リクエストURIの最大長からブラウザを判定するスクリプト.js

/*!
 * リクエストURIの最大長からブラウザを判定するスクリプト
 * (c) 2024 ActiveTK. / 2024/03/17
 * PoC: https://www.activetk.jp/public/browser_max_reqline.html
 */
const fetch_host = "https://no-such-a-doma.in/?browser_max_reqline=";
const Chrome_Max_Length = 2097152 - fetch_host.length;
const Firefox_Max_Length = 1048572 - fetch_host.length;

async function try_fetch(url) {
    try {
        const response = await fetch(url);
        const data = await response.text();
        return "Success";
    } catch (error) {
        return error;
    }
}

async function is_fetchable(len) {

    let start_performance = performance.now();
    try {
        let repeat_data = "a".repeat(len);
        let fetch_result = await try_fetch(fetch_host + repeat_data);
        if (
            // Request Done (unexpected)
            fetch_result == "Success" ||
            // CORS Error (firefox)
            fetch_result == "TypeError: NetworkError when attempting to fetch resource."
        ) {
            return true;
        }
    } catch (e) {}

    let time_taken = performance.now() - start_performance;
    if (time_taken > 300) {
        // Lookup Failed
        return true;
    } else {
        // RequestLine Too long
        return false;
    }

}

(async () => {

    let Firefox_Max_Length_= await is_fetchable(Firefox_Max_Length);
    let Firefox_Max_Length_plus_one = await is_fetchable(Firefox_Max_Length + 1);
    console.clear();
    console.log(
        "%cFirefox_Max_Length[" + Firefox_Max_Length + "]: " +
        Firefox_Max_Length_, "font-size:2rem;"
    );
    console.log(
        "%cFirefox_Max_Length+1[" + (Firefox_Max_Length + 1) + "]: " +
        Firefox_Max_Length_plus_one, "font-size:2rem;"
    );

    let Chrome_Max_Length_ = await is_fetchable(Chrome_Max_Length);
    let Chrome_Max_Length_plus_one = await is_fetchable(Chrome_Max_Length + 1);
    console.log(
        "%cChrome_Max_Length[" + Chrome_Max_Length + "]: " +
        Chrome_Max_Length_, "font-size:2rem;"
    );
    console.log(
        "%cChrome_Max_Length+1[" + (Chrome_Max_Length + 1) + "]: " +
        Chrome_Max_Length_plus_one, "font-size:2rem;"
    );

    let browser_name = "Unknown";
    if (Chrome_Max_Length_ && !Chrome_Max_Length_plus_one)
        browser_name = "Google Chrome";
    else if (Firefox_Max_Length_ && !Firefox_Max_Length_plus_one)
        browser_name = "Firefox";

    console.log(
        "%cブラウザ判定結果%c 汝のブラウザは %c" + browser_name + "%c に違いない！！",
        "font-size:2rem;color:white; background-color:purple; padding:2px; border-radius:4px;",
        "font-size:2rem;",
        "font-size:2rem;color:green;",
        "font-size:2rem;"
    );

})();