CSS: The bomb inside your inbox
The article examines how webmail clients render untrusted CSS and how attackers can exploit sanitization flaws to breach security. It details techniques such as abusing HTML labels, CSS pseudo‑elements, and image proxy bypasses to exfiltrate tokens, hijack accounts, and steal passwords across services like Fastmail, Gmail, and Outlook. The author also discusses defenses and future attack vectors.
- ▪Webmail sanitizers can be bypassed by creating discrepancies between what the sanitizer deems safe and how browsers render the HTML/CSS.
- ▪HTML label elements with a 'for' attribute can trigger UI actions in clients such as Outlook, enabling message pinning and UI manipulation.
- ▪CSS pseudo‑elements like :before and :after can be used to inject hidden content and indirect prompt injection in AI browsers like OpenAI's Atlas.
- ▪Image proxy bypasses allow attackers to track recipients and exfiltrate data even when CSP blocks external resources.
- ▪The paper outlines multiple novel attack techniques and suggests defensive measures to mitigate these CSS‑based exploits.
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Record
| Original publisher | PortSwigger Research |
| Canonical URL | https://portswigger.net/research/css-the-bomb-inside-your-inbox |
| Publication time | Sun, 09 Aug 2026 10:17:52 +0000 |
| Retrieval time | 2026-08-09T11:00:42.851Z |
| Last seen | 2026-08-09T11:00:42.851Z |
| Headline source | Publisher (no WeSearch rewrite) |
| Excerpt source | publisher body |
| Excerpt method | First ~120 words (~800 chars) of extracted publisher body, fair-use limited. |
| Summary | WeSearch · cerebras-chat (WeSearch summarizer) |
| Summary source text | contentText |
| Citation coverage | Summary is a WeSearch-generated derivative; primary citation is the original publisher URL. |
| Cluster | 1lh0P93OV7AR · 1 stories |
| Cluster logic | Grouped by semantic title/content similarity across sources within a rolling window. Same-publisher template collisions are excluded from coverage comparison. |
| Ranking reason | Story pages are not engagement-ranked. Hub feeds use recency, with optional source-diversified chronological ordering (cap consecutive stories per source). No personalized ranking. |
| Publisher visit | Yes — open original |
| Substitutes article? | No — link-out required for full text |
Rights status (four layers)
WeSearch handling by dimension
| Indexing | May the item be indexed (stored, ranked, made findable)? | Allowed |
| Snippet | May a short excerpt of the publisher's text be shown? | Allowed |
| AI summary | May WeSearch generate its own short summary of the article? | Limited |
| Retrieval / RAG | May the content be exposed for third-party retrieval-augmented generation? | Not asserted |
| Model training | May the content be used to train AI models? | Not asserted |
| Commercial reuse | May the content be reused commercially? | Not permitted |
Basis: Derived from the published RSS/Atom feed. Contact: [email protected]. Reviewed: 2026-07-24.
Opening excerpt (first ~120 words) tap to expand
CSS:the bomb inside your inbox Gareth Heyes Researcher @garethheyes Published: Thursday, 6 August 2026 at 22:00 UTC Updated: Thursday, 6 August 2026 at 22:00 UTC Gareth Heyes - [email protected] - @garethheyesIt's quite common for webmail clients to render untrusted CSS in a trusted UI. They attempt to make this safe using CSS sanitization. In this paper I'm going to show you how to break out of trust boundaries, exfiltrate tokens, compromise 3rd party websites and even steal passwords.Table of contentsIntroductionAbusing allowed HTML/CSSAbusing HTML labels to perform UI actionsControlling AI browsers via emailAccount takeover from pasting into a draft emailExfiltrating tokens when CSP is blocking all external resourcesBypassing CSS sanitizationMaking external requestsSyntax…
Excerpt limited to ~120 words for fair-use compliance. The full article is at PortSwigger Research.
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