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Canto v2 contest
Findings & Analysis Report


Table of contents


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 Canto v2 smart contract system written in Solidity. The audit contest took place between June 28—July 2 2022.

Note: this audit contest originally ran under the name New Blockchain v2.


51 Wardens contributed reports to the Canto v2 contest:

  1. Picodes
  2. 0x1f8b
  3. ladboy233
  4. __141345__
  5. Lambda
  6. Critical
  7. Chom
  8. cccz
  9. defsec
  10. csanuragjain
  11. 0x52
  12. TomJ
  13. rfa
  14. Funen
  15. Bnke0x0
  16. 0x29A (0x4non and rotcivegaf)
  17. c3phas
  18. fatherOfBlocks
  19. Soosh
  20. ynnad
  21. grGred
  22. mrpathfindr
  23. Sm4rty
  24. slywaters
  25. ignacio
  26. sach1r0
  27. samruna
  28. 0v3rf10w
  29. asutorufos
  30. m_Rassska
  31. Waze
  32. 0xArshia
  33. ajtra
  34. durianSausage
  35. Fitraldys
  36. Noah3o6
  37. RedOneN
  38. Rohan16
  39. zzzitron
  40. oyc_109
  41. AlleyCat
  42. Meera
  43. 0xKitsune
  44. TerrierLover
  45. JC
  46. hake
  47. aysha
  48. Limbooo
  49. Tomio
  50. cRat1st0s

This contest was judged by Alex the Entreprenerd.

Final report assembled by liveactionllama.


The C4 analysis yielded an aggregated total of 14 unique vulnerabilities. Of these vulnerabilities, 8 received a risk rating in the category of HIGH severity and 6 received a risk rating in the category of MEDIUM severity.

Additionally, C4 analysis included 34 reports detailing issues with a risk rating of LOW severity or non-critical. There were also 34 reports recommending gas optimizations.

All of the issues presented here are linked back to their original finding.


The code under review can be found within the C4 Canto v2 contest repository, and is composed of 15 smart contracts written in the Solidity programming language and includes 2,459 lines of Solidity code. One Cosmos SDK blockchain is also included.

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/non-critical.

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 (8)

[H-01] Total supply can be incorrect in ERC20

Submitted by Picodes

_totalSupply can be initialized to something different than 0, which would lead to an inaccurate total supply, and could easily break integrations, computations of market cap, etc.

Proof of Concept

If the constructor is called with _initialSupply = 1000, the totalSupply will be initialized to 1000.

As all the others computations are correct, there will be for ever a discrepancy of 1000 between the real total supply and the one of the contract.

Remove _initialSupply.

nivasan1 (Canto) acknowledged

Alex the Entreprenerd (judge) commented:

Same bug as from Canto V1. Recommend the sponsor to just set to 0 and remove the assignment from the constructor


Please note: the following additional discussions took place after judging and awarding were finalized. As such, this report will leave this finding in its originally assessed risk category as it simply reflects a snapshot in time.

shung (warden) commented:

In the provided contracts, v2 repo is included:

However, in this submission, the second line of code provided links to the v1 repo. The described issue only exists in v1 version. In v2 version the issue does not exist because msg.sender balance is updated along with the total supply:

Therefore this finding seems invalid.

Alex the Entreprenerd (judge) commented:

@shung - You’re right, I must have missed the line with the mitigation.

The current code will update the _totalSupply and will give the balance to the deployer.

This is a mistake on my part and the finding should have been closed as invalid as it was mitigated in the V2 code in scope.

Alex the Entreprenerd (judge) commented:

While a nitpick I’d recommend changing the code to use _mint as it the code in scope will not emit an event which may cause issues if you’re tracking via theGraph or similar.

Either way I made a mistake here, sorry about that.

[H-02] Deny of service in CNote.doTransferOut

Submitted by 0x1f8b, also found by Lambda

The CNote.doTransferOut method is susceptible to denial of service.

Proof of Concept

The logic of the doTransferOut method in CNote is as follows:

    function doTransferOut(address payable to, uint amount) virtual override internal {
        require(address(_accountant) != address(0));
        EIP20Interface token = EIP20Interface(underlying);
        if (to != address(_accountant)) {
            uint err = _accountant.supplyMarket(amount);
            if (err != 0) { revert AccountantRedeemError(amount); }
        token.transfer(to, amount);
        bool success;
        assembly {
            switch returndatasize()
                case 0 { success := not(0) }
                case 32 { 
                    returndatacopy(0, 0, 32)
                    success := mload(0)
                default { revert(0, 0) }
        require(success, "TOKEN_TRANSFER_OUT_FAILED");
        require(token.balanceOf(address(this)) == 0, "cNote::doTransferOut: TransferOut Failed"); // <-- ERROR

The doTransferOut method receives an amount which is transferred to to, after it the balance of the contract token is checked to be equal to zero or the transaction will be reverted.

In the following cases a denial of service will occur:

  • In the case that is used an amount different than the balance, the transaction will be reverted.
  • In the case that an attacker front-runs the transaction and sends one token more than the established by the _accountant.
  • In case of increasing balance tokens like mDai that constantly change their balance, the established by the _accountant will be different when the transaction is persisted.
  • Use balance differences instead of the 0 check.

nivasan1 (Canto) confirmed

Alex the Entreprenerd (judge) commented:

The warden has shown how, anyone, via a simple transfer of underlying can deny the functionality of doTransferOut.

Because the function is used in multiple functions inherited from CToken, and the griefing can be easily run by anyone, I believe High Severity to be appropriate.

[H-03] Underlying asset price oracle for CToken in BaseV1-periphery is inaccurate

Submitted by ladboy233

Underlying asset price oracle for CToken in BaseV1-periphery is inaccurate.

Proof of Concept

    function getUnderlyingPrice(CToken ctoken) external override view returns(uint price) {
        IBaseV1Pair pair;
        uint8 stable;
        bool stablePair;
        address underlying;

        if (compareStrings(ctoken.symbol(), "cCANTO")) {
            stable = 0;
            underlying = address(wcanto);
        //set price statically to 1 when the Comptroller is retrieving Price
        else if (compareStrings(ctoken.symbol(), "cNOTE") && msg.sender == Comptroller) {
            return 1; // Note price is fixed to 1

We should not be return 1. 1 is 1 wei. We should be 10 ** 18

Tools Used


We can return 10 ** 18

nivasan1 (Canto) confirmed

Alex the Entreprenerd (judge) commented:

The warden has shown what probably is a developer mistake, which will scale down the price of the cNOTE token to 1.

The sponsor confirms.

It should be noted that if cNOTE always returns 1e18 then the math for diff will always be zero, meaning the interest will exclusively be dictated by baseRatePerYear.

Because the sponsor confirms, and because the comments point to values “scaled by 1e18” I believe the finding to be valid, and since the “math is wrong”, I do agree with High Severity.

[H-04] Oracle periodSize is very low allowing the TWAP price to be easily manipulated

Submitted by 0x52, also found by __141345__, Chom, csanuragjain, and ladboy233

TWAP oracle easily manipulated.

Proof of Concept

periodSize is set to 0 meaning that the oracle will take a new observation every single block, which would allow an attacker to easily flood the TWAP oracle and manipulate the price.

Increase periodSize to be greater than 0, 1800 is typically standard.

nivasan1 (Canto) confirmed

Alex the Entreprenerd (judge) commented:

The warden has identified a constant set to zero for the time in between TWAP observations.

Because the code change:

  • Is a mistake (evidenced by the comments)
  • Causes the TWAP (already put into question in previous contest) to become a Spot Oracle
  • There’s no way to remediate as the variable is constant
  • The change will end up resulting in a manipulatable quote which will impact getUnderlyingPrice

I agree with High Severity.

[H-05] The LP pair underlying price quote could be manipulated

Submitted by __141345__

The underlying price for LP pool pair can be manipulated. This kind of price mainpulation happened before, can be found here: Warp Fincance event.

Which may lead to the exploit of the pool by a malicious user.

Proof of Concept

file: lending-market-v2/contracts/Stableswap/BaseV1-periphery.sol
522-526, 198-217:

            uint price0 = (token0 != USDC) ? IBaseV1Pair(pairFor(USDC, token0, stable0)).quote(token0, 1, 8) : 1;
            uint price1 = (token1 != USDC) ? IBaseV1Pair(pairFor(USDC, token1, stable1)).quote(token1, 1, 8) : 1;
            // how much of each asset is 1 LP token redeemable for
            (uint amt0, uint amt1) = quoteRemoveLiquidity(token0, token1, stablePair, 1);
            price = amt0 * price0 + amt1 * price1;

    function quoteRemoveLiquidity(
        address tokenA,
        address tokenB,
        bool stable,
        uint liquidity
    ) public view returns (uint amountA, uint amountB) {
        // create the pair if it doesn"t exist yet
        address _pair = IBaseV1Factory(factory).getPair(tokenA, tokenB, stable);

        if (_pair == address(0)) {
            return (0,0);

        (uint reserveA, uint reserveB) = getReserves(tokenA, tokenB, stable);
        uint _totalSupply = erc20(_pair).totalSupply();

        amountA = liquidity * reserveA / _totalSupply; // using balances ensures pro-rata distribution
        amountB = liquidity * reserveB / _totalSupply; // using balances ensures pro-rata distribution


The price of the LP pair is determined by the TVL of the pool, given by:
amt0 * price0 + amt1 * price1. However, when a malicious user dumps large amount of any token into the pool, the whole TVL will be significantly increased, which leads to inproper calculation of the price.

A differenct approach to calculate the LP price can be found here.

nivasan1 (Canto) confirmed

Alex the Entreprenerd (judge) commented:

The warden has shown how the LP Token Pricing math is incorrect, this is a mispricing that historically has resulted in total loss of funds and the subject is well known.

Remediation can be attained by following the guide linked:

Because the:

  • Math is incorrect
  • Exploit allows anyone to inflate prices within 1 block (no risk)

High Severity is appropriate.

[H-06] getBorrowRate returns rate per year instead of per block

Submitted by Lambda, also found by Chom

According to the documentation in InterestRateModel, getBorrowRate has to return the borrow rate per block and the function borrowRatePerBlock in CToken directly returns the value of getBorrowRate. However, the rate per year is returned for NoteInterest. Therefore, using NoteInterest as an interest model will result in completely wrong values.

Return baseRatePerBlock.

nivasan1 (Canto) confirmed

Alex the Entreprenerd (judge) commented:

The warden has shown that the borrowRate is returning per-year values instead of per-block values.

The effect of this is that the accounting will be magnified massively.

While impact should be mostly loss of value to interest and incorrect yield, due to the math being wrong I do agree with High Severity.

[H-07] Deny of service in AccountantDelegate.sweepInterest

Submitted by 0x1f8b, also found by Critical

The sweepInterest method is susceptible to denial of service.

Proof of Concept

The logic of the sweepInterest method relative to the treasury is as follows:

		bool success = cnote.transfer(treasury, amtToSweep);
		if (!success) { revert  SweepError(treasury , amtToSweep); }
		TreasuryInterface Treas = TreasuryInterface(treasury);
		require(cnote.balanceOf(treasury) == 0, "AccountantDelegate::sweepInterestError");

As you can see, amtToSweep is passed to it and redeem that amount. Later it is checked that the balance of cnote in the treasury address must be 0. However, all calculations related to amtToSweep come out of the balance of address(this) so if a third party sends a single token cnote to the address of treasury the method will be denied.

  • Check that the balance is the same after and before the bool success = cnote.transfer(treasury, amtToSweep);

nivasan1 (Canto) confirmed

Alex the Entreprenerd (judge) commented:

The warden has shown how, due to an incorrect invariant (treasury having zero cNote), any griefer can permanently brick the sweepInterest function.

The finding shows how a loss of yield can be achieved, so Medium Severity would be in order.

However, because:

  • an invariant was broken
  • the tokens cannot be withdrawn via an alternative method

I believe High Severity to be more appropriate.

[H-08] AccountantDelegate: The sweepInterest function sweeps an incorrect number of cnote

Submitted by cccz, also found by Critical

In the sweepInterest function of the AccountantDelegate contract, the number of cnote sent to treasury should be cNoteToSweep instead of amtToSweep, as amtToSweep will normally be smaller than cNoteToSweep, which will cause the interest to be locked in the in the contract.

		uint amtToSweep = sub_(cNoteAmt, noteDiff); // amount to sweep in Note, 
		uint cNoteToSweep = div_(amtToSweep, exRate); // amount of cNote to sweep = amtToSweep(Note) / exRate

		cNoteToSweep = (cNoteToSweep > cNoteBal) ? cNoteBal :  cNoteToSweep; 
		bool success = cnote.transfer(treasury, amtToSweep);
		if (!success) {
			revert  SweepError(treasury , amtToSweep); //handles if transfer of tokens is not successful

		TreasuryInterface Treas = TreasuryInterface(treasury);

Proof of Concept

		uint amtToSweep = sub_(cNoteAmt, noteDiff); // amount to sweep in Note, 
		uint cNoteToSweep = div_(amtToSweep, exRate); // amount of cNote to sweep = amtToSweep(Note) / exRate

		cNoteToSweep = (cNoteToSweep > cNoteBal) ? cNoteBal :  cNoteToSweep; 
-		bool success = cnote.transfer(treasury, amtToSweep);
+               bool success = cnote.transfer(treasury, cNoteToSweep);
		if (!success) {
-			revert  SweepError(treasury , amtToSweep); //handles if transfer of tokens is not successful
+                       revert  SweepError(treasury , cNoteToSweep); //handles if transfer of tokens is not successful

		TreasuryInterface Treas = TreasuryInterface(treasury);
-		Treas.redeem(address(cnote),amtToSweep);
+               Treas.redeem(address(cnote),cNoteToSweep);

nivasan1 (Canto) confirmed

Alex the Entreprenerd (judge) commented:

The warden has shown that the wrong variable is being used as the transferAmount.

Because cNoteToSweep << amtToSweep there will be many instances in which the function will revert.

Because the finding shows incorrect functionality, which can leave the tokens stuck indefinitely, I agree with High Severity.

Medium Risk Findings (6)

[M-01] Stableswap - Deadline do not work

Submitted by Picodes, also found by cccz, Funen, ladboy233, Soosh, and zzzitron

The ensure modifier is commented, so deadlines will not work when passing orders, breaking this functionality.

nivasan1 (Canto) acknowledged

Alex the Entreprenerd (judge) commented:

The warden has shown how, due to a comment, the modifier deadline doesn’t work.

Because Front-running is a key aspect of AMM design, deadline is a useful tool to ensure that your tx cannot be “saved for later”.

Due to the removal of the check, it may be more profitable for a miner to deny the transaction from being mined until the transaction incurs the maximum amount of slippage.

The lack of deadline means that the tx can be withheld indefinitely at the advantage of the miner.

For those reasons I agree with Medium Severity.

[M-02] Multiple initialization in NoteInterest

Submitted by 0x1f8b, also found by oyc_109

The initialize method of the contract NoteInterest can be initialized multiple times.

Proof of Concept

The method initialize of the contract NoteInterest looks like this:

    function initialize(address cnoteAddr, address oracleAddress) external {
        if (msg.sender != admin ) {
            revert SenderNotAdmin(msg.sender);
        address oldPriceOracle = address(oracle);
        cNote = CErc20(cnoteAddr);
        oracle = PriceOracle(oracleAddress);
        emit NewPriceOracle(oldPriceOracle, oracleAddress);

Nothing prevents it from being initialized again and altering the initial values of the contract. This allows the government, unnecessarily, to be able to perform attacks such as altering the logic of the updateBaseRate method.

Add a require to check that was not already initialized.

nivasan1 (Canto) disputed and commented:

It is not clear how governance would be able to modify the logic in updateBaseRate? All that could be changed is the price oracle that the NoteInterest Model references.

Alex the Entreprenerd (judge) commented:

The warden has shown that the function initialize can be called multiple times by governance.

This could cause undefined behaviour (e.g. change the token address), which could cause loss of funds.

Because of the system setup, it would be best to make sure that initialize can only be called once.

Because the finding can cause a loss, under specific circumstances, I believe Medium Severity is appropriate

[M-03] Non view function is called with staticcall in CErc20Delegator

Submitted by zzzitron

Lines of code

When using CToken implementation with CErc20Delegator, the functions borrowRatePerBlock and supplyRatePerBlock will revert when the underlying functions try to update some states.


The v1 of borrowRatePerBlock and supplyRatePerBlock were view functions, but they are not anymore. The CErc20Delegator is still using delegateToViewImplementation for those functions. Those functions can be used, as long as the implementation does not update any state variables, i.e. the block number increase since the last update is less or equal to the updateFrequency. However, when these functions are called after sufficient blocks are mined, they are going to revert.
Although one can still call the implementation using delegateToImplementation, it is not a good usability, especially if those functions are used for external user interface.

Proof Of Concept

Gist for the test

The gist shows a simple test. It calls borrowRatePerBlock and supplyRatePerBlock first time, it suceeds. Then, it mines for more than 300 times, which is the updateFrequency parameter. Then it calls again then fails.

Notes on the test file:

  • The setup is taken from tests/Treasury/Accountant.test.ts
  • using solidity from ethereum-waffle for chai to use reverted // in hardhat.config.js import chai from “chai”; import { solidity } from “ethereum-waffle”;


Tools Used


Instead of using delegateToViewImplementation use delegateToImplementation. Alternatively, implement view functions to query these rates in NoteInterest.sol and CToken.sol. It will enable to query the rates without spending gas.

nivasan1 (Canto) confirmed

Alex the Entreprenerd (judge) commented:

The warden has shown how, due to the usage of delegateToViewImplementation the function call to borrowRatePerBlock and supplyRatePerBlock will revert, provided some time has passed.

There is no more severe loss of availability than a function that doesn’t work, however, at this time, because the functions are view and “niche”, the actual underlying functionality is still available via delegateToViewImplementation, and the internal accounting of the protocol is still functioning, I believe Medium Severity to be appropriate.

[M-04] Missing zero address check can cause initialize to be called more than once

Submitted by oyc_109

There is potential to call initialize() on AccountantDelegate.sol more than once because the require statement only checks if the state variables != address(0)

If initialize() was called the first with these parameters as address(0x0), then initialize can be called again

The require statement should also check if the parameters != zero address, similar to the check performed in TreasuryDelegate.sol

Or use openzepplen initializable.

nivasan1 (Canto) acknowledged and commented:

This will be a function called by admin (Governance), as such, it is the community’s responsibility to review the parameters of the inititalize method.

Alex the Entreprenerd (judge) commented:

In contract to CantoV1, in this version of the code the initializer can be called multiple times.

This is contingent on the admin calling it, however the impact of changing any variable will most likely be complete breaking of all accounting (an exception may be a complete fresh start).

Given in the code in scope, and the conditionality (admin privilege), I believe Medium Severity to be appropriate.

[M-05] Admin Can Break All Functionality Through Weth Address

Submitted by defsec

On the protocol, almost all functionality is constructed through WETH address. However, if the admin is set to WETH address mistakenly, user could not claim through Comptroller.sol#L1381. Admin can break the protocol.

Proof of Concept

Set WETH address through initializer or change it through governance.

nivasan1 (Canto) disputed and commented:

The admin of the lending-market and LP will be cosmos-sdk governance, vis-a-vis, the community, as such it is expected that a malicious governance proposal will not be passed.

Alex the Entreprenerd (judge) commented:

Similarly to findings about admin re-initializing contracts, the warden has shown how, because the WETH address can be changed, accounting and functionality of the protocol and it’s interactions (in this case emission of rewards and all lending operations triggering a transfer of “COMP”), can be bricked.

Because this is contingent on malicious governance, I believe Medium Severity to be appropriate.

[M-06] A cap is needed on the amount of Note than can be borrowed

Submitted by Picodes

The fact that there is no cap on the amount of Note that can be borrowed makes the Oracle Extractable Value unlimited. But as you intend to rely on TWAP, you need to make sure the cost of oracle manipulation is lower than the Oracle Extractable Value.

Proof of Concept

By manipulating the TWAPs of the designated proxy used for Note (USDC ?) and its relative price to a given collateral(which would be highly costly), an attacker could borrow Note without limit, and empty all pools related to Note and all Note-related value, instantly killing the stablecoin.

The value extractable by Oracle Manipulations is usually easily computable as it is the size of the lending market, but here, it’s more difficult to evaluate as it could potentially be any value linked to Note. This makes risk management harder and increase significantly the risk of attack.

Therefore a cap on how many Notes can be borrowed needs to be added to mitigate this risk.

The attack would be:

  • Manipulate the USDC / Collateral TWAP to be able to borrow Note with less than 1$ of collateral, which would be costly.
  • Extract all the value possible linked to Note, for example:

    • by buying all the tokens from pools Note / token at a discount
    • by supplying Notes to the lending platform and borrow collaterals for which the Note price is still at 1$
    • etc

Essentially as you have no cap on the amount of Note that could be borrowed in such a scenario, you cannot be sure that the potential attack profits are lower than the attack cost.

The governance needs to set a limit on how much Note can be borrowed to mitigate risks, or add for example an “hourly” borrowing limit.

Easiest way to do this would be able to mint / burn from the accountant.

nivasan1 (Canto) acknowledged

Alex the Entreprenerd (judge) decreased severity to Medium and commented:

I don’t think you can manipulate the price of cNOTE per this code

        //set price statically to 1 when the Comptroller is retrieving Price
        else if (compareStrings(ctoken.symbol(), "cNOTE") && msg.sender == Comptroller) {
            return 1; // Note price is fixed to 1

However, you can manipulate the price of another token against USDC

        else {
            stablePair = (stable == 0) ? false : true;
            pair = IBaseV1Pair(pairFor(USDC, underlying, stablePair)); //get the pair for the USDC/underlying pool
            price = pair.quote(underlying, 1, 8); //how much USDC is this token redeemable for

The attack outlined by the warden would require an imbalance in the price of an asset against the given above code.

It would also require oracle manipulation, which requires no external arbitrage nor intervention It would require some value to be extractable from the system.

For those reasons, I think Medium Severity is more appropriate.

Low Risk and Non-Critical Issues

For this contest, 34 reports were submitted by wardens detailing low risk and non-critical issues. The report highlighted below by zzzitron received the top score from the judge.

The following wardens also submitted reports: 0x1f8b, TomJ, AlleyCat, Lambda, Picodes, Meera, aysha, c3phas, 0x29A, __141345__, oyc_109, Bnke0x0, defsec, Chom, fatherOfBlocks, cccz, ladboy233, slywaters, ignacio, JC, rfa, Funen, hake, sach1r0, samruna, ynnad, grGred, TerrierLover, 0v3rf10w, asutorufos, Limbooo, mrpathfindr, and Sm4rty.

[L-01] Price not scaled by the BASE in BaseV1Router01

  • BaseV1-periphery.sol:501

          else if (compareStrings(ctoken.symbol(), "cNOTE") && msg.sender == Comptroller) {
              return 1; // Note price is fixed to 1

The getUnderlyingPrice function returns 1, when Comptroller is calling with CNote. The Comptroller is using the given price as mantissa, which has 18 decimal precision.

[L-02] _acceptInitialAdminDelegated will revert in GovernorBravoDelegator

When the GovernorBravoDelegator is used with GovernorBravoDelegate as the implementation, the function _acceptInitialAdminDelegated will revert since the GovernorBravoDelegate does not have _acceptInitialAdmin function.

[L-03] Desirable underflow / overflow in BaseV1Pair

The BaseV1-core is using solidity version 0.8.11, so it will revert when overflow or underflow happens. Upon cumulating the time weighted reserve, it is desired to overflow. Likewise it is desired to underflow calculating price average. It also saves gas usage, to use unchecked block for those calculations.

[L-04] Lack of zero address check

  • NoteInterest.sol:104-105 in initialize cNote = CErc20(cnoteAddr); oracle = PriceOracle(oracleAddress);

    • the cNote and oracle was set without checking zero address
  • NoteInterest.sol:113 in setAdmin admin = newAdmin;

    • it is good to check admin for zero address, otherwise admin access will be lost forever. To renounce the admin, it is safer to add a separate function.

[L-05] Oracle failure check in NoteRateModel

  • NoteInterest.sol:145

              uint twapMantissa = oracle.getUnderlyingPrice(cNote); // returns price as mantissa

It is good to check the returned value from oracle is sane (i.e. if it is zero) and prepare for a fallback.

[L-06] Storage for delegate is outside of storage contract in CNote

When updating an implementation for a proxy, it is important to keep the storage structure. Putting non-constant state variables in a dedicated storage contract is less error-prone approach. Although CErc20Delegate has a storage contract in its inheritance tree, a state variable _accountant was introduced in CNote. Therefore, extra care should be paid to the state variable.

[N-01] Cast to int without checking in NoteRateModel

            int diff = BASE - int(twapMantissa); //possible annoyance if 1e18 - twapMantissa > 2**255, differ
            int InterestAdjust = (diff * int(adjusterCoefficient))/BASE;
            int ir = InterestAdjust + int(baseRatePerYear);

The uint256 variables are casted into int without checking for overflow. It is probably safe in this use case, but to be sure one may use SafeCast.

[N-02] Meaningless check in GovernorBravoDelegate

The check require(initialProposalId == 0, "GovernorBravo::_initiate: can only initiate once"); is not useful, as initialProposalId is never updated.

[N-03] Missing param in NatSpec in CToken

Although a new argument uint8 stable_ was added, the NatSpec for the parameter is missing.

[N-04] Misleading NatSpec in AccountantDelegator

[N-05] Typos

Alex the Entreprenerd (judge) commented:

[L-01] Price not scaled by the BASE in BaseV1Router01

[L-02] _acceptInitialAdminDelegated will revert in GovernorBravoDelegator
Valid Low (as the Sponsor uses another function to acceptAdmin)

[L-03] Desirable underflow / overflow in BaseV1Pair
Valid Low

[L-04] Lack of zero address check

[L-05] Oracle failure check in NoteRateModel

[L-06] Storage for delegate is outside of storage contract in CNote
Low, there will be no risk unless the Sponsor changes the Delegate contract

[N-01] Cast to int without checking in NoteRateModel
NC as it will never overflow but it’s a code-smell

[N-02] Meaningless check in GovernorBravoDelegate

[N-03] & [N-04] Natspec

[N-05] Typos

Really good report, there’s some personal opinion and style which helps it stand out. Also the quantity of findings is good, will give it bonus points.

Gas Optimizations

For this contest, 34 reports were submitted by wardens detailing gas optimizations. The report highlighted below by 0x1f8b received the top score from the judge.

The following wardens also submitted reports: rfa, 0xKitsune, Bnke0x0, 0x29A, defsec, Picodes, Tomio, TomJ, Funen, ladboy233, Meera, fatherOfBlocks, m_Rassska, oyc_109, TerrierLover, Waze, mrpathfindr, 0xArshia, ajtra, c3phas, Chom, cRat1st0s, durianSausage, Fitraldys, grGred, hake, JC, Lambda, Noah3o6, RedOneN, Rohan16, Sm4rty, and ynnad.

[G-01] Use abi.encodeWithSelector instead of abi.encodeWithSignature

abi.encodeWithSelector is much cheaper than abi.encodeWithSignature because it doesn’t require to compute the selector from the string.


Affected source code:

[G-02] Avoid creating unnecessary variables

The creation of variables has an extra cost due to the required opcodes, in addition to the fact that the evm virtual machine is limited to the number of elements.


Affected source code:

[G-03] Reorder structure layout

The following structs could be optimized moving the position of certains values in order to save slot storages:

struct Proposal {
    uint id;
    address proposer;
    uint eta;
    address[] targets;
    uint[] values;
    string[] signatures;
    bytes[] calldatas;
    bool canceled;  // <- Move these two bool close to the `proposer` address
    bool executed;

[G-04] Reduce the size of error messages (Long revert Strings)

Shortening revert strings to fit in 32 bytes will decrease deployment time gas and will decrease runtime gas when the revert condition is met.

Revert strings that are longer than 32 bytes require at least one additional mstore, along with additional overhead for computing memory offset, etc.

I suggest shortening the revert strings to fit in 32 bytes, or that using custom errors as described next.

Use Custom Errors instead of Revert Strings to save Gas

Custom errors from Solidity 0.8.4 are cheaper than revert strings (cheaper deployment cost and runtime cost when the revert condition is met)

Source Custom Errors in Solidity:

Starting from Solidity v0.8.4, there is a convenient and gas-efficient way to explain to users why an operation failed through the use of custom errors. Until now, you could already use strings to give more information about failures (e.g., revert(“Insufficient funds.”);), but they are rather expensive, especially when it comes to deploy cost, and it is difficult to use dynamic information in them.

Custom errors are defined using the error statement, which can be used inside and outside of contracts (including interfaces and libraries).

Below are some examples, but the code has a lot of them, it is recommended to change all require phrases to custom errors.

[G-05] ++i costs less gas compared to i++ or i += 1

++i costs less gas compared to i++ or i += 1 for unsigned integer, as pre-increment is cheaper (about 5 gas per iteration). This statement is true even with the optimizer enabled.

i++ increments i and returns the initial value of i. Which means:

uint i = 1;
i++; // == 1 but i == 2

But ++i returns the actual incremented value:

uint i = 1;
++i; // == 2 and i == 2 too, so no need for a temporary variable

In the first case, the compiler has to create a temporary variable (when used) for returning 1 instead of 2.
I suggest using ++i instead of i++ to increment the value of an uint variable. Same thing for --i and i--

Affected source code:

[G-06] Use external visibility

The following methods could be improved if external visibility is used:

[G-07] There’s no need to set default values for variables

If a variable is not set/initialized, the default value is assumed (0, false, 0x0 … depending on the data type). You are simply wasting gas if you directly initialize it with its default value.

Affected source code:

[G-08] Reduce boolean comparison

It’s compared a boolean value using == true or == false, instead of using the boolean value. NOT opcode, it’s cheaper to use EQUAL or NOTEQUAL when the value it’s false, or just the value without == true when it’s true, because it will use less opcodes inside the VM.

Change == false by ! in:

[G-09] Unused arguments

Is not required to send the nonce, only is required for compute the signature.

Affected source code:

[G-10] Gas saving using immutable

It’s possible to avoid storage access a save gas using immutable keyword for the following variables:

It’s also better to remove the initial values, because they will be set during the constructor.

Affected source code:

[G-11] Unused import

Importing pointless files costs gas during deployment and is a bad coding practice that is important to ignore.

Remove the following imports:

[G-12] unckecked keyword

It’s possible to save gas using the unckecked keyword. This will avoid the required checks to ensure that the variable won’t overflow.


The balance was checked before, it can be subtracted without checking overflows:

[G-13] use constant instead of storage

Use constant instead of storage and change from uint256 MAX_INT = 2**256-1; to uint256 private constant MAX_INT = type(uint256).max;:

[G-14] Unrequired method

Method not necessary since the implemented logic only calls to obtain the current block.

Affected source code:

[G-15] Optimize logic

The logic of the CToken.transferTokens method can bypass the startingAllowance condition and the allowanceNew variable if the spender and src checks are done at the same time the tokens are subtracted.

if (spender == src) {
    transferAllowances[src][spender] -= tokens;

Affected source code:

[G-16] Use memory instead of storage

Changing storage to memory we can save some storage access.

Affected source code:

Alex the Entreprenerd (judge) commented:

-> Will save in writing and reading, let’s say 2.1k * 2 for two saved SLOADs

9* 2100 (ignoring ERC20MinterBurnerDecimals as out of scope)

[G-13] Use constant instead of storage

Rest seems minor.

Big gas savings by reducing storage, nice finds.



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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.

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