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

Enum Default Value Allows Unchecked Users to Collect NFTs Without Santa's Approval

georgibuilds

Root + Impact

Description

The Status enum starts with NICE at position 0, which is Solidity's default value for uninitialized mappings. This allows any address never checked by Santa to pass the NICE status verification and collect NFTs without approval.

  • Describe the normal behavior in one or more sentences

  • Unchecked users default to Status value 0, which equals Status.NICE

Anyone never checked by Santa can collect NFTs

// src/SantasList.sol - Lines 25-30
enum Status {
NICE, // 0 - Default mapping value!
EXTRA_NICE, // 1
NAUGHTY, // 2
NOT_CHECKED_TWICE // 3
}
​
// src/SantasList.sol - Lines 110-121
function collectPresent() external {
// ...
if (s_theListCheckedOnce[msg.sender] == Status.NICE &&
s_theListCheckedTwice[msg.sender] == Status.NICE) {
_mintAndIncrement(); // Unchecked users pass this check!
return;
}
}

Risk

Likelihood:

  • Every address on the blockchain defaults to unchecked status (value 0)

No setup or special conditions required

  • Anyone can discover and exploit this

  • Unlimited number of addresses can claim

Impact:

  • Complete bypass of Santa's approval mechanism

Anyone never checked by Santa can claim NFTs

  • Protocol's core purpose (Santa's authority) is meaningless

  • Unlimited NFT minting possible

  • Economic model broken with uncontrolled supply

Proof of Concept

This test demonstrates that a user never checked by Santa can collect an NFT because Solidity's default mapping value (0) equals Status.NICE.

function test_uncheckedUserCanCollect() public {
address neverCheckedUser = makeAddr("neverCheckedUser");
// Verify: User was never checked (defaults to 0 = NICE)
assertEq(uint(santasList.getNaughtyOrNiceOnce(neverCheckedUser)), 0);
assertEq(uint(santasList.getNaughtyOrNiceTwice(neverCheckedUser)), 0);
// User collects without Santa's approval
vm.warp(santasList.CHRISTMAS_2023_BLOCK_TIME());
vm.prank(neverCheckedUser);
santasList.collectPresent();
// Verify: Unchecked user got NFT
assertEq(santasList.balanceOf(neverCheckedUser), 1);
}

Recommended Mitigation

Reorder the enum to place an invalid state at position 0, ensuring unchecked users default to a state that fails verification.

enum Status {
+ NOT_CHECKED, // 0 - New default state
NICE, // 1
EXTRA_NICE, // 2
NAUGHTY // 3
- NOT_CHECKED_TWICE
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 18 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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