CSS-Based Webmail Vulnerabilities Enable Password and Token Theft via UI Manipulation

New CSS-based attacks exploit webmail interfaces to steal credentials and tokens by manipulating UI elements and bypassing security measures, with proof-of-concept demonstrations across major email services. Researchers recommend sandboxing HTML emails and restricting CSS to mitigate risks.

New research shows 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.

PortSwigger researcher Gareth

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