Balance Deduction

​

The vulnerable pattern

1. We send tokens to the user
2. On success, we deduct the amount from the user’s balance

withdraw(token, amount):
    if balance[token, user] >= amount
        transfer token amount to the user 
        then callback withdraw_callback(user, token, amount)

private withdraw_callback(user,token, amount):
    if call succeeded
        balance[token, user] -= amount

​

How the exploit happens

1. The user calls withdraw and the contract sends the tokens to the user
2. The callback is scheduled, but the user can call withdraw again before the callback runs!
3. The user calls withdraw again, or multiple times as a batched transaction, the contract keeps sending tokens to the user
4. The callback runs and deducts the amount from the user’s balance only once, leaving the user with more tokens than they should have.

​

The safe pattern

pub withdraw(token, amount):
    if balance[token, user] >= amount
        balance[token, user] -= amount
        transfer token amount to the user
        then callback withdraw_callback(user, token, amount)

private withdraw_callback(user,token, amount):
    if call failed
        balance[token, user] += amount

​

The opposite rule for deposits

deposit_and_stake(amount):
    let user = predecessor()
    balance[user] += amount 
    call staking_pool.deposit_and_stake(amount)
        then callback deposit_callback(user, amount)

private deposit_callback(user, amount):
    if call failed
        balance[user] -= amount
        transfer amount NEAR tokens to user

deposit_and_stake(amount):
    let user = predecessor()
    call staking_pool.deposit_and_stake(amount)
        then callback deposit_callback(user, amount)

private deposit_callback(user, amount):
    if call succeeded
        balance[user] += amount
    else
        transfer amount NEAR tokens to user

• Withdrawals: deduct the balance before the call; add it back only if the call fails.
• Deposits: add the balance only if the call succeeds.