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Unsafe expect

Description​

In Rust, the expect method is often used for error handling. It returns the contained Ok value for a Result or Some value for an Option. If an error occurs, it calls panic! with a provided error message.

Why is this bad?​

.expect() might panic if the result value is an error or None. It is recommended to avoid the panic of a contract because it stops its execution, which might lead the contract to an inconsistent state if the panic occurs in the middle of state changes. Additionally, the panic could cause a transaction to fail.

Issue example​

Consider the following Soroban contract:


 pub fn balance_of(env: Env, owner: Address) -> i128 {
    let state = Self::get_state(env);
    state.balances.get(owner).expect("could not get balance")
}

In this contract, the balance_of function uses the expect method to retrieve the balance of an account. If there is no entry for this account in the balances mapping, the contract will panic and halt execution, which could be exploited maliciously to disrupt the contract's operation.

The code example can be found here.

Remediated example​

Instead of using expect, use a safer method for error handling. In this case, if there is no entry for an account in the balances mapping, return a default value (like 0).


pub fn balance_of(env: Env, owner: Address) -> i128 {
    let state = Self::get_state(env);
    state.balances.get(owner).unwrap_or(0)
}

The remediated code example can be found here.

How is it detected?​

Checks for usage of .expect().