In the sitewide notification settings, a surface meant for administrators. An access-control gap left the endpoint behind it answering ordinary logged-in accounts, and whatever was stored there rendered for every user who loaded the site. Neither weakness is remarkable on its own. The reach is what made the second one matter.
Our AI reached the notification settings from an ordinary customer account, stored script there, and confirmed it executed for other users of the site rather than only for the account that saved it. With a staff session running that script, the chain continued to administrative control of the store application.
Two weaknesses, neither decisive alone. Chained, total compromise.
Two checks, not one. Enforce the administrator requirement on the endpoint behind the notification settings, and escape stored notification content where it renders. We showed the exact handler for each, the fix, and steps a developer can replay in five minutes to watch the chain break.