Flash Loan

Flash Loan Attack Playbook

Flash loans give the attacker uncollateralized capital for a single transaction. They're not vulnerabilities themselves — they're a force multiplier for existing bugs. Sources: Aave, Balancer, Uniswap V2/V3, Maker, dYdX (deprecated).

Common attack patterns

1. Oracle amplification (most common)

Use loan to push a price, then trigger price-dependent action. Cross-reference: see oracle-manipulation/SKILL.md.

2. Unauthenticated callback handler

function executeOperation(
    address[] calldata assets,
    uint256[] calldata amounts,
    uint256[] calldata premiums,
    address initiator,                     // ← UNVALIDATED in many contracts
    bytes calldata params
) external returns (bool) {
    // Anyone can call this with fake `initiator`
    // and have the contract do whatever the params say
}

Bug: initiator and msg.sender == pool checks are missing. Attack: call executeOperation directly with no actual loan, malicious params → contract does the operation anyway.

3. Governance attack via flash loan

// Vulnerable governance:
function propose() external {
    require(getVotes(msg.sender) > THRESHOLD);
    // ...
}
function getVotes(address user) public view returns (uint256) {
    return token.balanceOf(user);  // ← reads SPOT balance, not snapshot
}

Attack: flash-loan governance tokens, propose malicious change (e.g., upgrade contract to drain), vote with the loaned tokens, execute, repay loan. MakerDAO had this pattern; mitigated by checkpoint-based voting.

4. Liquidity manipulation

Pool that uses totalSupply() or balanceOf(pool) for share math:

function deposit(uint256 amt) external {
    uint256 shares = (amt * totalSupply()) / underlying.balanceOf(address(this));
    _mint(msg.sender, shares);
    underlying.transferFrom(msg.sender, address(this), amt);
}

Attacker:

1. Flash-loan and deposit (becomes 99% of pool)
2. Donate underlying to inflate underlying.balanceOf artificially
3. Next depositor's shares = amt * totalSupply / inflatedBalance ≈ 0
4. Withdraw → drain This is the ERC4626 inflation attack (also covered in reentrancy/ overlap).

5. Liquidation arbitrage gone wrong

A liquidator that uses flash loans to repay debt before claiming collateral — usually benign. The bug: liquidation health check happens before the seizure, but the seizure happens via callback into a manipulable function. Reentrancy + oracle dependent.

Audit steps

1. Locate flash-loan integrations

grep -rn 'executeOperation\|flashLoan\|onFlashLoan\|flashCallback\|aaveFlashLoan' src/

2. For each handler

Check:

• msg.sender == known_pool (whitelist callback origin)
• initiator == address(this) (only respond to your own loans)
• ReentrancyGuard on the public function that takes the loan
• The fee is correctly accounted (amounts[i] + premiums[i] is repaid)

3. For each price-reading function

See oracle-manipulation/SKILL.md.

4. For each share-mint / share-burn function

Check ERC4626 virtual-shares mitigation:

// OpenZeppelin ERC4626 v4.7+
function _convertToShares(uint256 assets, MathUpgradeable.Rounding rounding) internal view virtual override returns (uint256) {
    return _initialConvertToShares(assets, rounding);  // adds 10**18 to total supply for share calc
}

If using legacy ERC4626 or custom vault math, write a Foundry test that:

1. Attacker deposits 1 wei → gets 1 share
2. Attacker direct-transfers 1e18 underlying to vault
3. Victim deposits 1e18 → gets 0 shares (rounding)
4. Attacker withdraws 1 share → gets 100% of pool

foundry_inflation_test(vault_address="...", underlying="...", target_path="src/Vault.sol")

Severity calibration

PoC template (Foundry)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "forge-std/Test.sol";
import "@aave-v3/interfaces/IPool.sol";

contract Test_FlashAttack is Test {
    IPool constant AAVE = IPool(0x87870Bca3F3fD6335C3F4ce8392D69350B4fA4E2);
    address constant TARGET = 0x...;
    address constant ATTACKER = address(0xBEEF);

    function setUp() public {
        vm.createSelectFork("https://eth-mainnet.../<block>");
    }

    function test_drain() public {
        vm.startPrank(ATTACKER);
        uint256 beforeBal = WETH.balanceOf(ATTACKER);

        // Request 10000 WETH flash loan
        address[] memory assets = new address[](1);
        assets[0] = address(WETH);
        uint256[] memory amts = new uint256[](1);
        amts[0] = 10000 ether;
        uint256[] memory modes = new uint256[](1);  // 0 = repay full

        AAVE.flashLoan(address(this), assets, amts, modes, address(this), "", 0);

        uint256 afterBal = WETH.balanceOf(ATTACKER);
        assertGt(afterBal, beforeBal + 100 ether, "should profit");
    }

    function executeOperation(
        address[] calldata assets,
        uint256[] calldata amts,
        uint256[] calldata premiums,
        address initiator,
        bytes calldata
    ) external returns (bool) {
        // 1. Use loaned funds to manipulate TARGET
        // 2. Profit
        // 3. Repay
        IERC20(assets[0]).approve(address(AAVE), amts[0] + premiums[0]);
        return true;
    }
}

OPSEC (operational, not blue team — this is on-chain)

Flash-loan attacks leave a single transaction trail visible to:

• Mempool watchers / Flashbots inspectors
• MEV bots that may front-run if not using private mempool
• Risk dashboards (DefiLlama, Forta, Cyfrin)

For audit-PoC purposes (not live exploit) just use a forked anvil.

Defender remediation

1. Snapshot-based governance — vote based on historical balance, not spot
2. TWAP oracles only — never spot price for trust-critical decisions
3. Validate flash callback origin — require(msg.sender == knownPool && initiator == address(this))
4. ERC4626 OpenZeppelin v4.7+ — virtual shares mitigation
5. Initial deposit by deployer — seed the vault with 10**18 underlying before opening, makes inflation impractical

Known exemplars

• bZx 1 (Feb 2020): $350k flash-loan oracle attack
• bZx 2 (Feb 2020): $645k Synthetix sUSD attack
• Beanstalk (Apr 2022): $182M flash-loan governance attack
• Euler (Mar 2023): $197M; donateToReserves bug + flash loan
• Mango (Oct 2022): $114M oracle + flash loan
• Cream (Oct 2021): $130M via yUSD oracle
• KyberSwap (Nov 2023): $48.5M via concentrated-liquidity edge case + flash loan