c4-submissions commented 11 months ago

Lines of code

https://github.com/code-423n4/2023-10-nextgen/blob/71d055b623b0d027886f1799739b7f785b5bc7cd/smart-contracts/XRandoms.sol#L15-L33

Vulnerability details

Impact

randomPool.randomWord() will mod 100 when calculating randomNum, so the range of randomNum is [0,99].

    function randomWord() public view returns (string memory) {
        uint256 randomNum = uint(keccak256(abi.encodePacked(block.prevrandao, blockhash(block.number - 1), block.timestamp))) % 100;
        return getWord(randomNum);
    }

After that, in getWord(), wordsList has 100 words, in the following code, since the range of randomNum is [0,99], the probability that "Acai" is selected is 2/100 (randomNum is 0 or 1), the probability that "Watermelon" is selected is 0, and the probability that other words are selected is 1/100, that is, randomWord() returns different words with unequal probability.

    function getWord(uint256 id) private pure returns (string memory) {

        // array storing the words list
        string[100] memory wordsList = ["Acai", "Ackee", "Apple", "Apricot", "Avocado", "Babaco", "Banana", "Bilberry", "Blackberry", "Blackcurrant", "Blood Orange", 
        "Blueberry", "Boysenberry", "Breadfruit", "Brush Cherry", "Canary Melon", "Cantaloupe", "Carambola", "Casaba Melon", "Cherimoya", "Cherry", "Clementine", 
        "Cloudberry", "Coconut", "Cranberry", "Crenshaw Melon", "Cucumber", "Currant", "Curry Berry", "Custard Apple", "Damson Plum", "Date", "Dragonfruit", "Durian", 
        "Eggplant", "Elderberry", "Feijoa", "Finger Lime", "Fig", "Gooseberry", "Grapes", "Grapefruit", "Guava", "Honeydew Melon", "Huckleberry", "Italian Prune Plum", 
        "Jackfruit", "Java Plum", "Jujube", "Kaffir Lime", "Kiwi", "Kumquat", "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mammee", "Mandarin", "Mango", 
        "Mangosteen", "Mulberry", "Nance", "Nectarine", "Noni", "Olive", "Orange", "Papaya", "Passion fruit", "Pawpaw", "Peach", "Pear", "Persimmon", "Pineapple", 
        "Plantain", "Plum", "Pomegranate", "Pomelo", "Prickly Pear", "Pulasan", "Quine", "Rambutan", "Raspberries", "Rhubarb", "Rose Apple", "Sapodilla", "Satsuma", 
        "Soursop", "Star Apple", "Star Fruit", "Strawberry", "Sugar Apple", "Tamarillo", "Tamarind", "Tangelo", "Tangerine", "Ugli", "Velvet Apple", "Watermelon"];

        // returns a word based on index
        if (id==0) {
            return wordsList[id];
        } else {
            return wordsList[id - 1];
        }
    }

Proof of Concept

https://github.com/code-423n4/2023-10-nextgen/blob/71d055b623b0d027886f1799739b7f785b5bc7cd/smart-contracts/XRandoms.sol#L15-L33 https://github.com/code-423n4/2023-10-nextgen/blob/71d055b623b0d027886f1799739b7f785b5bc7cd/smart-contracts/XRandoms.sol#L40-L43

Tools Used

None

Recommended Mitigation Steps

Change to

    function getWord(uint256 id) private pure returns (string memory) {

        // array storing the words list
        string[100] memory wordsList = ...;
-       if (id==0) {
            return wordsList[id];
-       } else {
-           return wordsList[id - 1];
-       }

Assessed type

Context

c4-pre-sort commented 11 months ago

141345 marked the issue as duplicate of #508

c4-judge commented 10 months ago

alex-ppg changed the severity to QA (Quality Assurance)