Santa's List
AI First Flight #3
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Submission Details
Severity: high
Valid

Default Enum Value Allows Bypassing First Check Entirely

lallanaad

Root + Impact

Every new address in the mapping starts out flagged as NICE without any explicit check, which means someone can skip the intended first verification step and still qualify for presents after only a second check, weakening the two‑step process and allowing unintended eligibility.

Description

Mappings in Solidity return the first enum value when unset. Since Status.NICE is the first enum value, any address that has never been checked appears as NICE by default.

This allows checkTwice to succeed even if checkList was never called.

enum Status {
NICE, // default value
EXTRA_NICE,
NAUGHTY,
NOT_CHECKED_TWICE
}
​
function checkTwice(address person, Status status) external onlySanta {
if (s_theListCheckedOnce[person] != status) {
revert SantasList__SecondCheckDoesntMatchFirst();
}
s_theListCheckedTwice[person] = status;
emit CheckedTwice(person, status);
}

Risk

Likelihood:

  • Affects all uninitialized users

Impact:

  • First check can be skipped entirely

  • Second check passes “by coincidence”

  • Completely invalidates verification logic

  • Allows minting without explicit approval

Proof of Concept

Set below function into the SantaListTest.t.solfile

function testCheckListTwice() public {
vm.startPrank(santa);
santasList.checkTwice(user, SantasList.Status.NICE);
vm.stopPrank();
​
assertEq(uint256(santasList.getNaughtyOrNiceOnce(user)), uint256(SantasList.Status.NICE));
assertEq(uint256(santasList.getNaughtyOrNiceTwice(user)), uint256(SantasList.Status.NICE));
}

Recommended Mitigation

Introduce a sentinel value as the first value to the enum and enforce it in the checkTwice function

enum Status {
+ NOT_CHECKED,
NICE,
EXTRA_NICE,
NAUGHTY,
NOT_CHECKED_TWICE
}
​
function checkTwice(address person, Status status) external onlySanta {
+ require(s_theListCheckedOnce[person] != Status.NOT_CHECKED);
if (s_theListCheckedOnce[person] != status) {
revert SantasList__SecondCheckDoesntMatchFirst();
}
s_theListCheckedTwice[person] = status;
emit CheckedTwice(person, status);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 5 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[H-02] All addresses are considered `NICE` by default and are able to claim a NFT through `collectPresent` function before any Santa check.

## Description `collectPresent` function is supposed to be called by users that are considered `NICE` or `EXTRA_NICE` by Santa. This means Santa is supposed to call `checkList` function to assigned a user to a status, and then call `checkTwice` function to execute a double check of the status. Currently, the enum `Status` assigns its default value (0) to `NICE`. This means that both mappings `s_theListCheckedOnce` and `s_theListCheckedTwice` consider every existent address as `NICE`. In other words, all users are by default double checked as `NICE`, and therefore eligible to call `collectPresent` function. ## Vulnerability Details The vulnerability arises due to the order of elements in the enum. If the first value is `NICE`, this means the enum value for each key in both mappings will be `NICE`, as it corresponds to `0` value. ## Impact The impact of this vulnerability is HIGH as it results in a flawed mechanism of the present distribution. Any unchecked address is currently able to call `collectPresent` function and mint an NFT. This is because this contract considers by default every address with a `NICE` status (or 0 value). ## Proof of Concept The following Foundry test will show that any user is able to call `collectPresent` function after `CHRISTMAS_2023_BLOCK_TIME` : ``` function testCollectPresentIsFlawed() external { // prank an attacker's address vm.startPrank(makeAddr("attacker")); // set block.timestamp to CHRISTMAS_2023_BLOCK_TIME vm.warp(1_703_480_381); // collect present without any check from Santa santasList.collectPresent(); vm.stopPrank(); } ``` ## Recommendations I suggest to modify `Status` enum, and use `UNKNOWN` status as the first one. This way, all users will default to `UNKNOWN` status, preventing the successful call to `collectPresent` before any check form Santa: ``` enum Status { UNKNOWN, NICE, EXTRA_NICE, NAUGHTY } ``` After modifying the enum, you can run the following test and see that `collectPresent` call will revert if Santa didn't check the address and assigned its status to `NICE` or `EXTRA_NICE` : ``` function testCollectPresentIsFlawed() external { // prank an attacker's address vm.startPrank(makeAddr("attacker")); // set block.timestamp to CHRISTMAS_2023_BLOCK_TIME vm.warp(1_703_480_381); // collect present without any check from Santa vm.expectRevert(SantasList.SantasList__NotNice.selector); santasList.collectPresent(); vm.stopPrank(); } ```

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