Argent Account & Argent Multisig Starknet Transaction V3 Updates

1 Executive Summary

This report presents the results of our engagement with Argent to review updates in Argent Account and Argent Multisig for Starknet Transaction v3.

The review was conducted from January 15–23, 2024, by George Kobakhidze and Heiko Fisch. A total of 14 person-days were spent. Due to Argent’s time constraints regarding the release, potential findings were reported informally during the first week of the engagement, with the client implementing fixes and improvements as discussed. At the end of the week, we reviewed these changes and finished the assessment. The report was compiled in the first half of the following week.

Argent is a provider of wallets in the Ethereum ecosystem, including its L2s. This audit focused on their smart contract wallet offering for Starknet, Argent Account (v0.3.1), and their multisig, Argent Multisig (v0.1.1), as well as the auxiliary libraries used for these. Starknet recently introduced the new transaction version 3, which Argent wanted to support. We had already reviewed an earlier version of the Argent Account and Multisig for Starknet, and it was our pleasure to work with Argent again to review the changes that were made to support the new Starknet transactions, along with a few smaller improvements that had become possible due to new releases of the Cairo language.

As last time, we found the code exceptionally well-written, -documented, and extensively tested. All our findings, as detailed in Section 4, have been fixed in the final version we considered in this audit with the commit hash 3b5e45cf28113f7389a716ef9faabfbd79ce45f9.

Users of Argent Account should be aware that the time window to react to a malicious Guardian’s takeover attempt is only one week and should take appropriate precautions.

2 Scope

We reviewed the client’s repository at the commit hash cb01c6a3c8f343ef195ebb7094361b674fe43a37. All findings that we identified have been fixed in the version with commit hash 3b5e45cf28113f7389a716ef9faabfbd79ce45f9. Only Cairo files – and most of them – have been in scope for this audit; a detailed list, together with their SHA-1 hashes in the initial and final version considered in this engagement, can be found in the Appendix. Naturally, we focused on the changes that have been introduced since our last review.

3 System Overview

Since the primary focus of this review was the changes introduced with new functionality available in Starknet and Cairo, they will be discussed first.

3.1 Transaction V3

The main anticipated change with this update is the support for v3 transactions. As can be found in Starknet documentation and the relevant SNIP-8, this new transaction type will allow users to utilize STRK as the gas token for transaction fees. As a result, some changes had to be made to Argent smart contracts, specifically in the functions validating invoking, deploying, and declaring transactions.

Additionally, since Argent accounts have Guardian addresses that can spend a limited amount of funds for transaction fees without approval of the owner during escapes, new logic had to be introduced to limit that for v3 transactions as well. Namely, v3 transactions introduce concepts such as max_price_per_unit and max_amount for L1 and L2 gas separately, as well as a tip for L2 gas, whereas v1 transactions just have a singular value max_fee. Therefore, new limits are needed to restrict the amount of gas a malicious Guardian can spend with a v3 transaction.

3.2 New Cairo Starknet Contract Functionalities

Along with new transaction types, Cairo Starknet contracts can now utilize new features that allow for a better developer experience. These include:

• Usage of selector!. This allows for easier access to a starknet_keccak of a string where needed, such as when computing selectors of functions and hashes for signature data.
• Usage of events. With this new version, events can now be emitted from a contract with a simple self.emit, as opposed to calling them like a function as in previous versions.
• Usage of self. Now smart contracts may utilize a special keyword called self that allows for easier access for a contract to interact with its internal functions, state variables, events, and more.

3.3 General System Overview

The rest of the system functionality outside of the changes remains the same as in the last version; a description may be found in our previous audit report here.

4 Findings

Each issue has an assigned severity:

• Minor issues are subjective in nature. They are typically suggestions around best practices or readability. Code maintainers should use their own judgment as to whether to address such issues.
• Medium issues are objective in nature but are not security vulnerabilities. These should be addressed unless there is a clear reason not to.
• Major issues are security vulnerabilities that may not be directly exploitable or may require certain conditions in order to be exploited. All major issues should be addressed.
• Critical issues are directly exploitable security vulnerabilities that need to be fixed.

/// Limits fee in escapes
const MAX_ESCAPE_MAX_FEE: u128 = 50000000000000000; // 0.05 ETH

} else if tx_info.version == TX_V1 || tx_info.version == TX_V1_ESTIMATE {
    // other fields not available on V1
    assert(tx_info.max_fee <= MAX_ESCAPE_MAX_FEE, 'argent/max-fee-too-high');

/// Limits tip in escapes
const MAX_ESCAPE_TIP: u128 = 1_000000000000000000; // 1 STRK

// Limit the maximum tip while escaping (max_fee returns 0 on TX_V3)
let max_l2_gas: u64 = loop {
    match tx_info.resource_bounds.pop_front() {
        Option::Some(r) => { if *r.resource == 'L2_GAS' {
            break *r.max_amount;
        } },
        Option::None => {
            // L2_GAS not found
            break 0;
        }
    };
};
let max_tip = tx_info.tip * max_l2_gas.into();
assert(max_tip <= MAX_ESCAPE_TIP, 'argent/tip-too-high');

// H('OutsideExecution(caller:felt,nonce:felt,execute_after:felt,execute_before:felt,calls_len:felt,calls:Call*)')
const OUTSIDE_EXECUTION_TYPE_HASH: felt252 = 0x11ff76fe3f640fa6f3d60bbd94a3b9d47141a2c96f87fdcfbeb2af1d03f7050;

selector!(
    "OutsideExecution(caller:felt,nonce:felt,execute_after:felt,execute_before:felt,calls_len:felt,calls:OutsideCall*)OutsideCall(to:felt,selector:felt,calldata_len:felt,calldata:felt*)"
)

#[inline(always)]
fn get_execution_info() -> Box<starknet::info::v2::ExecutionInfo> {
    starknet::syscalls::get_execution_info_v2_syscall().unwrap_syscall()
}

#[inline(always)]
fn get_tx_info() -> Box<starknet::info::v2::TxInfo> {
    get_execution_info().unbox().tx_info
}

fn __validate__(ref self: ContractState, calls: Array<Call>) -> felt252 {
    assert_caller_is_null();
    let tx_info = get_tx_info().unbox();
    assert_correct_invoke_version(tx_info.version);

fn __validate_declare__(self: @ContractState, class_hash: felt252) -> felt252 {
    let tx_info = get_tx_info().unbox();
    assert_correct_declare_version(tx_info.version);

fn __validate_deploy__(
    self: @ContractState, class_hash: felt252, contract_address_salt: felt252, owner: felt252, guardian: felt252
) -> felt252 {
    let tx_info = get_tx_info().unbox();
    assert_correct_invoke_version(tx_info.version);

#[inline(always)]
fn assert_correct_invoke_version(tx_version: felt252) {
    assert(
        tx_version == TX_V3 || tx_version == TX_V1 || tx_version == TX_V3_ESTIMATE || tx_version == TX_V1_ESTIMATE,
        'argent/invalid-tx-version'
    )
}

#[inline(always)]
fn assert_correct_declare_version(tx_version: felt252) {
    assert(
        tx_version == TX_V3 || tx_version == TX_V2 || tx_version == TX_V3_ESTIMATE || tx_version == TX_V2_ESTIMATE,
        'argent/invalid-declare-version'
    )
}

Appendix 1 - Files in Scope

This audit covered the following files:

Appendix 2 - Disclosure

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