iktok90-design/ai-smart-contract-auditor/fee-on-transfer

Fee-on-transfer token accounting detection

When this applies

Trigger on any of:

• Deposit/stake/addLiquidity that records the amount argument after transferFrom
• Vault/ERC-4626 deposit minting shares proportional to the requested assets, not received
• AMM/router updating reserves with the input amount rather than actual receipt
• Lending protocols crediting collateral by passed amount
• Bridges locking/burning amount then minting the same amount on the far side
• Any flow assuming received == amount for arbitrary ERC-20s

Detection patterns

Credit passed amount, not delta (HIGH)

function deposit(uint256 amount) external {
    token.transferFrom(msg.sender, address(this), amount);
    balanceOf[msg.sender] += amount;          // ← amount, but FoT token delivered less
    totalDeposited        += amount;
}

Signal: the contract believes it holds more than it does. On a fee-on-transfer token (USDT with fee switch enabled, PAXG, many meme tokens), totalDeposited exceeds the real balance — last withdrawers are insolvent, or an attacker mints excess shares/LP.

AMM reserve desync (HIGH)

Router records input amountIn into reserves but the pair received amountIn - fee. k invariant is computed on phantom reserves, mispricing the swap and letting an attacker drain the shortfall over time (the Uniswap-V2-style "supports FoT" *SupportingFeeOnTransferTokens functions exist precisely because of this).

Rebasing double-count (MEDIUM / HIGH)

uint256 shares = amount * totalShares / totalAssets;   // totalAssets from stored var

Signal: stored totalAssets vs live token.balanceOf(this) diverge as a positive-rebasing token (stETH, aTokens, OHM) grows. Crediting against the stale stored figure under- or over-mints shares; an attacker times deposits around rebase.

Bridge mint mismatch (CRITICAL)

Lock amount on chain A, emit message, mint amount on chain B — but only amount - fee was locked. Repeated deposits accumulate an unbacked mint surplus that can be withdrawn on A by honest users, draining the lockbox.

Severity rubric

Remediation patterns

1. Measure the delta: uint256 before = token.balanceOf(address(this)); token.transferFrom(...); uint256 received = token.balanceOf(address(this)) - before; and credit received.
2. Reject FoT on entry — if received != amount, revert (OpenZeppelin SafeERC20 does not do this for you).
3. Use live balance, not stored totals, for rebasing tokens, or wrap them (wstETH) so balances are non-rebasing.
4. Token allowlist for assets known not to take a transfer fee; document the assumption.
5. On bridges, lock the measured received amount and mint exactly that.

False-positive notes

• A protocol that only ever handles a fixed, audited, non-FoT token (e.g. a WETH-only vault) is safe — note the assumption rather than flagging High.
• *SupportingFeeOnTransferTokens router variants already measure deltas — don't flag.
• Read-only quoting that never credits balances is unaffected.

Related

• [[token-compatibility]] — broader weird-ERC20 behaviors (no return value, blocklists)
• [[erc4626-inflation]] — share-mint math interacts with miscounted assets
• [[cross-chain-messaging]] — bridge mint/lock mismatch surface