New CSS Attacks Break Webmail Defenses to Steal Passwords and Tokens

PortSwigger researcher Gareth Heyes presented at Black Hat USA 2026 showing that CSS content inside an email can escape its message boundary and interfere with the webmail interface. Across attack chains spanning Outlook, Gmail, Fastmail, Proton Mail, Yahoo Mail, and AOL Mail, the techniques can capture passwords, take over third-party accounts, leak tokens, hijack trusted UI actions, and manipulate AI tools that read email.

Overview

Attack Details

One demonstrated Outlook/Firefox chain spoofs a Microsoft sign-in screen and captures the password a recipient types. The attacks exploit CSS parsing behaviors specific to each webmail client:

• CSS mutation allows content to escape the email message boundary
• Password capture via spoofed sign-in screens rendered within the webmail interface
• Token hijacking by interfering with the webmail UI
• AI tool manipulation — AI agents that read email can be fed manipulated content

Mitigation

• Fastmail fixed two CSS mutation bugs
• Public PoCs remain available
• Users should exercise caution when opening emails in webmail interfaces
• Consider enabling MFA on all email-adjacent services
• Review email client security posture under ASD ISM and NZISM guidance

Related Pages

• Ai Assisted Http Terminator Finds Novel Http Desync Techniques And Apache Zero D — PortSwigger's other major web security research this week