CSS: The bomb inside your inbox

Webmail clients render untrusted HTML and CSS, relying on sanitizers to keep users safe. But sanitizers can be bypassed, and Gareth Heyes shows how. He demonstrates attacks on Outlook, Fastmail, Gmail, ProtonMail, and more, including abusing HTML labels to control UI, using CSS pseudo-elements to hide text from AI browsers like OpenAI's Atlas, stealing tokens via clipboard paste, and even exfiltrating passwords. The research reveals that CSS sanitization is far from bulletproof, with real-world impacts.