๐๐ฐ ๐ข๐ญ๐ญ ๐ต๐ฉ๐ฆ ๐๐๐๐๐ ๐๐ฐ๐ฏ๐ต๐ณ๐ข๐ค๐ต ๐๐ฆ๐ท๐ด ๐ฐ๐ถ๐ต ๐ต๐ฉ๐ฆ๐ณ๐ฆ, ๐ข
โ ๏ธ ๐๐๐๐๐ ๐๐๐๐๐ผ๐๐ ๐๐๐ ๐พ๐๐๐พ๐-๐๐๐๐๐พ๐๐-๐๐๐๐๐๐ผ๐พ๐๐๐๐ ๐๐ผ๐๐๐๐๐ โ ๏ธ
Building a secure smart contract does require adhering to the Best Practices.
And one of the most crucial practices to keep in mind is the ๐๐๐๐๐ ๐๐
๐
๐๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐ ๐๐๐ญ๐ญ๐๐ซ๐ง while making External Calls.
๐๐๐๐ ๐๐ฑ๐๐๐ญ๐ฅ๐ฒ ๐๐จ๐๐ฌ ๐ข๐ญ ๐ฆ๐๐๐ง?
In simple terms, it means that while designing a function in solidity, any state modification in the function must happen before an external call is made.
๐๐๐ ๐๐ฌ๐ ๐ญ๐ก๐ข๐ฌ ๐๐๐ญ๐ญ๐๐ซ๐ง?
Remember the DAO Hack of 2016 where the attacker drained 3.6 million ETH?
Well, one of the Imperative reasons behind that hack was the Violation of Check-Effects-Interaction patterns in function code.
๐๐๐ฎ ๐๐ ๐๐๐ง๐๐๐ช๐ก ๐ฌ๐๐๐ฃ ๐๐ญ๐๐๐ช๐ฉ๐๐ฃ๐ ๐๐ญ๐ฉ๐๐ง๐ฃ๐๐ก ๐พ๐๐ก๐ก๐จ?
An external call technically shifts the control over execution to another contract or a Third Party. This allows the Third-party contract to leverage from the fact that the Contract State didn't change before the external call.
It leads to an extremely undesirable scenario where a malicious actor can re-enter the contract and disturb the expected flow. Thus, leading to a potential Re-entrancy Scenario.
๐๐๐ ๐๐จ๐๐ฌ ๐ญ๐ก๐ข๐ฌ ๐๐๐ญ๐ญ๐๐ซ๐ง ๐๐๐๐ฎ๐ซ๐ ๐๐จ๐ง๐ญ๐ซ๐๐๐ญ๐ฌ?
Let's understand this by breaking down the 3 imperative steps in this pattern. (๐๐ฐ๐ด๐ต ๐ช๐ฎ๐ฑ๐ฐ๐ณ๐ต๐ข๐ฏ๐ต๐ญ๐บ, ๐๐ฏ ๐ต๐ฉ๐ฆ ๐ฆ๐น๐ข๐ค๐ต ๐ฐ๐ณ๐ฅ๐ฆ๐ณ)
๐๐๐๐๐
The first part is to implement a ๐พ๐๐๐พ๐ or input validations(๐ธ๐ช๐ต๐ฉ ๐ณ๐ฆ๐ฒ๐ถ๐ช๐ณ๐ฆ ๐ฐ๐ณ ๐ข๐ด๐ด๐ฆ๐ณ๐ต ๐ด๐ต๐ข๐ต๐ฆ๐ฎ๐ฆ๐ฏ๐ต๐ด) to ensure that arguments passed are valid and the function is ready to be executed.๐๐๐๐๐๐ง๐ฆ
Resolve all the ๐๐๐๐๐พ๐๐ to the State of the Contract. This part involves optimistically modifying the State Variables to a valid state in the protocol.๐๐ก๐ง๐๐ฅ๐๐๐ง๐๐ข๐ก
The final step should include any ๐๐๐๐๐๐ผ๐พ๐๐๐๐ with other external contracts. This is the step that should include any external call that is being made from the function, at the very end of the function.
โง ๐ ๐๐ป ๐ฎ ๐ก๐จ๐ง๐ฆ๐๐๐๐ ๐ โง
External calls must be the very last thing that you should do in a function. ๐ผ๐ฃ๐ฎ ๐จ๐ฉ๐๐ฉ๐ ๐ซ๐๐ง๐๐๐๐ก๐ ๐ข๐ค๐๐๐๐๐๐๐ฉ๐๐ค๐ฃ ๐ข๐ช๐จ๐ฉ ๐๐๐ฅ๐ฅ๐๐ฃ ๐๐๐๐ค๐ง๐ ๐๐ฃ ๐๐ญ๐ฉ๐๐ง๐ฃ๐๐ก ๐๐๐ก๐ก ๐๐จ ๐๐ญ๐๐๐ช๐ฉ๐๐ ๐๐ฃ ๐ค๐ง๐๐๐ง ๐ฉ๐ค ๐๐ซ๐ค๐๐ ๐ ๐ง๐-๐๐ฃ๐ฉ๐ง๐๐ฃ๐๐ฎ ๐จ๐๐๐ฃ๐๐ง๐๐ค.
Moreover, even if attackers try to re-enter a function that follows the CHECK-EFFECTS-INTERACTION pattern, they cannot really abuse the State of the contract as it has been already modified before the external call is made.
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