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Unsafe unwrap

Description​

In Rust, the unwrap method is commonly used for error handling. It retrieves the inner value of an Option or Result. If an error or None occurs, it calls panic! without a custom error message.

The usage of unwrap can lead to a panic and crash the program, which is not desired behavior in most cases, particularly in smart contracts.

Exploit Scenario​

Consider the following ink! contract:

#[ink::contract]
mod unsafe_unwrap {
    use ink::storage::Mapping;

    #[ink(storage)]
    pub struct UnsafeUnwrap {
        total_supply: Balance,
        balances: Mapping<AccountId, Balance>,
    }

    // ...

    impl UnsafeUnwrap {
        /// Returns the balance of a given account.
        #[ink(message)]
        pub fn balance_of(&self, owner: AccountId) -> Balance {
            self.balances.get(owner).unwrap()
        }

        // ...
    }
}

In this contract, the balance_of function uses the unwrap 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, potentially leading to malicious exploitation to disrupt the contract's operation.

The vulnerable code example can be found here.

Remediation​

Instead of using unwrap, 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).

#[ink::contract]
mod unsafe_unwrap {
    use ink::storage::Mapping;

    #[ink(storage)]
    pub struct UnsafeUnwrap {
        total_supply: Balance,
        balances: Mapping<AccountId, Balance>,
    }

    // ...

    impl UnsafeUnwrap {
        /// Returns the balance of a given account.
        #[ink(message)]
        pub fn balance_of(&self, owner: AccountId) -> Balance {
            self.balances.get(owner).unwrap_or(0)
        }

        // ...
    }
}

The remediated code example can be found here.

References​

Rust documentation: unwrap