still maintain the original balance of rewards and again transfer the SHER tokens.

As _sendSherRewardsToOwner() is internal the attack can be initiated through the external function ownerRestake() see here.

Steps to produce the attack:

  1. Deploy attack contract to handle reenterancy
  2. Call initialStake() from the attack contract with the smallest period
  3. Wait for period amount of time to pass
  4. Have the attack contract call ownerRestake(). The attack contract will gain control of the (See note above about control flow). This will recursively call ownerRestake() until the balance of Sherlock is 0 or less than the user’s reward amount. Then allow reentrancy loop to unwind and complete.

Reentrancy can be mitigated by one of two solutions.

The first option is to add a reentrancy guard like nonReentrant the is used in SherlockClaimManager.sol.

The second option is to use the checks-effects-interactions pattern. This would involve doing all validation checks and state changes before making any potential external calls. For example the above function could be modified as follows.

  function _sendSherRewardsToOwner(uint256 _id, address _nftOwner) internal {
    uint256 sherReward = sherRewards_[_id];
    if (sherReward == 0) return;

    // Deletes the SHER reward mapping for this NFT ID
    delete sherRewards_[_id];

    // Transfers the SHER tokens associated with this NFT ID to the address of the NFT owner
    sher.safeTransfer(_nftOwner, sherReward);
  }

Additionally the following functions are not exploitable however should be updated to use the check-effects-interations pattern.

  • Sherlock._redeemShares() should do _transferTokensOut() last.
  • Sherlock.initialStake() should do token.safeTransferFrom(msg.sender, address(this), _amount); last
  • SherClaim.add() should do sher.safeTransferFrom(msg.sender, address(this), _amount); after updating userClaims
  • SherlockProtocolManager.depositToActiveBalance() should do token.safeTransferFrom(msg.sender, address(this), _amount); after updating activeBalances

Evert0x (Sherlock) confirmed, but disagreed with High severity and commented:

Good find. I think it’s med-risk as it depends on the implementation of SHER token (does it allow callbacks).

Jack the Pug (judge) decreased severity to Medium and commented:

Downgrade to Med as the SHER token is a known token that currently comes with no such hooks like ERC777.

Evert0x (Sherlock) resolved

Low Risk Findings (9)

Non-Critical Findings (33)

Gas Optimizations (24)

Disclosures

C4 is an open organization governed by participants in the community.

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.