Reentrancy through stale contract balance checks

Severity: High ID: reentrancy-balance

Flags reentrant external calls between saving address(this).balance and checking the current contract balance against that saved value.

What it does

Warns when a public or external entry point saves address(this).balance in a local value, performs an external call that can forward enough gas to re-enter, and then compares the saved value with a fresh contract-balance read in a require, assert, or exiting branch. Casts, tuple assignments, derived locals, fresh post-call balance locals, internal helper parameters and returns, and modifiers are tracked when their bodies are available.

This detector intentionally covers native ETH held by the current contract. It is not the later token balanceOf(address) detector with a similar name. It does not report token balances, balances of other addresses, mutable state or storage baselines, view or static calls, calls capped at a total callee budget of 2,300 gas or less after accounting for the value-transfer stipend, checks on directly mutually exclusive branches, baselines overwritten after the call, or expressions that do not compare a fresh contract-balance read with the stale local value. It also recognizes standard state-lock nonReentrant modifiers when the same lock guards every mutable entry point on each concrete deployment of the function.

Why is this bad?

A callback can re-enter the function several times before any invocation reaches its balance check. Nested invocations can therefore share the same pre-call balance and make one payment appear to satisfy several operations. A non-strict inequality does not prevent the attack because the saved baseline, rather than the comparison operator, is stale.

Example

Bad

function mint(IPayer payer, uint256 amount) external {
    uint256 balanceBefore = address(this).balance;
    payer.pay();
    require(address(this).balance >= balanceBefore + amount, "insufficient payment");
    _mint(msg.sender, amount);
}

Good

function mint(uint256 amount) external payable nonReentrant {
    require(msg.value >= amount, "insufficient payment");
    _mint(msg.sender, amount);
}