Puppy Raffle
AI First Flight #1
Beginner FriendlyFoundrySolidityNFT
EXP
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Submission Details
Severity: low
Valid

Rarity calculation is off by one, reducing the chance to win Legendary puppies

gravesagain

Description

  • The selectWinner function assigns rarities based on a random number modulo 100, which produces a value between 0 and 99.

  • The if statements use <= (less than or equal to), which accounts for 71 numbers (0-70) for Common, 25 numbers (71-95) for Rare, and only 4 numbers (96-99) for Legendary.

// Root cause in the codebase with @> marks to highlight the relevant section
uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
if (rarity <= COMMON_RARITY) {
tokenIdToRarity[tokenId] = COMMON_RARITY;
@> } else if (rarity <= COMMON_RARITY + RARE_RARITY) {
tokenIdToRarity[tokenId] = RARE_RARITY;
} else {
tokenIdToRarity[tokenId] = LEGENDARY_RARITY;
}

Risk

Likelihood:

  • Always occurs when rarity is calculated.

  • The distribution logic is fundamentally flawed due to a zero-index offset.

Impact:

  • Legendary puppies are minted with a 4% probability instead of the intended 5%.

  • Common puppies are minted with a 71% probability instead of 70%.

Proof of Concept

uint256 rarity = 70; // This falls under COMMON, taking up 71 slots (0-70)
uint256 rarity = 95; // This falls under RARE, taking up 25 slots (71-95)
// Only 96, 97, 98, and 99 are left for LEGENDARY (4 slots = 4%)

Recommended Mitigation

Use < instead of <= to achieve the correct distributions.

- if (rarity <= COMMON_RARITY) {
+ if (rarity < COMMON_RARITY) {
tokenIdToRarity[tokenId] = COMMON_RARITY;
- } else if (rarity <= COMMON_RARITY + RARE_RARITY) {
+ } else if (rarity < COMMON_RARITY + RARE_RARITY) {
tokenIdToRarity[tokenId] = RARE_RARITY;
} else {
tokenIdToRarity[tokenId] = LEGENDARY_RARITY;
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[L-03] Participants are mislead by the rarity chances.

## Description The drop chances defined in the state variables section for the COMMON and LEGENDARY are misleading. ## Vulnerability Details The 3 rarity scores are defined as follows: ``` uint256 public constant COMMON_RARITY = 70; uint256 public constant RARE_RARITY = 25; uint256 public constant LEGENDARY_RARITY = 5; ``` This implies that out of a really big number of NFT's, 70% should be of common rarity, 25% should be of rare rarity and the last 5% should be legendary. The `selectWinners` function doesn't implement these numbers. ``` uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100; if (rarity <= COMMON_RARITY) { tokenIdToRarity[tokenId] = COMMON_RARITY; } else if (rarity <= COMMON_RARITY + RARE_RARITY) { tokenIdToRarity[tokenId] = RARE_RARITY; } else { tokenIdToRarity[tokenId] = LEGENDARY_RARITY; } ``` The `rarity` variable in the code above has a possible range of values within [0;99] (inclusive) This means that `rarity <= COMMON_RARITY` condition will apply for the interval [0:70], the `rarity <= COMMON_RARITY + RARE_RARITY` condition will apply for the [71:95] rarity and the rest of the interval [96:99] will be of `LEGENDARY_RARITY` The [0:70] interval contains 71 numbers `(70 - 0 + 1)` The [71:95] interval contains 25 numbers `(95 - 71 + 1)` The [96:99] interval contains 4 numbers `(99 - 96 + 1)` This means there is a 71% chance someone draws a COMMON NFT, 25% for a RARE NFT and 4% for a LEGENDARY NFT. ## Impact Depending on the info presented, the raffle participants might be lied with respect to the chances they have to draw a legendary NFT. ## Recommendations Drop the `=` sign from both conditions: ```diff -- if (rarity <= COMMON_RARITY) { ++ if (rarity < COMMON_RARITY) { tokenIdToRarity[tokenId] = COMMON_RARITY; -- } else if (rarity <= COMMON_RARITY + RARE_RARITY) { ++ } else if (rarity < COMMON_RARITY + RARE_RARITY) { tokenIdToRarity[tokenId] = RARE_RARITY; } else { tokenIdToRarity[tokenId] = LEGENDARY_RARITY; } ```

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