New CSS attack chains break webmail boundaries
🔒 New research shows HTML and CSS can escape email message boundaries to interfere with webmail UIs across major providers. PortSwigger researcher Gareth Heyes presented proof-of-concept chains at Black Hat USA 2026 targeting Outlook, Gmail, Fastmail, Proton Mail, Yahoo Mail, and AOL Mail. The techniques can capture passwords, leak tokens, hijack UI actions, and manipulate AI-connected tools; some PoCs remained public as of August 8.
