Write-up: CORS vulnerability with basic origin reflection @ PortSwigger Academy

This write-up for the lab “CORS vulnerability with basic origin reflection” is part of my walk-through series for PortSwigger’s Web Security Academy.

Learning path: Client-side topics → Cross-origin resource sharing (CORS)

Python script: script.py

Lab description

  • Lab application has an insecure CORS configuration trusting all origins
  • Known good credentials wiener:peter


Create some JavaScript that

  • retrieves the admin API key
  • uploads it to the exploit server



The web application in question is a shop website. I log in with the provided credentials and see the API key of wiener:

Looking in the Burp HTTP history, I can that the /my-account page uses a request to /accountDetails to obtain the API key.

The response contains a CORS header ( Access-Control-Allow-Credentials), so I put the request into Repeater to see how this plays with the Origin header in the request:

The origin is reflected within the Access-Control-Allow-Origin header. This basically tells the browser that any origin value is allowed to access resources of the vulnerable site.

Get the details

So my JavaScript needs to do three things:

  1. call the /accountDetails page
  2. get the apikey from the response
  3. issue a request to the exploit server containing this value

As a first attempt I try to access the response from /accountDetails within a JavaScript on the exploit server and simply write it to the page. Fortunately, the documentation on mozilla.org for XMLHttpRequest shoes everything that is required:

Sure enough, testing the exploit writes the structure obtained from /accountDetails on the screen:

Build the exploit

So I update the script to generate a second request to my exploit server, containing username and APIkey. I also use JSON.parse() to have a cleaner log entry:

Once delivered to the victim, the log shows the required data:

After submitting the apikey, the lab updates to

Originally published at https://github.com.



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Frank Leitner

Frank Leitner

Tech nerd, doing security stuff for fun and some as a job | CISSP, OSCP. Read all stories on medium and support me: https://medium.com/@frank.leitner/membership