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

Unchecked addresses default to NICE and can mint presents without Santa approval

rvoffindings

Root + Impact

Description

The zero value of the Statusenum isNICE, while both checklist mappings return zero for every address that has never been written. collectPresent()treats twoNICE` values as sufficient authorization, so any previously unseen address is implicitly approved even though Santa never checked it.

enum Status {
NICE,
EXTRA_NICE,
NAUGHTY,
NOT_CHECKED_TWICE
}
​
mapping(address person => Status naughtyOrNice) private s_theListCheckedOnce;
mapping(address person => Status naughtyOrNice) private s_theListCheckedTwice;
​
function collectPresent() external {
if (block.timestamp < CHRISTMAS_2023_BLOCK_TIME) revert SantasList__NotChristmasYet();
if (balanceOf(msg.sender) > 0) revert SantasList__AlreadyCollected();
​
if (
s_theListCheckedOnce[msg.sender] == Status.NICE
&& s_theListCheckedTwice[msg.sender] == Status.NICE
) {
_mintAndIncrement();
return;
}
// ...
}

After the Christmas timestamp, an attacker can create arbitrarily many fresh addresses and mint one NFT from each. This bypasses Santa's authorization model and makes the NFT supply permissionlessly inflationary.

Risk

Likelihood: High

  • The behavior is deterministic for every unmapped address.

  • No privilege, token balance, or capital is required; Sybil addresses are trivial to create.

Impact: High

  • Anyone can mint without either Santa check.

  • The attacker can repeat the flow across unlimited new addresses, destroying scarcity and bypassing the protocol's core access control.

Proof of Concept

Add this Foundry test to the existing SantasListTest setup:

function test_UncheckedAddressDefaultsToNiceAndMints() public {
address attacker = makeAddr("attacker");
​
assertEq(
uint256(santasList.getNaughtyOrNiceOnce(attacker)),
uint256(SantasList.Status.NICE)
);
assertEq(
uint256(santasList.getNaughtyOrNiceTwice(attacker)),
uint256(SantasList.Status.NICE)
);
​
vm.warp(santasList.CHRISTMAS_2023_BLOCK_TIME());
vm.prank(attacker);
santasList.collectPresent();
​
assertEq(santasList.ownerOf(0), attacker);
}

Recommended Mitigation

Make the default enum value an explicit unchecked state and require both checks to have been recorded:

enum Status {
+ NOT_CHECKED_TWICE,
NICE,
EXTRA_NICE,
- NAUGHTY,
- NOT_CHECKED_TWICE
+ NAUGHTY
}

Alternatively, keep separate boolean flags for the first and second checks. Add regression tests proving that a never-checked address reverts and only addresses explicitly checked twice by Santa can collect.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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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