Webmail CSS Attacks Expose a New Risk for AI-Powered Email Tools
Brief
CSS attacks on major webmail services can steal credentials, hijack sessions and manipulate AI tools connected to users’ inboxes.
PortSwigger researcher Gareth Heyes demonstrated something that should make every webmail team a little nervous: plain CSS, the styling language that’s supposed to just make text look nice, can be weaponized to steal passwords, hijack sessions, and manipulate AI tools reading your inbox. The research covers real attack chains against Outlook, Gmail, Fastmail, Proton Mail, Yahoo Mail, and AOL Mail.
The core idea is that email clients let HTML and CSS through with the assumption that styling can’t reach outside the message it’s attached to.