dos-revert.md

DoS with (Unexpected) Revert

Preconditions

• Critical contract logic depends on an external call succeeding
• A single revert in the external call blocks the entire function
• OR: strict equality checks on contract balance can be violated by force-sent ETH
• OR: division by zero is possible due to unvalidated denominators

Vulnerable Pattern

// Push-payment: one reverting recipient blocks all payments
function payAll() external {
    for (uint256 i = 0; i < recipients.length; i++) {
        // If ANY recipient reverts (e.g., contract with no receive()),
        // the entire function reverts — no one gets paid
        require(payable(recipients[i]).send(amounts[i]), "transfer failed");
    }
}

// Strict balance check broken by force-sent ETH
function withdraw() external {
    // Attacker sends ETH via selfdestruct, breaking this check
    require(address(this).balance == expectedBalance, "invariant");
    _processWithdrawal();
}

// Division by zero
function distribute(uint256 totalShares) external {
    // If totalShares == 0, this reverts and blocks the function
    uint256 perShare = totalRewards / totalShares;
}

Detection Heuristics

1. Search for loops containing require or assert on external call results — one failure blocks all iterations
2. Search for push-payment patterns: contract iterating over recipients and sending ETH/tokens in one transaction
3. Search for strict balance equality checks (address(this).balance ==) — these can be broken by selfdestruct or coinbase rewards force-sending ETH
4. Search for division operations and check if the denominator can be zero
5. Check for require(success) after .send() or .call() inside loops — this turns a single recipient failure into a full DoS
6. Look for "highest bidder" or "king of the hill" patterns where the current leader's refund must succeed for a new leader to be set

False Positives

• Pull-payment pattern is used (each recipient withdraws individually)
• The external call target is a trusted, known contract that will not revert
• Division denominator is guaranteed non-zero by prior checks or invariants
• Balance checks use >= instead of ==
• The function handles individual failures gracefully (try/catch, continue on failure)

Remediation

• Replace push-payment with pull-payment: let recipients withdraw individually
• Use >= instead of == for balance checks to tolerate force-sent ETH
• Validate all denominators before division: require(totalShares > 0)
• In loops, handle individual call failures without reverting the whole transaction
• Use try/catch for external calls where failure should not be fatal

// Pull-payment pattern
mapping(address => uint256) public pendingWithdrawals;

function claimPayment() external {
    uint256 amount = pendingWithdrawals[msg.sender];
    require(amount > 0, "nothing to claim");
    pendingWithdrawals[msg.sender] = 0;
    (bool success,) = msg.sender.call{value: amount}("");
    require(success);
}