Sigvex
EVMcriticalDetector2 min read

Unprotected Ether Withdrawal

Detects functions that transfer Ether without verifying the caller's authorization, allowing anyone to drain the contract's ETH balance.

evmcriticalaccess-controlstatic-analysisdetectordata-flow

Unprotected Ether Withdrawal

Overview

The unprotected ether withdrawal detector identifies functions that send ETH (via CALL with non-zero value, transfer, or send) without checking that msg.sender matches a privileged address. Any user who discovers such a function can drain the contract’s entire ETH balance.

Sigvex locates CALL instructions with non-zero value operands and checks whether the function contains a CALLER comparison against a stored address before the call.

Why This Is an Issue

This is the simplest form of access control failure: a public function exists that sends ETH to msg.sender or to an address parameter, without any ownership or role check. The Parity multi-sig wallet freeze involved a similar pattern where critical functions lacked access control.

How to Resolve

// Before: Vulnerable — anyone can withdraw
function withdraw() external {
    payable(msg.sender).transfer(address(this).balance);
}

// After: Fixed — restrict to owner
modifier onlyOwner() {
    require(msg.sender == owner, "Not owner");
    _;
}

function withdraw() external onlyOwner {
    payable(owner).transfer(address(this).balance);
}

Detection Methodology

  1. ETH transfer detection: Identifies CALL instructions where the value parameter is non-zero, plus SELFDESTRUCT which forwards remaining balance.
  2. Authorization check search: Looks for CALLER (msg.sender) loaded and compared (EQ) against a storage value before the transfer.
  3. Destination analysis: Checks whether the ETH destination is msg.sender (higher risk) or a hardcoded address (lower risk).

Limitations

False positives: Functions protected by access control in a separate modifier or parent contract may be flagged if the check is not inlined. False negatives: Contracts that use assembly-level balance manipulation may be missed.

References