DatingDapp
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Submission Details
Severity: low
Invalid

Incorrect token existence Check in `tokenUri` function

damboy

Summary

The tokenURI function in the SoulboundProfileNFT contract uses ownerOf(tokenId) == address(0) to check if a token exists. However, this check is incorrect because ownerOf(tokenId) reverts if the token does not exist, instead of returning address(0). As a result, calling tokenURI with an invalid tokenId will cause a transaction failure instead of handling the error gracefully.

Vulnerability Details

In the tokenURI function, the following check is used to verify whether a token exists:

if (ownerOf(tokenId) == address(0)) { revert ERC721Metadata__URI_QueryFor_NonExistentToken(); }

However, this approach is flawed because:

  1. ownerOf(tokenId) Does Not Return address(0) for Invalid Tokens

    • The ownerOf function in OpenZeppelin’s ERC721 contract is designed to revert if the token does not exist.

    • It does not return address(0).

    • This means the if-condition will never execute; instead, the function will revert immediately when calling ownerOf with a non-existent tokenId.

  2. How the Issue Manifests

    • If a user queries tokenURI(nonExistentTokenId), the transaction will fail with a revert error from ownerOf, rather than the intended ERC721Metadata__URI_QueryFor_NonExistentToken() error.

    • This makes error handling inconsistent and unexpected.

Proof of Concept (PoC)

Add the test below in the testSoulboundProfileNFT.t.sol and run the command forge test --mt testTokenURI_ShouldRevertForNonExistentToken -vvvv

function testTokenURI_ShouldRevertForNonExistentToken() public {
​
uint256 nonExistentTokenId = 999;
​
vm.expectRevert();
​
soulboundNFT.tokenURI(nonExistentTokenId);
​
}

Your Output should look like the one below:

​
Ran 1 test for test/testSoulboundProfileNFT.t.sol:SoulboundProfileNFTTest
[PASS] testTokenURI_ShouldRevertForNonExistentToken() (gas: 12714)
Traces:
[12714] SoulboundProfileNFTTest::testTokenURI_ShouldRevertForNonExistentToken()
├─ [0] VM::expectRevert(custom error f4844814:)
│ └─ ← [Return]
├─ [2633] SoulboundProfileNFT::tokenURI(999) [staticcall]
│ └─ ← [Revert] ERC721NonexistentToken(999)
└─ ← [Stop]
​
Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 690.37µs (98.58µs CPU time)
​
Ran 1 test suite in 5.84ms (690.37µs CPU time): 1 tests passed, 0 failed, 0 skipped (1 total tests)

Looking at the Logs, you will see that the contract reverted with ERC721NonexistentToken(999), proving that ownerOf(tokenId) == address(0) does not correctly check token existence.

Impact

  • Unexpected Reverts: Calling tokenURI with an invalid token ID will revert due to ownerOf, rather than executing the intended error-handling logic.

  • Inconsistent Error Handling: The contract's ERC721Metadata__URI_QueryFor_NonExistentToken() error is never actually used, making debugging more difficult.

Tools Used

  • Manual Code Review

  • OpenZeppelin ERC721 Documentation

Recommendations

Use _exists(tokenId) Instead of ownerOf(tokenId) == address(0)

OpenZeppelin provides an _exists(tokenId) function that safely checks if a token exists without reverting. Update tokenURI as follows:

function tokenURI(uint256 tokenId) public view override returns (string memory) {
if (!_exists(tokenId)) {
revert ERC721Metadata__URI_QueryFor_NonExistentToken();
}
string memory profileName = _profiles[tokenId].name;
uint256 profileAge = _profiles[tokenId].age;
string memory imageURI = _profiles[tokenId].profileImage;
​
return string(
abi.encodePacked(
_baseURI(),
Base64.encode(
abi.encodePacked(
'{"name":"', profileName, '", ',
'"description":"A soulbound dating profile NFT.", ',
'"attributes": [{"trait_type": "Age", "value": ', Strings.toString(profileAge), "}], ",
'"image":"', imageURI, '"}'
)
)
)
);
}
​
Updates

Appeal created

n0kto Lead Judge 7 months ago
Submission Judgement Published
Invalidated
Reason: Non-acceptable severity
Assigned finding tags:

Informational or Gas

Please read the CodeHawks documentation to know which submissions are valid. If you disagree, provide a coded PoC and explain the real likelyhood and the detailed impact on the mainnet without any supposition (if, it could, etc) to prove your point.

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