On: I created an ephemeral group chat app for stranger...

[someuser54541]

Some user was able to open a browser modal with a custom message in everyones browser. Any ideas how they did that?

[bean-weevil → someuser54541]

https://en.wikipedia.org/wiki/Cross-site_scripting

The author of the site was naughty and didn't do any sanitization :) They said they've fixed it in another comment, though.