ERC-3234: Batch Flash Loans
Simple Summary
This ERC provides standard interfaces and processes for multiple-asset flash loans.
Motivation
Flash loans of multiple assets, or batch flash loans, are a common offering of flash lenders, and have a strong use case in the simultaneous refinance of several positions between platforms. At the same time, batch flash loans are more complicated to use than single asset flash loans (ER3156). This divergence of use cases and user profiles calls for independent, but consistent, standards for single asset flash loans and batch flash loans.
Specification
A batch flash lending feature integrates two smart contracts using a callback pattern. These are called the LENDER and the RECEIVER in this EIP.
Lender Specification
A lender
MUST implement the IERC3234BatchFlashLender interface.
The maxFlashLoan
function MUST return the maximum loan possible for each token
. If a token
is not currently supported maxFlashLoan
MUST return 0, instead of reverting.
The flashFee
function MUST return the fees charged for each loan of amount
token
. If a token is not supported flashFee
MUST revert.
The batchFlashLoan
function MUST include a callback to the onBatchFlashLoan
function in a IERC3234BatchFlashBorrower
contract.
The batchFlashLoan
function MUST transfer amounts[i]
of each tokens[i]
to receiver
before the callback to the borrower.
The batchFlashLoan
function MUST include msg.sender
as the initiator
to onBatchFlashLoan
.
The batchFlashLoan
function MUST NOT modify the tokens
, amounts
and data
parameters received, and MUST pass them on to onBatchFlashLoan
.
The lender
MUST verify that the onBatchFlashLoan
callback returns the keccak256 hash of "ERC3234BatchFlashBorrower.onBatchFlashLoan".
The batchFlashLoan
function MUST include a fees
argument to onBatchFlashLoan
with the fee to pay for each individual token
and amount
lent, ensuring that fees[i] == flashFee(tokens[i], amounts[i])
.
After the callback, for each token
in tokens
, the batchFlashLoan
function MUST take the amounts[i] + fees[i]
of tokens[i]
from the receiver
, or revert if this is not successful.
If successful, batchFlashLoan
MUST return true
.
Receiver Specification
A receiver
of flash loans MUST implement the IERC3234BatchFlashBorrower interface:
For the transaction to not revert, for each token
in tokens
, receiver
MUST approve amounts[i] + fees[i]
of tokens[i]
to be taken by msg.sender
before the end of onBatchFlashLoan
.
If successful, onBatchFlashLoan
MUST return the keccak256 hash of "ERC3156BatchFlashBorrower.onBatchFlashLoan".
Rationale
The interfaces described in this ERC have been chosen as to cover the known flash lending use cases, while allowing for safe and gas efficient implementations.
flashFee
reverts on unsupported tokens, because returning a numerical value would be incorrect.
batchFlashLoan
has been chosen as a function name as descriptive enough, unlikely to clash with other functions in the lender, and including both the use cases in which the tokens lended are held or minted by the lender.
receiver
is taken as a parameter to allow flexibility on the implementation of separate loan initiators and receivers.
Existing flash lenders (Aave, dYdX and Uniswap) all provide flash loans of several token types from the same contract (LendingPool, SoloMargin and UniswapV2Pair). Providing a token
parameter in both the batchFlashLoan
and onBatchFlashLoan
functions matches closely the observed functionality.
A bytes calldata data
parameter is included for the caller to pass arbitrary information to the receiver
, without impacting the utility of the batchFlashLoan
standard.
onBatchFlashLoan
has been chosen as a function name as descriptive enough, unlikely to clash with other functions in the receiver
, and following the onAction
naming pattern used as well in EIP-667.
An initiator
will often be required in the onBatchFlashLoan
function, which the lender knows as msg.sender
. An alternative implementation which would embed the initiator
in the data
parameter by the caller would require an additional mechanism for the receiver to verify its accuracy, and is not advisable.
The amounts
will be required in the onBatchFlashLoan
function, which the lender took as a parameter. An alternative implementation which would embed the amounts
in the data
parameter by the caller would require an additional mechanism for the receiver to verify its accuracy, and is not advisable.
The fees
will often be calculated in the batchFlashLoan
function, which the receiver
must be aware of for repayment. Passing the fees
as a parameter instead of appended to data
is simple and effective.
The amount + fee
are pulled from the receiver
to allow the lender
to implement other features that depend on using transferFrom
, without having to lock them for the duration of a flash loan. An alternative implementation where the repayment is transferred to the lender
is also possible, but would need all other features in the lender to be also based in using transfer
instead of transferFrom
. Given the lower complexity and prevalence of a "pull" architecture over a "push" architecture, "pull" was chosen.
Security Considerations
Verification of callback arguments
The arguments of onBatchFlashLoan
are expected to reflect the conditions of the flash loan, but cannot be trusted unconditionally. They can be divided in two groups, that require different checks before they can be trusted to be genuine.
- No arguments can be assumed to be genuine without some kind of verification.
initiator
,tokens
andamounts
refer to a past transaction that might not have happened if the caller ofonBatchFlashLoan
decides to lie.fees
might be false or calculated incorrectly.data
might have been manipulated by the caller. - To trust that the value of
initiator
,tokens
,amounts
andfees
are genuine a reasonable pattern is to verify that theonBatchFlashLoan
caller is in a whitelist of verified flash lenders. Since often the caller ofbatchFlashLoan
will also be receiving theonBatchFlashLoan
callback this will be trivial. In all other cases flash lenders will need to be approved if the arguments inonBatchFlashLoan
are to be trusted. - To trust that the value of
data
is genuine, in addition to the check in point 1, it is recommended that thereceiver
verifies that theinitiator
is in some list of trusted addresses. Trusting thelender
and theinitiator
is enough to trust that the contents ofdata
are genuine.
Flash lending security considerations
Automatic approvals for untrusted borrowers
The safest approach is to implement an approval for amount+fee
before the batchFlashLoan
is executed.
Including in onBatchFlashLoan
the approval for the lender
to take the amount + fee
needs to be combined with a mechanism to verify that the borrower is trusted, such as those described above.
If an unsuspecting contract with a non-reverting fallback function, or an EOA, would approve a lender
implementing ERC3156, and not immediately use the approval, and if the lender
would not verify the return value of onBatchFlashLoan
, then the unsuspecting contract or EOA could be drained of funds up to their allowance or balance limit. This would be executed by a borrower
calling batchFlashLoan
on the victim. The flash loan would be executed and repaid, plus any fees, which would be accumulated by the lender
. For this reason, it is important that the lender
implements the specification in full and reverts if onBatchFlashLoan
doesn't return the keccak256 hash for "ERC3156FlashBorrower.onBatchFlashLoan".
Flash minting external security considerations
The typical quantum of tokens involved in flash mint transactions will give rise to new innovative attack vectors.
Example 1 - interest rate attack
If there exists a lending protocol that offers stable interests rates, but it does not have floor/ceiling rate limits and it does not rebalance the fixed rate based on flash-induced liquidity changes, then it could be susceptible to the following scenario:
FreeLoanAttack.sol
- Flash mint 1 quintillion DAI
- Deposit the 1 quintillion DAI + $1.5 million worth of ETH collateral
- The quantum of your total deposit now pushes the stable interest rate down to 0.00001% stable interest rate
- Borrow 1 million DAI on 0.00001% stable interest rate based on the 1.5M ETH collateral
- Withdraw and burn the 1 quint DAI to close the original flash mint
- You now have a 1 million DAI loan that is practically interest free for perpetuity ($0.10 / year in interest)
The key takeaway being the obvious need to implement a flat floor/ceiling rate limit and to rebalance the rate based on short term liquidity changes.
Example 2 - arithmetic overflow and underflow
If the flash mint provider does not place any limits on the amount of flash mintable tokens in a transaction, then anyone can flash mint 2^256-1 amount of tokens.
The protocols on the receiving end of the flash mints will need to ensure their contracts can handle this. One obvious way is to leverage OpenZeppelin's SafeMath libraries as a catch-all safety net, however consideration should be given to when it is or isn't used given the gas tradeoffs.
If you recall there was a series of incidents in 2018 where exchanges such as OKEx, Poloniex, HitBTC and Huobi had to shutdown deposits and withdrawls of ERC20 tokens due to integer overflows within the ERC20 token contracts.
Flash minting internal security considerations
The coupling of flash minting with business specific features in the same platform can easily lead to unintended consequences.
Example - Treasury draining
In early implementations of the Yield Protocol flash loaned fyDai could be redeemed for Dai, which could be used to liquidate the Yield Protocol CDP vault in MakerDAO:
- Flash mint a very large amount of fyDai.
- Redeem for Dai as much fyDai as the Yield Protocol collateral would allow.
- Trigger a stability rate increase with a call to
jug.drip
which would make the Yield Protocol uncollateralized. - Liquidate the Yield Protocol CDP vault in MakerDAO.
Copyright
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