Bakery

This was a web-based challenge featured in the KPMG Recruitment CTF 2023.

Date : 22 Dec 2023 Friday

Hey Hackers, I forgot to capture screenshots while i was solving the challenge but i will provide as much as possible context to create the whole scenario with words. <<

Table of Content

1. Background
2. Recon
3. Exploitation
4. Conclusion

Background

KPMG team is expert in giving useless descriptions for the challenges but still according to the description :

This is KPMG Backery

Recon

Nmap

By running Nmap against the server as expected there is no intersting port open just “80” and “443”.

Directory Busting - Gobuster

Nothing interesting returned bu the gobuster only the /js directory having a single file with non-juicy information.

Analysing the Application

I Tried to create a diagram so that it will be easy to imagine the application x00

• There is a Clickable “Bakery” Button.
• A Search Box.
• An area where our searched query will be displayed.

Bakery Button

When we click on the button it will take us to a page where two lines of code is revealed

$pattern = "/script/i";
$payload = preg_replace($pattern, '', $payload);

From the above code we can easily understand its a PHP code that is using the preg_replace function to replace the “Script” keyword blank space ’ ’.

Let’s Try to Enter the Script keyword in the Search box

Yaah Our understanding was accurate script is filtered.

Exploitation - preg_replace bypass

As the application is filtering the “Script” keyword we will try to bypass it and execute the XSS attack.

In the Exposed code /i flag is used with the keyword that means application will filter the word in-case-sensitive in simple words making the word upper or lower will not work for us.

Test case

When I am entering any word concatenated with the “script” keyword application only filters the “script” keyword and the other word remains the same :

Example : scriptscri -> scri

I think you got the idea !!! YESS we can just create a long word having script concatenate with the half of the script itself what i mean to say is

• script+scrip+script+t
• So when the application filters the word it will became scriptscripscriptt (script)
• After further analysing the application i saw that there is not even a single cookie in the application.
• so lets retrieve cookie using the DOM.

Payload : alert(document.cookie)

There is a cookie disclosing the path of a file and upon accessing the file we got the flag.

Conclusion

In this writeup we understood how we can bypass the PHP function preg_replace() and execute the XSS attack.