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Asymmetry USA.d
Electisec USA.d Review
Review Resources:
Auditors:
- Panda
- Adriro
Table of Contents
{: .no_toc }
Review Summary
USA.d
USD.a is a synthetic dollar, part of a collateralized debt position (CDP), backed by a stablecoin, ETH, and BTC basket.
The codebase is forked from Liquity V2 (codename bold) and adjusted to include the collateral changes.
The contracts of the USA.d Repo were reviewed over five days. The code review was performed by two auditors between 27 January and 31 January 2025. The repository was under active development during the review, but the review was limited to the latest commit d74f1532535e6471bbe437d16616ffe12fc36ecf for the USA.d repo.
Scope
The scope of the review consisted of the following contracts at the specific commit:
contracts/script/DeployUSA.D.s.sol
contracts/src/PriceFeeds/MainnetPriceFeedBase.sol
contracts/src/PriceFeeds/USA.D/BaseOracle.sol
contracts/src/PriceFeeds/USA.D/ERC4626Oracle.sol
contracts/src/PriceFeeds/USA.D/Fallbacks/BaseFallbackOracle.sol
contracts/src/PriceFeeds/USA.D/Fallbacks/CrvUsdFallbackOracle.sol
contracts/src/PriceFeeds/USA.D/Fallbacks/SfrxEthFallbackOracle.sol
contracts/src/PriceFeeds/USA.D/Fallbacks/TbtcFallbackOracle.sol
contracts/src/PriceFeeds/USA.D/Fallbacks/WbtcFallbackOracle.sol
contracts/src/PriceFeeds/USA.D/ScrvUsdOracle.sol
contracts/src/PriceFeeds/USA.D/SdaiOracle.sol
contracts/src/PriceFeeds/USA.D/SfrxEthOracle.sol
contracts/src/PriceFeeds/USA.D/TbtcOracle.sol
contracts/src/PriceFeeds/USA.D/WbtcOracle.sol
contracts/src/TroveNFT.sol
contracts/src/Zappers/USAZapper.sol
After the findings were presented to the USA.d team, fixes were made and included in several PRs.
This review is a code review to identify potential vulnerabilities in the code. The reviewers did not investigate security practices or operational security and assumed that privileged accounts could be trusted. The reviewers did not evaluate the security of the code relative to a standard or specification. The review may not have identified all potential attack vectors or areas of vulnerability.
Electisec and the auditors make no warranties regarding the security of the code and do not warrant that the code is free from defects. Electisec and the auditors do not represent nor imply to third parties that the code has been audited nor that the code is free from defects. By deploying or using the code, USA.d and users of the contracts agree to use the code at their own risk.
Code Evaluation Matrix
| Category | Mark | Description |
|---|---|---|
| Access Control | Good | Most changes don't require access control. MockInterestRouter and MetadataNFT functions are restricted to the Asymmetry multisig. |
| Mathematics | Low | One severe issue was found in the oracle math. |
| Complexity | Average | While the changes do not involve an increase in complexity, it is important to note that Liquity itself is a complex protocol.. |
| Libraries | Good | Standard, well-audited libraries used. It relies on the OpenZeppelin library and the Uniswap V3 periphery contracts to interface with the oracles. |
| Decentralization | Good | The protocol inherits the decentralized nature of Liquity v2. |
| Code stability | Good | No changes were made to the contract during the review. |
| Documentation | Good | Being a Liquity fork, plenty of documentation is available. |
| Monitoring | Good | Sufficient event emissions for tracking operations. |
| Testing and verification | Average | More tests around oracles are suggested. |
Findings Explanation
Findings are broken down into sections by their respective impact:
- Critical, High, Medium, Low impact
- These are findings that range from attacks that may cause loss of funds, impact control/ownership of the contracts, or cause any unintended consequences/actions that are outside the scope of the requirements.
- Gas savings
- Findings that can improve the gas efficiency of the contracts.
- Informational
- Findings including recommendations and best practices.
Critical Findings
1. Critical - Incorrect price calculation in WBTC Oracle
The WBTC Oracle uses the WBTC/BTC and BTC/USD Chainlink price feeds to calculate the price. However, the underlying calculation is incorrect, as it divides the BTC/USD price by the WBTC/BTC price.
Technical Details
The WbtcOracle.sol implementation fetches both prices and divides them to get the WBTC/USD price.
59: int256 wbtcUsdPrice = btcUsdPrice * int256(10 ** decimals()) / wbtcBtcPrice;
Since the WBTC/BTC price represents the amount of WBTC in terms of BTC, and the BTC/USD represents the amount of BTC in terms of USD, the calculation should multiply both prices and adjust the scale.
Impact
Critical. The WBTC Oracle price is incorrect.
Recommendation
The calculation should multiply both prices and divide by the scale.
- int256 wbtcUsdPrice = btcUsdPrice * int256(10 ** decimals()) / wbtcBtcPrice;
+ int256 wbtcUsdPrice = wbtcBtcPrice * btcUsdPrice / int256(1e8) ;
Developer Response
Fixed e7dffc2.
High Findings
None
Medium Findings
None
Low Findings
1. Low - Incorrect staleness threshold for sfrxETH Oracle
The Oracle is configured with a 24-hour threshold, while the underlying Chainlink Oracle uses one hour.
Technical Details
The SfrxEthOracle.sol contract is configured with a 24-hour staleness threshold in the deployment script.
323: } else if (address(_collToken) == SFRXETH) {
324: _stalenessThreshold = _24_HOURS; // CL FRAX/ETH heartbeat (Fallback). Primary FRAX/USD is 1 hour, but falls back
325: SfrxEthFallbackOracle fallbackOracle = new SfrxEthFallbackOracle();
326: contracts.oracle = address(new SfrxEthOracle(address(fallbackOracle)));
327: sfrxEthFallbackOracle = fallbackOracle;
The comment indicates the fallback uses a 24-hour threshold due to the FRAX/ETH Chainlink price feed. However, the fallback is based on Curve's Price Aggregator, which doesn't expose a last update timestamp.
Impact
Low.
Recommendation
Change the threshold to one hour.
Developer Response
Fixed f606f9f
2. Low - Do not set the same value for MCR and SCR
Technical Details
In the DeployUSA.D.s.sol script, the TroveManager parameters are set with equal MCR (Minimum Collateral Ratio) and SCR (System Critical Ratio) values for all collateral types:
troveManagerParamsArray[0] = TroveManagerParams(150e16, 110e16, 110e16, 5e16, 10e16); // scrvUSD
troveManagerParamsArray[1] = TroveManagerParams(150e16, 110e16, 110e16, 5e16, 10e16); // sDAI
troveManagerParamsArray[2] = TroveManagerParams(150e16, 110e16, 110e16, 5e16, 10e16); // sfrxETH
troveManagerParamsArray[3] = TroveManagerParams(150e16, 110e16, 110e16, 5e16, 10e16); // tBTC
troveManagerParamsArray[4] = TroveManagerParams(150e16, 110e16, 110e16, 5e16, 10e16); // WBTC
For each collateral type, both MCR and SCR are set to 110e16 (110%). When these values are equal, a single trove that becomes liquidatable (falls below MCR) can trigger a system-wide shutdown of that particular branch, as the total collateral ratio would fall below the SCR threshold.
If a branch has only one trove and that trove becomes liquidatable, the entire branch will be forced to shut down. This creates a vulnerability where a single borrower could intentionally or unintentionally cause a branch-wide shutdown, affecting all potential users of that collateral type.
Impact
Low.
Recommendation
There are two possible solutions:
`. Set the SCR lower than the MCR to create a buffer zone between when individual troves become liquidatable and when the system enters recovery mode. For example:
troveManagerParamsArray[0] = TroveManagerParams(150e16, 110e16, 105e16, 5e16, 10e16); // SCR < MCR
- As part of the deployment process:
- Open a trove on each branch
- Redeem it down to 0 debt
- Keep it open for each branch
Developer Response
Acknowledged.
Gas Saving Findings
None
Informational Findings
1. Informational - Consider using sUSDS as a collateral
Sky.money has introduced a new stablecoin called USDS, which can be exchanged 1:1 with DAI.
Technical Details
The Total Value Locked (TVL) of Sky.money has grown rapidly, to the detriment of DAI. Currently, the TVL of USDS is bigger than that of DAI. As more funds transition to USDS, incorporating it as an additional collateral type will maintain the ability to use the Sky.money/MakerDAO stablecoin effectively.
As more funds are moving to USDS, having USDS as another collateral will keep the possibility of using the sky/MakerDao stable coin.
Impact
Informational.
Recommendation
Add sUSDS as a collateral
Developer Response
USDS added as a collateral 62e1ef
2. Informational - Incorrect constant name
Technical Details
The constants CL_TBTC_BTC_PRICE_FEED and _CL_TBTC_BTC_HEARTBEAT are incorrectly named as they suggest a tBTC/BTC price feed, but the address 0x8350b7De6a6a2C1368E7D4Bd968190e13E354297 actually corresponds to a tBTC/USD Chainlink price feed.
Impact
Informational.
Recommendation
Rename the constants to reflect that it's an accurate USD price feed.
Developer Response
Fixed 97a971a
3. Informational - FallbackOracle price could rely on chainlink response.
The SfrxEthFallbackOracle, TbtcFallbackOracle, and WbtcFallbackOracle contracts rely on CryptoWithStablePriceSfrxeth, CryptoWithStablePriceTBTC and CryptoWithStablePriceWBTC. The three contracts from curve have the option to use a chainlink oracle as part of the price() function calculation. The call to the Chainlink Oracle is currently disabled but can be turned on.
Technical Details
We are going to focus on the SfrxEthOracle but this issue is similar with on the other oracles.
The SfrxEthOracle has a fallback oracle if the oracle response is zero. The fallback oracle uses the contract CryptoWithStablePriceSfrxeth this contract can use chainlink as an oracle. Both CryptoWithStablePriceSfrxeth and SfrxEthOracle. When enabled in CryptoWithStablePriceSfrxeth. If the answer value from the oracle is zero but the oracle is properly updated, the likely wrong price from Chainlink will be used, and the fallback oracle will also return a zero value.
Impact
Informational
Recommendation
- Monitor if the
CryptoWithStablePriceSfrxethcontractuse_chainlinkis set to true. - Disable the fallback oracle if it is set to use chainlink
Developer Response
Will be monitored.
4. Informational - Replace assets for logos
The deployment script still references Liquity's assets.
Technical Details
The MetadataDeployment.sol contract loads the logos for the original Liquity assets (BOLD, WETH, rETH, wstETH, and geist).
Impact
Informational.
Recommendation
Replace the assets for the logos used in the protocol: USA.D, scrvUSD, sDAI, sfrxETH, tBTC, and WBTC.
Developer Response
5. Informational - Revert in unimplemented Zapper functions
There are several functions in USAZapper.sol, inherited from the BaseZapper interface, that are left unimplemented.
Technical Details
closeTroveFromCollateral()receiveFlashLoanOnCloseTroveFromCollateral()receiveFlashLoanOnOpenLeveragedTrove()receiveFlashLoanOnLeverUpTrove()receiveFlashLoanOnLeverDownTrove()
Impact
Informational.
Recommendation
Consider reverting these functions to state that these shouldn't be called explicitly. Alternatively, the inherited interface could be adjusted to remove them.
Developer Response
Won't implement.
Final remarks
USA.d is a fork of Liquity with limited modifications built to create a stablecoin backed by BTC, ETH derivatives, and DAI/USDS collaterals. It contains new oracles relying on chainlink with fallback oracles in case of a failure to get a value from the chainlink oracles. The codebase is well documented, and the changes are isolated from the original codebase. The team has been responsive to the findings and has fixed the critical issue found during the review. The auditors recommend further testing around the oracles and the zappers.