msg.value in Loop

Overview

The msg.value-in-loop detector identifies contracts that reference msg.value inside a loop body. Since msg.value is constant throughout a transaction, using it inside a loop that processes multiple items means the same ETH amount is credited to each iteration. An attacker sending 1 ETH can have it counted N times if the loop runs N iterations, effectively multiplying their deposit.

Why This Is an Issue

This is a straightforward accounting flaw. If a batch deposit function loops over recipients and credits each with msg.value, the contract believes it received N times the actual ETH sent. The attacker deposits once but receives credit for N deposits, allowing them to withdraw far more than they sent. This pattern has appeared in multi-call contracts, batch minting functions, and aggregator protocols.

How to Resolve

// Before: Vulnerable — msg.value counted per iteration
function batchDeposit(address[] calldata recipients) external payable {
    for (uint i = 0; i < recipients.length; i++) {
        deposits[recipients[i]] += msg.value;  // Same msg.value each iteration
    }
}

// After: Fixed — divide msg.value or use explicit amounts
function batchDeposit(address[] calldata recipients, uint256[] calldata amounts) external payable {
    uint256 total = 0;
    for (uint i = 0; i < recipients.length; i++) {
        deposits[recipients[i]] += amounts[i];
        total += amounts[i];
    }
    require(total == msg.value, "Amount mismatch");
}

Detection Methodology

1. Loop identification: Detects loop structures (back-edges in the CFG) in the contract bytecode.
2. CALLVALUE in loop body: Checks whether the CALLVALUE opcode (which reads msg.value) appears within loop-body blocks.
3. Accumulation pattern: Flags cases where the CALLVALUE result feeds into an addition or storage write that executes on each iteration.
4. Single-iteration filter: Loops that provably execute exactly once (e.g., length-1 arrays) are filtered out.

Limitations

False positives: Loops that read msg.value for comparison (e.g., require(msg.value >= minPrice)) without accumulating it are safe but may be flagged. False negatives: Indirect references to msg.value through local variables assigned before the loop are not traced into the loop body.

Related Detectors

• Integer Overflow — detects arithmetic overflow broadly
• Business Logic Error — detects logic flaws in token accounting

References

• Solidity Docs — Special Variables
• SWC-104: Unchecked Call Return Value