[HFCTF2020]EasyLogin.md

---

/login

注册一个账号，在/home发现一个 "GET FLAG" 按钮

按下之后提示 "permission denied"

/static/js/app.js

/**
 *  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);
    });
}

注释提示使用了koa-static处理静态文件访问，并且把目录设置为web根目录

getflag通过/api/flag获取内容，对于MVC架构，业务逻辑的处理一般放在controllers中

因而猜测对api的处理由/controllers/api.js完成

/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();
    }
};

node.js的jwt.verify在secret为空时无视第三个参数给定的algorithm，转而采用none作为签名方式

于是需要构造sid使得globals.secrets[sid]为空

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

利用js弱类型比较，[]在数值比较中隐式转换为0的特性实现绕过

更改alg为none，secretid为[]，username为admin，最终形式如下

成功登录

/api/flag

#Web #nodejs #koa #MVC #源码泄漏 #jwt