Here is the short definition about fallback function in solidity:

Solidity fallback function does not have any arguments, has external visibility and does not return anything.

What happens if there is no fallback function and someone calls non-existing function in the smart contract? Is there security complication without a fallback function?

2 Answers 2


The call will simple fail and revert. There are no security implications as everything will be reverted.

  • May the contract not collect some ether associated with the call?
    – user938363
    Commented Jan 16, 2021 at 7:36
  • No. The whole transaction reverts. All possible Ethers are returned. Commented Jan 16, 2021 at 15:11
  • @LauriPeltonen what happens if there is fallback function then? It wont cause an error even though the function doesnt exist?
    – andsilver
    Commented Jan 26, 2022 at 1:57
  • Sorry but the questions doesn't make sense. Either it exists or it doesn't exist. Commented Jan 26, 2022 at 6:26

In most cases, the call will fail and revert but there are exceptions. A contract could self-destruct and forcefully send ether to another contract, even if that contract is missing a fallback function. This could cause security implications resulting in unexpected behavior whenever it checks address(this).balance.

In the following example, sending 1 ETH would end voting prematurely:

contract Election {
    uint public yesVotes;
    uint public noVotes;

    // stop after 100 votes
    function vote(bool _forOrAgainst) public payable {
        // Each voter pays 0.01 ETH
        require(msg.value == 1e16, "Please send exactly 0.01 ETH");

        // A hacker could end voting before 100 votes by sending ether so
        // that the contract's balance >= 1 ether
        require(address(this).balance < 1e18, "Voting has closed");
        _forOrAgainst ? yesVotes++ : noVotes++;

Warning: Spoiler ahead for those who are attempting Ethernaut as it is the solution to one of the levels.

Calling the code below would allow you to send ether to a contract without a fallback function. It can be used against the contract above to stop the voting process prematurely.

 contract Attack {
     function attack(address payable addr) public payable {

This article also gives another example of how to hack a contract using selfdestruct.

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