Preliminary research

CSS: the bomb inside your inbox

Gareth Heyes · PortSwigger Research · 6 August 2026

AI-collected research leads through 22 September 2026, including targeted additions between broader sweeps. Unranked, incomplete, not community-vetted, and subject to change.

Webmail sanitizers and browsers disagree about CSS. Nested href attribute selectors, sped by a clipboard-paste race, leak a Medium login token five characters at a time; when CSP blocks requests, @font-face descent-override plus animations form a font-height oracle and inset reveals the matching link. Fastmail's CSSOM filter re-serialises \7d\2a into }*, Outlook's accepts @media --narrow-window;/*"*/ arbitrary CSS, giving click hijacking, prompt injection and a keylogger.

Record

Researcher

Gareth Heyes

Published by

PortSwigger Research

Date

6 August 2026

Topic

Other

In the archive

• Collected in the 2026 preliminary research sweep

Related sources

• CSS: the bomb inside your inbox (Whitepaper) Whitepaper
• CSS: the bomb inside your inbox (Slides) Whitepaper
• Code Repository
• Additional tested vectors

Tags

• css-injection
• css
• sanitizer-bypass
• email
• prompt-injection
• info-leak
• ui-redress
• xsleak
• llm
• phishing
• owasp-a03-2021
• owasp-a04-2021
• owasp-a05-2021

This page is the archive's own catalogue record. The research is the work of Gareth Heyes, first published at the original source. Preserved copies are kept so the citation survives its host; this one was last captured on 14 August 2026.