Exploiting Misconfigured CORS via Wildcard Subdomains

In last post about CORS i explained the cases where and how we can detect the presence of CORS misconfiguration, so this post will cover the one of specific case from them.

So last week while testing one of web application for CORS misconfiguration, i came across a scenario and this is how it looks like:

Request 1#

Response 1#

Request 2#

Response 2#

Request 3#

Response 3#

Request 4#

Response 4#

so from request 1-3 we can see, the ACAO is properly set no matter from what Origin its requested, but in case 4 we can see the anything after domain name site.com is getting reflected back to ACAO header.

So i thought to exploit it, and after bit of confusion and quick tip from James cleared my doubts. so i enabled the wildcard entry for my domain geekboy.ninja.

And idea is of wildcard will work like this, now if i request anything in this manner: https://site.com.geekboy.ninja/exploit.html, it will valid request and exploit.html will be served from domain geekboy.ninja/exploit.html  and other side, Origin will be set as: https://site.com.geekboy.ninja which is the requirement of this case.

And this is how a small misconfiguration allows attacker to bypass the SOP of website.

Takeaways for hackers: check for every variations of Origin header, for dev: use predefined ACAO dynamically.  


do let me know if you have any question in comment section.

Exploiting Misconfigured CORS (Cross Origin Resource Sharing)

Hey frnds 🙂

few days before noticed a blog post for exploiting facebook chat and reading all the chats of users so that made me to interested to know about the issues, and basically it was misconfigured CORS configuration where null origin is allowed with credentials true,  it was not something heard for the 1st time, @albinowax from the portswigger explained it very well in his blog post, so after reading that messenger blog post i went to test for the same issue for some targets where i allowed to test it.

but before that here are some tips about CORS where it can be exploitable from attackers point of view:

  • Poorly implemented, Best case for Attack:

Access-Control-Allow-Origin: https://attacker.com

Access-Control-Allow-Credentials: true

  • Poorly implemented, Exploitable:

Access-Control-Allow-Origin: null

Access-Control-Allow-Credentials: true

  • Bad implementation but not exploitable:

Access-Control-Allow-Origin: *

Access-Control-Allow-Credentials: true

or just

Access-Control-Allow-Origin: *

even this is not good from development point of view but due to own rules of CORS if Access-Control-Allow-Origin set to * we don’t get benefit Access-Control-Allow-Credentials: true means no cookie access of the victim.

When you can’t exploit even if above misconfigurations are present:

  • Presence of any custom header in request.
  • Presence of any unique/authentication/key in the request URI  

am not going to more deep about CORS, as earlier blog post covered it very well.
so in above i mentioned 3 cases where first two cases is exploitable in that eg of 2nd case is that Facebook Messenger chat issue which i mentioned in earlier section of the post, and eg of 1st case is mine which i found 2 days before only where any arbitrary Origin is allowed and same Origin get reflected back to Access-Control-Allow-Origin with Credentials set to True, the best way i found to check for CORS issue is using CURL.

eg : curl https://test.victim.com -H "Origin: https://geekboy.ninja" -I and check the response if Origin is reflected in the response or not.

OR if your burp pro user, Burp Active Scan may find this for you, but in mine case it didnt, idk the reason, when i CURLed my target manully curl https://my.target.com -H "Origin: https://geekboy.ninja" -I , the Origin didnt got reflected but when i curled specifc endpoint where all users data getting back into response curl https://my.target.com/api/web/user -H "Origin: https://geekboy.ninja" -I it reflected back with my host with Credentials set to True and that’s enough to make this work and steal all that data.

i made quick poc code for it

function cors() {
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
document.getElementById("demo").innerHTML =
xhttp.open("GET", "https://my.target.com/api/web/user", true);
xhttp.withCredentials = true;

And here how it worked 🙂

Sources for better understanding of CORS:


Views/Suggestions/Edits always welcome 🙂