Rust智能合約中的拒絕服務攻擊

拒絕服務(DoS)攻擊會使智能合約在一段時間內無法被正常使用。主要有以下幾種原因:

1. 合約邏輯中存在計算復雜度過高的缺陷,導致Gas消耗超出限制。

2. 跨合約調用時,合約執行依賴不可靠的外部合約狀態,造成阻塞。

3. 合約所有者丟失私鑰,導致關鍵的特權函數無法執行。

下面通過具體例子來分析這些DoS漏洞。

1. 遍歷可被外部更改的大型數據結構

以下是一個簡單的"分紅"合約,存在DoS風險:

rust #[near_bindgen] #[derive(BorshDeserialize, BorshSerialize)] pub struct Contract { pub registered: Vec, pub accounts: UnorderedMap<accountid, balance="">, }

pub fn register_account(&mut self) { if self.accounts.insert(&env::predecessor_account_id(), &0).is_some() { env::panic("The account is already registered".to_string().as_bytes()); } else { self.registered.push(env::predecessor_account_id()); } log!("Registered account {}", env::predecessor_account_id()); }

pub fn distribute_token(&mut self, amount: u128) { assert_eq!(env::predecessor_account_id(), DISTRIBUTOR, "ERR_NOT_ALLOWED");

for cur_account in self.registered.iter() {
    let balance = self.accounts.get(&cur_account).expect("ERR_GET");
    self.accounts.insert(&cur_account, &balance.checked_add(amount).expect("ERR_ADD"));
    log!("Try distribute to account {}", &cur_account);
    
    ext_ft_token::ft_transfer(
        cur_account.clone(),
        amount,
        &FTTOKEN,
        0,
        GAS_FOR_SINGLE_CALL  
    );
}

}

這裏self.registered可被無限擴大,導致遍歷時Gas不足。

應改用"提現"模式,讓用戶主動提取獎勵:

rust pub fn withdraw(&mut self) { let account_id = env::predecessor_account_id(); let amount = self.accounts.get(&account_id).expect("No reward");

self.accounts.insert(&account_id, &0);

ext_ft_token::ft_transfer(
    account_id,
    amount, 
    &FTTOKEN,
    0,
    GAS_FOR_SINGLE_CALL
);

}

2. 跨合約狀態依賴導致阻塞

以下是一個"競價"合約:

rust #[near_bindgen] #[derive(BorshDeserialize, BorshSerialize)] pub struct Contract { pub registered: Vec, pub bid_price: UnorderedMap<accountid, balance="">, pub current_leader: AccountId, pub highest_bid: u128, pub refund: bool }

pub fn bid(&mut self, sender_id: AccountId, amount: u128) -> PromiseOrValue { assert!(amount > self.highest_bid);

if self.current_leader == DEFAULT_ACCOUNT {
    self.current_leader = sender_id;
    self.highest_bid = amount;
} else {
    ext_ft_token::account_exist(
        self.current_leader.clone(),
        &FTTOKEN,
        0,
        env::prepaid_gas() - GAS_FOR_SINGLE_CALL * 4,
    ).then(ext_self::account_resolve(
        sender_id,
        amount,
        &env::current_account_id(),
        0,
        GAS_FOR_SINGLE_CALL * 3,
    ));
}

log!(
    "current_leader: {} highest_bid: {}",
    self.current_leader,
    self.highest_bid
);

PromiseOrValue::Value(0)

}

#[private] pub fn account_resolve(&mut self, sender_id: AccountId, amount: u128) { match env::promise_result(0) { PromiseResult::NotReady => unreachable!(), PromiseResult::Successful(_) => { ext_ft_token::ft_transfer( self.current_leader.clone(), self.highest_bid, &FTTOKEN, 0, GAS_FOR_SINGLE_CALL * 2, ); self.current_leader = sender_id; self.highest_bid = amount; } PromiseResult::Failed => { ext_ft_token::ft_transfer( sender_id.clone(), amount, &FTTOKEN, 0, GAS_FOR_SINGLE_CALL * 2, ); log!("Return Back Now"); } }; }

如果前一出價者帳戶被銷毀,新的出價將被阻塞。

應考慮外部調用失敗的情況,可以將無法退回的代幣暫存,供用戶後續提取:

rust pub fn withdraw_lost_funds(&mut self) { let account_id = env::predecessor_account_id(); let amount = self.lost_funds.get(&account_id).expect("No lost funds");

self.lost_funds.remove(&account_id);

ext_ft_token::ft_transfer(
    account_id,
    amount,
    &FTTOKEN,
    0,
    GAS_FOR_SINGLE_CALL
);

}

3. 所有者私鑰丟失

某些關鍵函數只能由合約所有者調用。如果所有者丟失私鑰,這些函數將無法執行。

應採用多籤名方案來管理合約,避免單點故障:

rust pub struct MultiSigContract { owners: Vec, required_confirmations: u32, }

pub fn submit_transaction(&mut self, transaction: Transaction) { assert!(self.owners.contains(&env::predecessor_account_id())); // 添加交易到待確認列表 }

pub fn confirm_transaction(&mut self, transaction_id: u64) { assert!(self.owners.contains(&env::predecessor_account_id())); // 增加確認數 // 如果確認數達到要求,執行交易 }

通過以上方法可以有效防範智能合約中的DoS攻擊風險。

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