TLDR Tech / InfoSec
CSS: The Bomb Inside Your Inbox
Portswigger demonstrates CSS and HTML attacks against webmail sanitizers. Outlook labels can trigger interface actions, while a CSS parser flaw enables arbitrary CSS injection and a spoofed login screen that captures passwords in Firefox. Yahoo Mail and AOL Mail allowed pasted CSS to race sanitization, exposing Medium email-login tokens. Fastmail image-proxy flaws enabled view tracking, ProtonMail could reveal an IP address, and hidden prompt instructions in email content directing Atlas browser actions. The recommended security controls include sandboxed iframes, restrictive allowlists, blocked image requests, and filtering dangerous selectors.
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