SKF write-ups
NodeJS - URL Redirection - Harder

Running the app nodeJs

First make sure nodejs and npm are installed on your host machine. After installation, we go to the folder of the lab we want to practice. "i.e /skf-labs/XSS, /skf-labs/RFI/" and run the following commands:
$ npm install
$ npm start
Now that the app is running let's go hacking!


Step 1

The application shows that there is a new version of the website available somewhere, and a click on the button "Go to new website" will redirect you to it.
If we click on the button we will be redirected on the new page http://localhost:5000/newsite

Step 2

Intercepting the traffic generated by the application, we note that the redirection is performed using the following call
GET /redirect?newurl=newsite
that will generate a 302 Redirect response from the server
Exactly like in the previous example (KBID-67-Url-redirection). If we look at the code we discover a tiny difference: a blacklist!
let newurl = req.query.newurl;
if (blacklist(newurl)) {
res.render("index.ejs", {
content: 'Sorry, you cannot use "." in the redirect',
If we look at the blacklist definition, we can immediately see that the URL, in order to be valid, must not contain any "." (dot).
const blacklist = (newurl) => {
if (newurl.includes(".")) {
return true;
return false;

Step 3

Let's verify the effectiveness of this blacklist. If we try to exploit the unvalidated redirect using an external website, we see that the application blocks us, returning an error in the page.
If we URL encode the dot the application is smart enough to decode it and recognise it in the URL, blocking us again.


Although we cannot explicitly use the dot character, we can find different ways to bypass the blacklist. In example we could use the following techniques:
  • double encoding: https://google%252ecom
  • UTF-8 encoding:
  • Can you find more?
Using the payload above we will be able to successfully redirect a user to a malicious website

Additional sources

WSTG - v4.1 | OWASP Foundation