Santa's List
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Submission Details
Severity: high
Valid

Missing Access Control on `SantasList::checkList` function allows anyone to call the function, breaking the protocol logic

0x0nullian

Missing Access Control on SantasList::checkList function allows anyone to call the function, breaking the protocol logic

Description

There is a missing access control that allows the attacker to call the SantasList::checkList function, changing the Status of the s_theListCheckedOnce list users addresses' to whatever he wants

Impact

The "Poisoned First Step" Attack:
The SantasList::checkTwice function relies entirely on the data stored by checkList. The attacker can break the whole protocol logic, gaining access to manipulate its data for his own sake, minting the NFTs and gaining the rewards for himself.

Proof of Concept

  1. Missing access control allows anyone to change the state of the protocol

  2. The SantasList::checkTwice function relies entirely on the data stored by SantasList::checkList

  3. An attacker can exploit the missing access control in checkList to manufacture a valid state that passes the second check.

  4. The threat actor can manipulate the state of the SantasList::s_theListCheckedOnce, like changing the status to be EXTRA_NICE, so he can get both the NFT and SantaToken which is used to purchase an additional NFTs. Threat actor forces Santa to execute a reward for a user the attacker chooses

  5. This also breaks the logic of the protocol, as Only Santa should be able to call the checkList function.

Proof of Code Add this to your test suite:
function testAnyoneCanCheckList() public {
// 1. Attacker maliciously sets the first check to EXTRA_NICE
vm.prank(user);
santasList.checkList(extraNiceGuy, SantasList.Status.EXTRA_NICE);
​
// 2. Attacker maliciously sets the first check to EXTRA_NICE
vm.prank(santa);
santasList.checkTwice(extraNiceGuy, SantasList.Status.EXTRA_NICE);
​
// 3. Proving that the status is EXTRA_NICE, which is equavilent to 1 in the `enum`
assertEq(uint256(santasList.getNaughtyOrNiceOnce(extraNiceGuy)), 1);
}

Recommended Mitigation

You can use any of these:

  • Add a check to the SantasList::checkList function to control the access:

function checkList(address person, Status status) external {
+ if(msg.sender != i_santa) {
+ revert SantasList__NotSanta();
+ }
s_theListCheckedOnce[person] = status;
emit CheckedOnce(person, status);
}

  • Add the onlySanta modifier to the SantasList::checkList function:

- function checkList(address person, Status status) external {
+ function checkList(address person, Status status) external onlySanta {
s_theListCheckedOnce[person] = status;
emit CheckedOnce(person, status);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[H-01] Anyone is able to call `checkList` function in SantasList contract and prevent any address from becoming `NICE` or `EXTRA_NICE` and collect present.

## Description With the current design of the protocol, anyone is able to call `checkList` function in SantasList contract, while documentation says only Santa should be able to call it. This can be considered as an access control vulnerability, because not only santa is allowed to make the first check. ## Vulnerability Details An attacker could simply call the external `checkList` function, passing as parameter the address of someone else and the enum Status `NAUGHTY`(or `NOT_CHECKED_TWICE`, which should actually be `UNKNOWN` given documentation). By doing that, Santa will not be able to execute `checkTwice` function correctly for `NICE` and `EXTRA_NICE` people. Indeed, if Santa first checked a user and assigned the status `NICE` or `EXTRA_NICE`, anyone is able to call `checkList` function again, and by doing so modify the status. This could result in Santa unable to execute the second check. Moreover, any malicious actor could check the mempool and front run Santa just before calling `checkTwice` function to check users. This would result in a major denial of service issue. ## Impact The impact of this vulnerability is HIGH as it results in a broken mechanism of the check list system. Any user could be declared `NAUGHTY` for the first check at any time, preventing present collecting by users although Santa considered the user as `NICE` or `EXTRA_NICE`. Santa could still call `checkList` function again to reassigned the status to `NICE` or `EXTRA_NICE` before calling `checkTwice` function, but any malicious actor could front run the call to `checkTwice` function. In this scenario, it would be impossible for Santa to actually double check a `NICE` or `EXTRA_NICE` user. ## Proof of Concept Just copy paste this test in SantasListTest contract : ``` function testDosAttack() external { vm.startPrank(makeAddr("attacker")); // any user can checList any address and assigned status to naughty // an attacker could front run Santa before the second check santasList.checkList(makeAddr("user"), SantasList.Status.NAUGHTY); vm.stopPrank(); vm.startPrank(santa); vm.expectRevert(); // Santa is unable to check twice the user santasList.checkTwice(makeAddr("user"), SantasList.Status.NICE); vm.stopPrank(); } ``` ## Recommendations I suggest to add the `onlySanta` modifier to `checkList` function. This will ensure the first check can only be done by Santa, and prevent DOS attack on the contract. With this modifier, specification will be respected : "In this contract Only Santa to take the following actions: - checkList: A function that changes an address to a new Status of NICE, EXTRA_NICE, NAUGHTY, or UNKNOWN on the original s_theListCheckedOnce list." The following code will resolve this access control issue, simply by adding `onlySanta` modifier: ``` function checkList(address person, Status status) external onlySanta { s_theListCheckedOnce[person] = status; emit CheckedOnce(person, status); } ``` No malicious actor is now able to front run Santa before `checkTwice` function call. The following tests shows that doing the first check for another user is impossible after adding `onlySanta` modifier: ``` function testDosResolved() external { vm.startPrank(makeAddr("attacker")); // checklist function call will revert if a user tries to execute the first check for another user vm.expectRevert(SantasList.SantasList__NotSanta.selector); santasList.checkList(makeAddr("user"), SantasList.Status.NAUGHTY); vm.stopPrank(); } ```

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