A Hidden Threat Lurking in Your Inbox: Researchers Warn CSS Can Exfiltrate Data from Webmail
Cybersecurity researchers have discovered a previously unknown threat lurking in plain sight: Cascading Style Sheets (CSS), used for web page design, can be exploited to exfiltrate sensitive user information from email platforms. This alarming finding has left many vendors scrambling to address the issue, but some are not taking it seriously enough.
Gareth Heyes, a web security researcher at PortSwigger, has been experimenting with CSS and discovered its potential for malicious use. According to him, CSS is “almost like a programming language now,” capable of building working keyloggers that can steal confidential user information without the need for JavaScript or attachments. This is particularly concerning as it bypasses traditional email-based exploits.
The problem lies in the fact that webmail vendors are not prepared to handle this new threat. While some have acknowledged the issue and are working on fixes, others have dismissed it entirely, only to quietly implement patches later without public acknowledgment. This lack of transparency raises serious concerns about the security posture of these companies.
Heyes believes that CSS-based attacks will become a major concern in the near future due to their ability to bypass traditional defenses. Browsers continually add new features to HTML and CSS, but this also expands the attack surface, making it easier for malicious actors to exploit vulnerabilities. It’s essential for webmail vendors to take immediate action and filter/sanitize user displays to prevent data exfiltration.
The root of the problem lies with the vendors themselves, who have a responsibility to ensure their platforms are secure against these emerging threats. While users can’t do much on their end without advanced knowledge of HTML and CSS, it’s crucial for companies to take proactive measures to protect their customers. Heyes suggests using image proxies as an effective way to guardrails against these attacks.
The research community is urging webmail vendors to prioritize security disclosures and implement robust filtering/sanitization mechanisms to prevent data breaches. As the threat landscape continues to evolve, it’s essential for companies to stay ahead of emerging threats and take proactive measures to protect their users.
In conclusion, the discovery of CSS-based attacks highlights a critical vulnerability in email platforms that requires immediate attention from vendors. It’s time for them to step up and address this issue before it’s too late. Users can only hope that these companies will prioritize security and transparency to prevent data exfiltration and protect their sensitive information.
Source: Dark Reading — 2026-08-05