Standards

supportsInterface

supportsInterface is the ERC165 function that returns whether a contract claims support for a given interface ID.

supportsInterface is how other contracts ask, "Do you implement this interface?"

supportsInterface Explained in Detail

supportsInterface(bytes4 interfaceId) returns true or false for an interface ID. It is the main function defined by ERC165.

A good implementation is predictable, cheap to call, and honest about what the contract can actually do.

Smart contract example

return interfaceId == type(IERC721).interfaceId || super.supportsInterface(interfaceId);

The super call matters when the contract inherits support from parent contracts.

supportsInterface in Auditing

Contracts and off-chain systems may use supportsInterface before choosing how to interact with a target. A false answer can make integrations skip safety logic or call the wrong path.

Auditors check both the function and the behavior behind each claimed interface.

Red flags in code

  • The function returns true for every input.

  • It forgets inherited interfaces.

  • It depends on msg.sender or mutable caller context.

  • It claims ERC721 or ERC1155 support while missing required functions.

  • It is tested directly on the implementation but not through the proxy.

How to test or review it

  • Assert every expected interface ID explicitly.

  • Assert 0xffffffff returns false.

  • Test random unknown IDs.

  • Check interface support after upgrades.

  • Verify the advertised interface functions exist and behave correctly.

ERC165

ERC165 is the Ethereum standard for interface detection through supportsInterface(bytes4).

Selector Collision

A selector collision happens when two function signatures share the same 4-byte selector or when routing code maps a selector to the wrong function.

Upgradeable Proxy

An upgradeable proxy is a smart contract pattern where users call a stable proxy address while execution is delegated to replaceable implementation logic.

Practice this in real audit scenarios

Definitions help, but auditors need reps. SCH turns concepts like supportsInterface into exploit labs, code review habits, and report-writing practice.

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Sources