Canto Identity Subprotocols contest - J4de's results

Lines of code

https://github.com/code-423n4/2023-03-canto-identity/blob/main/canto-bio-protocol/src/Bio.sol#L43-L117

Vulnerability details

Summary

// https://github.com/code-423n4/2023-03-canto-identity/blob/main/canto-bio-protocol/src/Bio.sol#L43-L117

The function will generate SVG data and insert the bioText of the specified NFT into it. Front-end developers are likely to insert this SVG data to the webpage, and attackers may use this to carry out XSS attack.

Impact

Attackers may use this to carry out XSS attack.

Proof of Concept

    function testXssString() public {
        string memory XssString = unicode"<script>alert('XSS');</script>";
        uint256 len = bytes(XssString).length;
        bio.mint(XssString);
        uint256 tokenId = bio.numMinted();
        string memory uri = bio.tokenURI(tokenId);
        string memory json = string(Base64.decode(slice(uri, 29, bytes(uri).length)));
        string memory svg = string(Base64.decode(slice(json, 74 + bytes(XssString).length, bytes(json).length - 2)));
    }

The return of tokenURI function is:

data:application/json;base64,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

The base64 decoded SVG data is:

<svg xmlns="http://www.w3.org/2000/svg" preserveAspectRatio="xMinYMin meet" viewBox="0 0 400 100"><style>text { font-family: sans-serif; font-size: 12px; }</style><text x="50%" y="50%" dominant-baseline="middle" text-anchor="middle"><tspan x="50%" dy="20"><script>alert('XSS');</script></tspan></text></svg>

After opening this data in a brower, an XSS bullet box pops up.

Tools Used

Manual

Recommended Mitigation Steps

Do XSS verification when converting to uri or implement the conversion logic externally (such as the front-end).

Lines of code

https://github.com/code-423n4/2023-03-canto-identity/blob/main/canto-namespace-protocol/src/Tray.sol#L150-L168

Vulnerability details

Summary

// https://github.com/code-423n4/2023-03-canto-identity/blob/main/canto-namespace-protocol/src/Tray.sol#L150-L168

    /// @notice Buy a specifiable amount of trays
    /// @param _amount Amount of trays to buy
    function buy(uint256 _amount) external {
        uint256 startingTrayId = _nextTokenId();
        if (prelaunchMinted == type(uint256).max) {
            // Still in prelaunch phase
            if (msg.sender != owner) revert OnlyOwnerCanMintPreLaunch();
            if (startingTrayId + _amount - 1 > PRE_LAUNCH_MINT_CAP) revert MintExceedsPreLaunchAmount();
        } else {
            SafeTransferLib.safeTransferFrom(note, msg.sender, revenueAddress, _amount * trayPrice);
        }
        for (uint256 i; i < _amount; ++i) {
            TileData[TILES_PER_TRAY] memory trayTiledata;
            for (uint256 j; j < TILES_PER_TRAY; ++j) {
                lastHash = keccak256(abi.encode(lastHash));
                trayTiledata[j] = _drawing(uint256(lastHash));
            }
            tiles[startingTrayId + i] = trayTiledata;
        }
        _mint(msg.sender, _amount); // We do not use _safeMint on purpose here to disallow callbacks and save gas
    }

Each NFT tiles is hashed from the previous NFT tile, so all tiles are predictable.

Impact

Tiles are used to generate SVG with a fixed algorithm. If tiles are predictable, so are all SVGs. The SVG of each Tray NFT is the only factor that distinguishes it from other NFTs. Whether the SVG looks good or not is an important factor affecting the price of each NFT.

If all SVGs are predictable, the attacker can buy their favorite (higher value) NFT at a base price based on the predicted value.

Proof of Concept

Tools Used

Manual

Recommended Mitigation Steps

Add some random factors when generating tiles.