Exploiting CSS for Tracking and Evasion: A Growing Threat to Security and Privacy

Cascading Style Sheets (CSS), a cornerstone in web design, is widely used to manage the layout and presentation of HTML elements on web pages. While CSS has enabled developers to create visually compelling and responsive websites, its capabilities are increasingly being abused by cybercriminals. Cisco Talos has recently highlighted a growing concern: the use of CSS by threat actors to evade detection, track user behavior, and even conduct fingerprinting attacks. This evolving technique has significant implications for both security and privacy, especially in the realm of email and web interactions.

Summary: How Attackers Abuse CSS for Malicious Purposes

Cisco Talos has observed a surge in the use of CSS by cybercriminals to hide malicious content and track users’ actions. While CSS is primarily a tool for controlling the appearance of web content, it can also be exploited for nefarious purposes. Attackers are utilizing CSS properties to make phishing content invisible or bypass security filters, targeting email clients with techniques like:

Text Concealment: By using properties like text-indent: -9999px, attackers can push phishing text far out of view. Additionally, setting the font-size to an extremely small value makes the text almost invisible on most screens.
Invisible Text via Color & Opacity: Attackers often use the color: transparent property to hide text by blending it into the background. Similarly, the opacity property is employed to make content fully transparent, further concealing harmful information.

An example cited in the Talos report involved a phishing email impersonating the Blue Cross Blue Shield organization. In this case, the attacker used several CSS tricks to make the email’s preheader text invisible. By manipulating properties like height, max-height, and mso-hide for Outlook users, the attacker made the preheader appear harmless (e.g., “FOUR yummy soup recipes just for you!”), reducing the likelihood of detection by spam filters.

Moreover, CSS is being abused for user tracking and fingerprinting. The @media rule enables attackers to gather data on recipients’ email clients, screen resolution, and even language preferences. By observing these behaviors, cybercriminals can fine-tune their attacks, increasing the chances of success.

While these malicious tactics are increasingly sophisticated, the good news is that advanced filtering mechanisms and email privacy proxies can help mitigate the risks. Email clients that rewrite emails to strip away CSS-based tracking can enhance user privacy and prevent data exfiltration.

What Undercode Says:

The latest trend of using CSS for tracking and evasion presents a major shift in the tactics employed by cybercriminals. Traditionally, phishing and malware attempts relied heavily on content that was visually obvious to the user. However, the subtlety of CSS-based manipulation has allowed attackers to bypass traditional spam filters and detection systems, hiding malicious content and tracking users without their knowledge.

The growing reliance on CSS in these attacks is particularly concerning because it exploits a fundamental web technology that is usually considered benign. CSS is intended to improve the user experience by managing the aesthetics of a webpage or email. Unfortunately, cybercriminals have found creative ways to manipulate it for surveillance and obfuscation. This could lead to a rise in fingerprinting, where attackers gather detailed information about users’ devices and behaviors, creating a more individualized attack profile.

Moreover, these threats

One aspect that stands out in the Talos report is the use of CSS for “invisible” phishing, where the attacker hides crucial parts of the email. This includes preheader text and other hidden elements, which, though benign-sounding (like “FOUR yummy soup recipes”), are designed to avoid detection by spam filters and trick users into clicking malicious links.

The potential for privacy breaches cannot be overstated. While browser and email client developers are rolling out security features to mitigate these risks, the abuse of CSS poses an ongoing challenge. Companies and individual users alike must stay vigilant and be aware of the evolving threat landscape. Cybersecurity professionals must enhance detection mechanisms that specifically look for these kinds of attacks, as traditional methods may no longer be enough.

Ultimately, CSS-based tracking and evasion are signs of a broader trend in cybersecurity: attackers are leveraging everyday technologies, like CSS, to silently exploit vulnerabilities. As this threat evolves, so too must our strategies to defend against it.

Fact Checker Results:

1. Cisco

The use of CSS for fingerprinting and tracking is a growing concern among cybersecurity experts.
Mitigation strategies, like advanced filtering and email privacy proxies, are effective but need further implementation across platforms.

References:

Reported By: https://securityaffairs.com/175512/security/attackers-use-css-to-create-evasive-phishing-messages.html
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