Fei Protocol Audit – Phase 2

The Fei Protocol team asked us to review and audit updates to their protocol smart contracts. This engagement builds upon a previous audit as well as a post-mortem investigation we performed for their protocol. We looked at this new code and now publish our results.

Scope

We audited commit f54d7bb07c55adb78e2e142e7044f60090bb7602 of the fei-protocol/fei-protocol-core-internal repository. It is the intent that these updated contracts will be integrated into the Fei Protocol by their governance process.

In scope were the following contracts:

bondingcurve/BondingCurve.sol
bondingcurve/EthBondingCurve.sol
bondingcurve/IBondingCurve.sol
oracle/CompositeOracle.sol
pcv/IPCVDeposit.sol
pcv/IPCVDripController.sol
pcv/IPCVSwapper.sol
pcv/IUniswapPCVController.sol
pcv/IUniswapPCVDeposit.sol
pcv/PCVDripController.sol
pcv/PCVSplitter.sol
pcv/PCVSwapperUniswap.sol
pcv/RatioPCVController.sol
pcv/UniswapPCVController.sol
pcv/UniswapPCVDeposit.sol
refs/CoreRef.sol
refs/ICoreRef.sol
refs/IOracleRef.sol
refs/IUniRef.sol
refs/OracleRef.sol
refs/UniRef.sol
stabilizer/EthReserveStabilizer.sol
stabilizer/IReserveStabilizer.sol
stabilizer/ITribeReserveStabilizer.sol
stabilizer/ReserveStabilizer.sol
stabilizer/TribeReserveStabilizer.sol
utils/Incentivized.sol
utils/Timed.sol

Note: the repository in question is private at the time of writing, so many hyperlinks will only work for the Fei team.

High level delta of changes

  • Removal of genesis, direct incentives, and orchestrator code
  • Updated contracts for ERC20 support
  • Added a chainlink oracle “oracle/ChainlinkOracleWrapper.sol” (out of scope)
  • Simplified Bonding Curve, fixed discount before scale and fixed premium after
  • Updated reweight algorithm to be two-sided, and use rebasing below the peg
  • Added ReserveStabilizers which sell PCV for FEI at a fixed price <= $1 – FEI, TRIBE, Core, and the DAO remain unchanged, although there are no FEI incentive contracts planned for use We consider the major updates to be the addition of ERC20 support, the simplified Bonding Curve, and the ReserveStabilizer.

ERC20 Support

Previously Fei Protocol only supported the use of ETH as Protocol Controlled Value (PCV). The BondingCurve allowed FEI to be purchased for ETH, and a Uniswap ETH/FEI pool was maintain by the protocol to help keep the FEI price stable. In this updated version of the protocol, the contracts have been rewritten to allow for generic ERC20 PCV throughout the system. A new BondingCurve allows users to purchase FEI using ERC20 tokens, new ERC20/FEI Uniswap pools will be used, and ReserveStabilizers (see below) will help to stabilize the FEI price by selling any ERC20 PCV token.

Simplified Bonding Curve

In the previous version of the Fei Protocol, the amount of FEI received for a purchase on the BondingCurve contract was calculated using a formula that included (1) how much FEI had been purchased already, and (2) how large the scale cap on the curve was. This calculation has now been simplified such that all purchases before scale get the same fixed price of peg-discount, and once scale has been hit all purchases are at a price of peg+buffer.

ReserveStabilizers

The ReserveStabilizers provide price support for FEI below the peg by exposing an exchangeFei function exchanging discounted FEI for PCV from the held reserves. This way, arbitrageurs can absorb FEI trading below peg on external markets and exchange it for a profit with the ReserveStabilizers. The ReserveStabilizers will be deployed by governance to hold reserves in various ERC20 tokens. There are two main mechanisms in place to automate the flow of PCV into the ReserveStabilizers: – The incentivized external allocate function of the BondingCurves will distribute their respective PCV amongst set PCVDeposits, some of which will be ReserveStabilizers. – The incentivized external drip function of the PCVDripController transfers PCV between PCVDeposits. PCVDripControllers can be deployed and set by governance, where the target of the drip can be a ReserveStabilizer.

Privileged roles

Since the initial audit, there has been a reassignment of roles to the Timelock contract: – The Timelock contract is a now a Governor, PCVController, and a Minter. Upon deployment of this upgrade, the following roles are planned to be set:

  • EthBondingCurve and BondingCurve‘s will have Minter role.
  • PCVDripController will have PCVController role.
  • PCVSwapperUniswap will have Minter role.
  • RatioPCVController will have PCVController role.
  • UniswapPCVController will have Minter, and Burner roles.
  • UniswapPCVDeposit will have Minter, and Burner roles.
  • EthReserveStabilizer, and ReserveStabilizer will have Burner role.
  • TribeReserveStabilizer will have Burner, and Tribe Minter roles.

Security model and trust assumptions

The Fei protocol access control system is limited to 5 roles: Minter, Burner, PCVController, Governor, and Guardian. In the words of the Fei Protocol team:

We assume none of these roles are malicious, and only the Guardian will ever be held by a multisig, the others by contracts. The roles are maintained by Core which is out of scope. There is also a special role “Tribe Minter” which can only be held by a single contract and has the ability to mint TRIBE. It is currently held by the Timelock (managed by Fei DAO/GovernorAlpha) and will be transitioned to the TribeReserveStabilizer. The primary oracle will be either the ChainlinkOracleWrapper or UniswapOracle depending on community preference. Neither is in scope for the audit, both are assumed to be accurate and not malicious or manipulable.

It must be noted that given the Fei documentation stating that the “Tribe Minter” role can only be held by a single contract, when this role is transitioned to the TribeReserveStabilizer this “Tribe Minter” role will effectively be shared between the TribeReserveStabilizer and governance. We consider this discrepancy between the documentation and implementation an issue and it is covered in our report.

In contrast to early drafts of their whitepaper, where TRIBE had a hard cap, the TRIBE token is now designed to be inflationary without a cap.

Most contracts relying on an oracle, do so through OracleRef which has the added benefit of a backup oracle. The lone contract that doesn’t use a backup oracle is the TribeReserveStabilizer.

Also, it is worth noting that the rebasing mechanism of the UniswapPCVController triggered by the reweight function actually disincentivizes liquidity providers, whom are not FEI protocol contracts, from entering the Uniswap FEI liquidity pools. This disincentive comes from the fact that this rebasing mechanism burns FEI from the corresponding Uniswap FEI liquidity pool so that liquidity providers will always suffer a loss in combined liquidity when they exit the pool. Since this is a feature and not a bug, we are sharing this fact here so that the community is aware but we are not otherwise considering it an issue.

Findings

Here we present our findings.

Critical severity

[C01] Protocol does not support token decimals

This iteration of the Fei Protocol is designed to be a generic version of the original protocol – now instead of supporting just ETH as the PCV of the system, the contracts are designed to be able to use any ERC20 token as PCV. However, the code is not designed to handle tokens that have different numbers of decimals, and many of the contracts break in their entirety when a token is used with a different number of tokens than 18. Examples of widely used tokens with a different number of decimals include WBTC (8), USDC (6), and USDT (6).

The root of the issue stems from the fact that the readOracle function returns the FEI price as FEI per X where FEI and X are both whole tokens not token units. Due to the fact that Solidity actually handles tokens in their smallest units, and not as whole tokens, when this price is used for a token with a differing number of decimals to FEI, the calculation result is out by 10 ** |feiDecimals - tokenDecimals|.

Below we explore some ways in which this affects the protocol. Throughout we use an example where the token being used as PCV is WBTC with 8 decimals. A price of 30,000 WBTC/FEI is used.

Exchanging FEI on the ReserveStabilizer

The ReserveStabilizer contract allows users to burn FEI in exchange for tokens. The exchangeFei function calls the getAmountOut function to calculate how many tokens the user should receive for their FEI. However getAmountOut uses the readOracle function without adjusting for the number of decimals that token has.

In our example, the WBTC ReserveStabilizer would allow a user to deposit 1 FEI (worth ~1 USD), and receive over 300,000 WBTC, worth over $10 billion. This would allow a user to drain all token PCV from the contract for a fraction of a dollar.

Purchasing on the BondingCurve

Each instantiation of the BondingCurve contract allows users to purchase FEI in exchange for a specified ERC20 token. The purchase function allows the user to specify how many tokens to use, and calls getAmountOut to calculate the amount of FEI they should receive in return. However, as in the ReserveStabilizer, getAmountOut uses the readOracle function without adjusting for the number of decimals that token has.

In our example WBTC BondingCurve, a user could provide 1 WBTC to the BondingCurve.purchase function before scale, which is specified as 1 * 10**8. The contract then calculates the amount of FEI to return as 31,579 * 10**8, which given that FEI has 18 decimals is significantly less than 1 FEI. In this case the user received less than $1 of FEI in return for their $30,000 deposit.

Other examples

The implications of this issue are far reaching throughout the entire protocol, and so we will not explain every vulnerability caused. At a high level, other vulnerabilities include:

  • The allocate function of the BondingCurve, which allows PCV to be removed from the BondingCurve and into the rest of the protocol uses readOracle to calculate whether the value of the PCV held is large enough to allow the call to be successful. However the call will always fail until the amount of PCV held is 10 ** |feiDecimals - tokenDecimals| larger than it should need to be. In the case of WBTC this would mean collecting billions of dollars to be able to remove the PCV from the contract – effectively locking it up until that happens.
  • The reweight function of the UniswapPCVController, which rewards users for moving the FEI price back to the peg, incorrectly calculates whether a reweight should be allowed. The “distance to the peg” will be out by factors of 10, meaning that reweights will virtually always be possible.
  • Additionally, when performing the reweight, the _rebase and _reverseReweight functions will be reweighting towards an incorrect peg price for FEI.

Consider updating the readOracle function to return the price in terms of token units, and not whole tokens. Alternatively, consider always adjusting the price returned from readOracle to account for the difference in token decimal amounts of the tokens in question. Additionally, consider updating your test suite to ensure that you are testing using a diverse set of tokens reflective of a mainnet environment.

Update: Fixed in PR#69.

High severity

[H01] Fei reweight and stabilization may not react to price swings

In the TribeReserveStabilizer contract, the exchangeFei function determines whether to exchange a users FEI for TRIBE by calling the isFeiBelowThreshold function. This function reads the current FEI price from an oracle, however this price is not ever updated by this contract, so a stale price is used to determine whether the exchange can occur.

Similarly, in the UniswapPCVController, the reweight function determines whether a reweight is allowed by immediately calling the reweightEligible function. This fetches the current uniswap price, and compares it to a price that it reads from an oracle. If the difference between the 2 prices is large enough, a reweight is permitted to occur. However the update function of the oracle is not called prior to reading the price, meaning that stale oracle prices can be used without detection.

Consider always updating oracle prices before reading their values, to avoid making incorrect decisions in critical protocol operations.

Update: Partially fixed in PR#70. The reweight issue outlined above has been fixed, however exchangeFei issue has not been fixed. The Fei Protocol team stated:

Did not add for TribeReserveStabilizer because feiOracle will be a chainlink oracle without need for update

[H02] Introduction of additional BondingCurves creates period of volatility

The BondingCurve contract enables use of generic bonding curves for any ERC20 token, in addition to the ETH bonding curve already being used in the protocol. The BondingCurve mints new FEI tokens in exchange for a specified ERC20 token at a discounted price, until a scale number of FEI have been minted. Once the curve is at scale, the price will increase and maintain a buffer price.

The Fei team have stated that they intend for scale to be 1m-100m, and the discount to be 0-5%. Assuming the most extreme of these parameters, 100M FEI will be available to purchase for $95M of token. This large incentivization will likely lead to bots rapidly purchasing discounted FEI and immediately selling it for a profit on various exchanges. This means that every time a new BondingCurve is launched, the price of FEI will drop across the market, causing volatility in the FEI price. Additionally, users can take out flashloans to maximize the profits they can make from the launch of a new BondingCurve – making profit for free from the protocol’s PCV stored in Uniswap pools.

Take for example a new DAI bonding curve, with a 5% discount, and a scale of at least 5M. Additionally assume that the ETH/FEI and ETH/DAI UniswapV2 pools have the same reserves and prices as at the time of writing. A user can:

  • Flash-borrow $4.8M in DAI
  • Buy $5.053M FEI from the BondingCurve
  • Sell this FEI for 2326 ETH using the ETH/FEI UniswapV2 pool
  • Sell this ETH for $4.93M DAI using the ETH/DAI UniswapV2 pool
  • Pay back the flash loan and keep $137,000 as profit

While $137k of profit isn’t as large as many of the attacks seen in the ecosystem, this method can be repeated across many different exchanges and pools to make considerably more money when scale is as large as 100M. Additionally with bots performing the trades, this will lead to the price of FEI crashing immediately after each new BondingCurve contract is deployed, and day to day users being disadvantaged by their stablecoin holding crashing in price. This crash in price will ultimately also lead to increased use of, and draining of PCV from, the various ReserveStabilizer contracts.

Consider removing the offering of discounted FEI for new tokens, to remove the added volatility of the price of FEI. Alternatively, consider significantly reducing the parameters of 100M and 5% so that the effect of launching a new BondingCurve contract will not be so large.

Update: Not fixed. The Fei Protocol team states:

This is expected behavior. The parameters used will be conservative and account for risks of volatility in FEI.

[H03] Inconsistent use of oracles

Throughout the protocol, oracles are relied upon to keep FEI stable, calculate payouts to users, and judge whether actions are eligible to be carried out. In addition to the vulnerabilities caused by oracles described in C01, and H01, we found the use of oracles throughout the code to be inconsistent and confusing.

Oracle updates

The functions IOracle.update and OracleRef.updateOracle return a boolean. However the definition of what this boolean signals is ambiguous. The UniswapOracle contract defines it as a signal whether an update was needed – false is not a failure, but instead just a signal that the oracle didn’t need to be updated. Whereas the OracleRef contract defines it as a signal of whether the update was effective – here false is the signal of a failure to update the price.

This has lead to inconsistent handling of this boolean value: The PCVSwapperUniswap.swap function reverts if false is returned, whereas the remainder of the codebase ignores the return value entirely. If the definition found within OracleRef is correct, functions throughout the codebase are not reverting when the update fails, which they should be.

Use of isOutdated

All oracles of type IOracle implement a function isOutdated which signals whether or not the current oracle price is outdated. This function is utilized in PCVSwapperUniswap, however the rest of the codebase ignores the function, and never checks whether an oracle needs to be updated.

Inverting oracle prices

Oracles return a price of a currency X, in terms of a currency Y, which can then be inverted if what is desired is the price of Y in terms of X. However the possibilities of inverting token prices is inconsistent throughout the codebase. The PCVSwapperUniswap has a boolean flag invertOraclePrice which signals whether the oracle being used needs to be inverted or not, whereas the ReserveStabilizer always inverts the price, and the remainder of the codebase never inverts the price.

Consider updating the codebase to remove the above inconsistencies and vulnerabilities, and updating all comments and documentation to reflect the intended use of all functions and booleans.

Update: Partially fixed in PR#69. The implementations of updating and inverting oracle prices have been fixed in this PR. The PR additionally makes the use of isOutdated consistent throughout the codebase – the function is now not used at all. However, given that the purpose of isOutdated is to flag whether an oracle price is stale, reading oracle prices without first calling isOutdated could lead to incorrect prices being used throughout the protocol to determine critical conditions.

Medium severity

[M01] Errors and omissions in events throughout codebase

Throughout the codebase, events are used to signify when changes are made to the contracts. However many events lack indexed parameters, are missing important parameters, or emit erroneous parameters. Additionally collisions between events exist, and some sensitive actions are lacking events altogether.

Events emitting erroneous parameters include:

Events missing important parameters include:

Events lacking indexed parameters include:

Sensitive actions that are lacking events include, but are not limited to:

  • The relevant “update” events in the constructors of BondingCurve, PCVSwapperUniswap, and TribeReserveStabilizer.

Additionally, some contracts have events with identical names. For example the WithdrawERC20 event in RatioPCVController and IPCVDeposit. A call to RatioPCVController.withdrawRatioERC20 emits 2 different events with different parameters, but both named WithdrawERC20. Another occurrence is the Timed contract and the IUniswapOracle interface both define a DurationUpdate event. This may cause confusion for off-chain clients and users alike.

Finally, the ReweightIncentiveUpdate event is defined but never used.

Consider making all of the above changes to enable off-chain clients to correctly track changes to the Fei Protocol.

Update: Partially fixed in PR#104. The following items were not fixed:

  • The dripAmount of the Dripped event remains potentially inaccurate.
  • 2 of the 4 events in PCVSwapperUniswap do not emit the previous variable value.
  • TimerReset does not emit the previous value.
  • No indexing has been added to the list of events lacking indexed parameters provided.

[M02] UniswapPCVDeposit can burn PCV FEI without Burner role

The external deposit and withdraw functions of the UniswapPCVDeposit contract each respectively have underlying calls to the _burnFeiHeld function of the CoreRef contract. This _burnFeiHeld function has no modifier or check that the caller has the Burner role.

In the case the UniswapPCVDeposit has its Burner role revoked, calls to the deposit and withdraw functions will in fact continue to burn PCV FEI. While the deposit function is callable by anyone, it will only burn FEI dust remaining after adding liquidity to the Uniswap pool. However the withdraw function will burn non-trivial PCV FEI resulting from its underlying removeLiquidity call to the Uniswap pool. While the withdraw function is modified by onlyPCVController, it can be triggered by anyone by calling the drip function of the PCVDripController contract. This means, in the case the Burner role of UniswapPCVDeposit is revoked as an emergency incident response, the safeguard can be circumvented.

Consider refactoring the _burnFeiHeld function to ensure the caller has the Burner role.

Update: Not fixed. The Fei Protocol team state:

Any address can burn its own FEI, this is expected behavior

[M03] Lack of input validation

Throughout this codebase we found there to be an overall lack of input validation. The functions lacking input validation are either modified by the onlyGovernance modifier or are constructors. However simple human error in entering these values, by perhaps entering too many or too few 0s, can have far reaching negative consequences.

Some points where a lapse in input validation could be particularly problematic include:

  • There is no check in the callstack of the setAllocation function of the BondingCurve contract to ensure that the token featured in the BondingCurve is the token handled by the PCVDeposit. In the case this mismatch would occur, the PCVController would have to manually reallocate these stray tokens using the withdrawERC20 function.

  • The PCVDripController contract drips tokens from one PCVDeposit contract to another, but it never validates that the token each of them handle are the same. This means it could drip a token into a deposit contract that handles a different token. This mismatch could affect the accounting dictating the logic of the drip function. In particular, the check of dripEligible may wind up considering the balance of the wrong token.

  • The constructor of PCVSwapperUniswap contract does not check that the two tokens in the pair are tokenSpent and tokenReceived. This can affect the calculations of the inputs to the uniswap swap, leading to accounting errors in pcv.

Examples of onlyGovernor modified functions lacking input validation are:

  • The setFeiOracle function of the TribeReserveStabilizer contract doesn’t check newFeiOracle is not the zero-address.

  • The setDuration function of the UniswapPCVController contract does not validation the _duration is non-zero or within sensible bounds.

The functions that could benefit from input validation in this codebase are numerous and there are many more than are listed here.

Consider implementing programmatic safeguards validating input parameters to ensure all function calls and contract constructions would “fail early and loudly” on erroneous inputs. This is needed especially in the case of functions or contracts vetted by governance, where subtle bugs in parameters that pass the governance process can have far reaching impacts on a system.

Update: Partially fixed in PR#75. The first 2 points outlined in this issue were not addressed, however the remaining issues were fixed.

[M04] ReserveStabilizer performs unsafe token transfer

The exchangeFei function of the ReserveStabilizer contract burns a user’s FEI and in exchange transfers them tokens at a discounted rate. However, the boolean return value of the token transfer performed is not checked. This means that ERC20-compatible tokens that return false on failure, such as ZRX, will not cause the call to revert. In turn the user will receive no tokens for their FEI.

Consider updating the token transfer to instead use OpenZeppelin’s SafeERC20, which will handle all edge cases of token transfers for you.

Update: Fixed in PR#71.

[M05] Incorrect use of Time-Weighted Average Prices

Throughout the codebase, oracles of type IOracle are used to update, and read asset prices. These prices are then used to inform decisions that together try to keep the price of FEI stable. The oracle interface is designed such that the protocol can use any combination of oracles from different protocols, but currently only those from Chainlink, and Uniswap have been integrated – with the live protocol solely using Uniswap at launch in January, and now solely Chainlink. However, the way that the protocol inverts the price read from IOracle is not safe when the price is a Time-Weighted Average Price (TWAP) calculated with an arithmetic mean, as is the case with Uniswap V2.

To protect against price manipulation, UniswapV2 uses Time-Weighted Average Prices (TWAPs). These TWAPs enable contracts to fetch an average price over a given time interval, instead of using the asset spot price at the current instant. This means that prices are significantly harder to manipulate, which is a great safety feature for protocols using these prices. However, while the spot price of token0/token1 can be inverted to get the spot price of token1/token0, the same is not true of TWAPs: the TWAP of token0/token1 cannot be inverted to get the TWAP of token1/token0. The more volatile the price of token0/token1, the more extreme this result is.

For example, assume we have tokens ABC and DEF, where the price of ABC/DEF at time 1 is 100, and the price at time 5 is 700. With some simplification of scaling, the cumulative prices tracked in UniswapV2 would be:

cumulativeABC: (1*100)+(4*700) = 2900
cumulativeDEF: (1*1/100)+(4*1/700) = 11/700

The average prices over the window from 0 to 5 are therefore calculated as:

averagePriceABC: 2900/5 = 580
averagePriceDEF: 11/700/5 = 0.00314 (approx.)

If instead of using the DEF price of 0.00314, the inverted ABC price is used, this is a price of 1/580 = 0.00172. This is approximately 54% of the correct price.

The codebase twice uses the IOracle.invert function to calculate the reciprocal of a price like this – without knowledge of whether the oracle is using Chainlink spot prices, Uniswap TWAPs, or something else. If these prices are TWAPs, the amount returned to a caller by the ReserveStabilizer, and the slippage parameter in the PCVSwapperUniswap will be incorrect by potentially large percentages.

Consider updating the code to ensure that Uniswap TWAPs are never inverted using a reciprocal, and instead the inverse accumulator is used to correctly calculate the inverse TWAP.

Update: Partially fixed in PR#69. The updated code makes it possible to use oracles correctly and safely, and is a complete fix as long as one condition is never broken: If the oracle or backupOracle in OracleRef returns a Uniswap TWAP as the price, the doInvert variable should never be set to true. If this condition is broken, the prices used throughout the codebase will be incorrect, and could lead to vulnerabilities.

Low severity

[L01] Duplicate definitions

The WETH address is set as a public immutable state variable in:

The uint256 constant BASIS_POINTS_GRANULARITY is defined in various parts of the protocol, all having the same value of 10_000:

While these definitions were made with consistent values across the codebase, the multiple definitions could introduce inconsistencies in future updates to the protocol

Consider defining values shared across the protocol once in a common library.

Update: Not fixed.

[L02] ERC20 BondingCurve accepts ETH

The BondingCurve contract has a purchase function which allows users to purchase FEI using ERC20 tokens. Due to the fact that EthBondingCurve inherits BondingCurve, and requires its purchase function to be payable, BondingCurve‘s purchase is also payable, despite not handling the case where ETH is sent with a call. Furthermore, once ETH is in the BondingCurve, there is no way to withdraw the ETH again.

Consider requiring that the msg.value of a call to BondingCurve.purchase is 0 to prevent ETH getting stuck inside the contract.

Update: Fixed in PR#72.

[L03] Using old Solidity version

Throughout the contract, Solidity ^0.8.0 is used. However, at the time of writing there are 2 known bugs in version 0.8.0. Consider upgrading your contracts to use the latest version of Solidity, 0.8.7.

Update: Fixed in PR#73.

[L04] Role definition differs from implementation

The “Tribe Minter” role is defined in the Fei protocol to only be held by a single contract. This role, as the name suggests, controls the ability of an entity to mint TRIBE. Currently this role is held by the Timelock contract but it is planned that this “Tribe Minter” role will be transitioned to the TribeReserveStabilizer contract. The TribeReserveStabilizer needs the “Tribe Minter” role since it mints TRIBE to exchange for FEI in its exchangeFei function.

However the TribeReserveStabilizer also has as one of its methods a mint function, modified by the onlyGovernor modifier, that makes an internal call to mint TRIBE. This means that the Timelock contract retains its ability to mint TRIBE by way of this onlyGovernor modified mint function despite passing this distinct role to the TribeReserveStabilizer. In other words, the Timelock contract effectively shares the “Tribe Minter” role with the TribeReserveStabilizer despite the role being documented to be held by a single contract. This discrepancy can be misleading to users in understanding the security model of the Fei protocol.

Consider either updating the documentation so that it is clear which entities can mint TRIBE, or removing the onlyGovernor modified mint function from the TribeReserveStabilizer.

Update: The Fei Protocol team state:

We will update the documentation.

[L05] Uninitialized global variable

The OracleRef contract relies on 2 oracle addresses – the main oracle, and a backupOracle. However, only the oracle can be initialized upon construction of the contract – the backupOracle remains uninitialized even if the creator wants the contract to have a backup oracle.

Consider adding a parameter to the constructor of OracleRef to allow the backupOracle to be initialized on creation of the contract.

Update: Fixed in PR#67.

[L06] Variable shadowing in ReserveStabilizer

The ReserveStabilizer contract has a global variable token, which stores the address of the token exchanged on this stabilizer. The ReserveStabilizer.withdrawERC20 function additionally defines a local variable named token, shadowing the global variable definition. Creating shadowed variable names can lead to bugs where the writer intends to use the global variable, but the compiler uses the local shadow variable instead. Additionally it makes the code harder to read, and confusing to understand.

Consider renaming the local token variable within withdrawERC20.

Update: Fixed in PR#74.

Notes & Additional Information

[N01] _getAmountToPeg lacks comments

The _getAmountToPeg function of the UniswapPCVController contract performs a key calculation in the reweight algorithm. If the Uniswap price of Fei in relation to the other token is below the peg the reweight algorithm rebases the Uniswap pool, but otherwise performs a _reverseReweight of the Uniswap pool.

The _reverseReweight relies on calculations from the _getAmountToPeg function involving a fairly simple formula. The derivation of this formula is not immediately straight-forward but appears in the Fei protocol’s documentation. Since this derivation appears in external documentation but not commented in the code, it is easy for readers to become confused and miss the connection.

To aid in understanding and readability, consider documenting as comments in code derivations of all mathematical formulae.

Update: Fixed in PR#76.

[N02] Interfaces missing functions

Many of the contracts in the codebase have a matching interface which defines the function signatures for the contract. However some of the interfaces are missing functions which are defined in the respective implementing contract. Examples of this are:

Consider updating these interfaces to define the missing function signatures.

Additionally, ITribeReserveStabilizer is missing all of the functions defined in IReserveStabilizer. Consider having ITribeReserveStabilizer inherit IReserveStabilizer.

Update: Not fixed.

[N03] Missing Natspec

While the majority of the codebase is well-commented, with Natspec used for most functions, there are some places that Natspec is missing or incomplete. Some examples of this are:

Additionally, some of the existent Natspec contains erroneous information. Examples of this include:

Consider updating the above examples and checking that complete Natspec exists for all functions throughout the codebase.

Update: Partially fixed in PR#79. The NatSpec of the following functions was not fixed: PCVSwapperUniswap.setInvertOraclePrice, and PCVSwapperUniswap._getDecimalNormalizer.

[N04] Renaming suggestions

Good naming is one of the keys for readable code, and to make the intention of the code clear for future changes. There are some names in the code that make it confusing or hard to understand.

Consider the following suggestions:

In the UniswapPCVController contract:

  • magnitude to meetsMagnitudeRequirement.
  • time to meetsTimeRequirement.

In the BondingCurve contract:

Update: Fixed in PR#77.

[N05] Inconsistent style

There are various occurrences of style inconsistency within this codebase:

Taking into consideration how much value a consistent coding style adds to the project’s readability, enforcing a standard coding style is recommended.

Update: Partially fixed in PR#77. However both the indentations throughout PCVSwapperUniswap.sol, and the use of underscores in parameters throughout the codebase remain inconsistent.

[N06] Typos

The codebase contains the following typos:

Consider correcting these typos to improve code readability.

Update: Fixed in PR#77.

[N07] Unnecessary return value

In the PCVSplitter contract, the checkAllocation function returns a boolean. However the function always reverts on failure, and the only call to this function does not check its return value.

Similarly, the _getUniswapPrice function in UniswapPCVController returns 3 parameters, but only the first of these is ever used.

Consider removing the unused return value from the function to simplify the codebase.

Update: Fixed in PR#78.

[N08] Unnecessary imports

Consider removing the following import statements, as they are never used in their corresponding contracts:

Update: Fixed in PR#77.

[N09] Using ERC20 not IERC20

In the PCVSwapperUniswap contract, ERC20 is imported to allow the contract to interact with a variety of tokens. Throughout the rest of the codebase, as is customary, the interface IERC20 is instead used to interact with tokens.

Consider updating PCVSwapperUniswap to be consistent with the rest of the codebase, by using IERC20 to interact with tokens.

Update: Fixed in PR#69. IERC20MetaData was used instead of IERC20, as the metadata functions were required to fix another issue.

Conclusions

1 critical and 3 high severity issues were found. Some changes were proposed to follow best practices and reduce potential attack surface.