apegurus/weird-tokens

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Weird ERC20 Tokens Reference

Tokens that don't behave like standard ERC20. Integrating with these requires special handling.

Quick Reference Table

| Pattern | Example Tokens | Risk | Mitigation | |---------|---------------|------|------------| | Missing return values | USDT, BNB, OMG | Silent failures | SafeERC20 | | Fee on transfer | STA, PAXG, SAFEMOON | Balance mismatch | Measure before/after | | Rebasing | AMPL, stETH, OHM | Cached balance wrong | Don't cache balances | | Pausable | BNB, ZIL | DOS | Handle gracefully | | Blocklist | USDC, USDT | User funds frozen | Document risk | | Upgradeable | USDC, USDT | Behavior can change | Monitor upgrades | | Flash mintable | DAI | Infinite supply attacks | Check total supply | | Multiple addresses | TUSD | Accounting errors | Verify canonical address | | Low decimals | USDC (6), GUSD (2) | Precision loss | Handle decimals explicitly | | High decimals | YAM-V2 (24) | Overflow risk | Check bounds | | Approval race | USDT, KNC | Front-running | Set to 0 first | | Revert on zero | LEND | Unexpected reverts | Check amount > 0 | | Non-string metadata | MKR | Type errors | Handle bytes32 | | ERC777 hooks | imBTC | Reentrancy | Treat as untrusted |

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Detailed Patterns

1. Missing Return Values

Tokens: USDT, BNB, OMG, BADGER

Problem:

// Returns nothing, but ERC20 interface expects bool
function transfer(address to, uint value) public {
    // ... no return statement
}

Exploit:

// This compiles but fails at runtime for USDT
IERC20(usdt).transfer(to, amount);  // Reverts: expected return data

Fix:

import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
using SafeERC20 for IERC20;

IERC20(usdt).safeTransfer(to, amount);  // Works

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2. Fee on Transfer

Tokens: STA, PAXG, SAFEMOON, most "reflection" tokens

Problem:

function transfer(address to, uint value) public returns (bool) {
    uint fee = value * feePercent / 100;
    _transfer(msg.sender, feeCollector, fee);
    _transfer(msg.sender, to, value - fee);
    return true;
}

Exploit:

// User deposits 100, protocol credits 100
// But only 98 actually arrived (2% fee)
// User can withdraw 100, stealing 2 from protocol
function deposit(uint amount) external {
    token.transferFrom(msg.sender, address(this), amount);
    balances[msg.sender] += amount;  // WRONG!
}

Fix:

function deposit(uint amount) external {
    uint balanceBefore = token.balanceOf(address(this));
    token.transferFrom(msg.sender, address(this), amount);
    uint received = token.balanceOf(address(this)) - balanceBefore;
    balances[msg.sender] += received;  // Credit actual amount
}

---

3. Rebasing Tokens

Tokens: AMPL, stETH (rebases), OHM, AAVE aTokens

Problem:

// Balance changes without any transfers
function rebase(int supplyDelta) external {
    totalSupply = totalSupply + supplyDelta;
    // All balances scale proportionally
}

Exploit:

// Protocol caches balance
uint cachedBalance = steth.balanceOf(address(this));

// ... time passes, rebase happens ...

// Cached balance is now wrong
// Can lead to accounting errors, insolvency

Fix:

• Don't cache balances
• Use wrapper tokens (wstETH instead of stETH)
• Track shares, not absolute amounts

---

4. Pausable Tokens

Tokens: BNB, ZIL, USDC (admin can pause)

Problem:

modifier whenNotPaused() {
    require(!paused, "Paused");
    _;
}

function transfer(address to, uint value) public whenNotPaused {
    // ...
}

Impact:

• Protocol can be DOS'd if token pauses
• User funds stuck
• Liquidations fail

Fix:

• Have emergency withdrawal that doesn't rely on transfers
• Document risk to users
• Consider timelock on protocol side

---

5. Blocklist/Blacklist Tokens

Tokens: USDC, USDT (compliance blacklists)

Problem:

function transfer(address to, uint value) public {
    require(!blacklisted[msg.sender], "Blacklisted");
    require(!blacklisted[to], "Blacklisted");
    // ...
}

Impact:

• Blacklisted users can't withdraw
• Protocol holding funds for blacklisted user is stuck

Fix:

• Document risk
• Consider allowing admin rescue to different address
• Use wrapper tokens where possible

---

6. Upgradeable Tokens

Tokens: USDC, USDT, many newer tokens

Problem:

• Token logic can change after integration
• New fee mechanisms could be added
• Pausability could be added

Impact:

• Your integration assumptions may break post-upgrade

Fix:

• Monitor token upgrades
• Have upgrade handlers in your protocol
• Test against upgraded implementations

---

7. Flash Mintable Tokens

Tokens: DAI (flash mint), some wrapped tokens

Problem:

function flashLoan(uint amount) external {
    _mint(msg.sender, amount);
    IFlashBorrower(msg.sender).onFlashLoan(amount);
    _burn(msg.sender, amount);
}

Impact:

• Attacker can temporarily have infinite tokens
• Breaks governance (voting with flash minted tokens)
• Price manipulation

Fix:

• Use TWAP for prices
• Snapshot voting tokens before proposals
• Check totalSupply before/after for sanity

---

8. Approval Race Condition

Tokens: USDT requires setting to 0 first

Problem:

// USDT's approve reverts if allowance > 0 and newValue > 0
function approve(address spender, uint value) public {
    require(value == 0 || allowance[msg.sender][spender] == 0);
    // ...
}

Fix:

// Always set to 0 first
token.approve(spender, 0);
token.approve(spender, newAmount);

// Or use SafeERC20
token.safeIncreaseAllowance(spender, amount);

---

9. ERC777 Hooks (Reentrancy)

Tokens: imBTC, any ERC777

Problem:

// ERC777 calls hooks on sender and receiver
function _send(address from, address to, uint256 amount) internal {
    _callTokensToSend(from, to, amount);  // HOOK - reentrancy!
    _transfer(from, to, amount);
    _callTokensReceived(from, to, amount);  // HOOK - reentrancy!
}

Impact:

• Reentrancy even without ETH transfers
• imBTC was exploited via this

Fix:

• Treat ERC777 transfers as untrusted external calls
• Apply CEI pattern
• Use ReentrancyGuard

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Integration Checklist

When integrating any token:

• [ ] Check for missing return values (SafeERC20)
• [ ] Check for fee-on-transfer (measure actual received)
• [ ] Check for rebasing (don't cache balances)
• [ ] Check for pausability (emergency paths)
• [ ] Check for blocklists (document risk)
• [ ] Check for upgradeability (monitor changes)
• [ ] Check for flash minting (use TWAP)
• [ ] Check for ERC777 hooks (reentrancy guard)
• [ ] Check decimals (6, 8, 18, 24 all exist)
• [ ] Check approval behavior (set to 0 first for USDT)

Resources

• weird-erc20: https://github.com/d-xo/weird-erc20
• Token Integration Checklist: https://github.com/crytic/building-secure-contracts/blob/master/development-guidelines/token_integration.md