Can a security expert explain to me how the following XSS changed a cookie on a completely different computer? I can't reproduce it and I'm not even sure how was that possible in the first place.

The web application uses ASP.NET that is already secured extremely well by default. I tried to find XSS vulnerability and when I couldn't, I changed <select> element's value to <script>alert (1);</script> and selected it. Selection change sets a cookie value and refreshes the page. This time the cookie value was set and page refreshed to ASP.NET's error page that prevented a dangerous query from executing. All of this happened on my laptop in my apartment.

At that point I said fuck it and concluded the website isn't vulnerable to XSS. However, the next day at work I got the same error and the cookie's value was the same value I entered on my laptop. Plus, only my work computer had that cookie magically set.

So, let's recap:
- I manually set a cookie on my laptop in my apartment.
- A computer in my office also had the same exact cookie set.

  • 1
    @linuxxx I thought maybe you'd know so I'll tag you.
  • 1
    I’m not a security expert by any means but were you logged into a google account at the time?
  • 0
    @for-Each Nope. I don't login to any of my personal accounts on the work computer.
  • 1
    @hobbyte sorry bud that’s all I had. Don’t know enough about security to provide you any form of logical insight
  • 3
    same session id and it shared cookies?

    don't fully understand your explanation
  • 3
    @zlice Session IDs are always unique so I don't really know how it could happen. Maybe my co-worker fucked something up in regards to session IDs then. I'll waddle through all of the spaghetti code to check that next week.
  • 2
    @zlice Thanks for the suggestion! Btw, the only reason why I care is because there are a lot of clients who will use this web app so I'm just trying to see if this XSS thing is possible to bring down the web app for even more users than only myself.
  • 2
    I literally have zero experience with asp.net due to always having refused to use it so idk 😬
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