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  • How to Explain External Call Risks in Solidity Interviews?

    Synthill Brown

    Member

    Updated: Mar 18, 2025
    Views: 1.6K

    Had a Solidity dev interview recently, and they threw this at me: What are the risks of using external calls in smart contracts, and how do you mitigate them?

    I covered the usual suspects—reentrancy attacks, gas efficiency, and external contract reliability—but now I’m wondering if I missed anything important.

    For those who've tackled this in interviews, how did you structure your response? Any key points that helped you stand out? Trying to refine my answer and make it as clear and solid as possible. Would love to hear your thoughts!

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  • Varun Mehta

    Member • 2mos

    Interviewers ask about external calls in Solidity to test security knowledge. Focus on reentrancy, unexpected control flow, and gas issues. A strong answer explains these risks and how to prevent them.

    External calls in Solidity allow interactions with other contracts but introduce security risks. The biggest risk is reentrancy, where an attacker repeatedly calls back into the contract before the first call completes. Prevent this using the checks-effects-interactions pattern—update state variables before making external calls. Another issue is unexpected control flow, where an external contract executes arbitrary code. Use pull payments instead of direct transfers to reduce this risk. Gas issues also matter; external calls may fail if the target contract runs out of gas. Avoid fixed gas stipends and use try-catch blocks to handle failures safely. Keeping external calls minimal and verifying contract addresses before interaction adds security.

    This structured response covers risks and mitigation clearly, making it easy to understand.

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