Cybercriminals Exploit CSS to Bypass Spam Filters and Track User Activity in Emails

 

CSS exploited to bypass spam filters and track users.

Cybercriminals have discovered a new method to bypass spam filters and track users' actions through Cascading Style Sheets (CSS), a core technology used to design and format web pages. According to the latest research from Cisco Talos, these malicious actors are taking advantage of CSS properties to manipulate content and compromise users' security and privacy. This alarming development highlights the need for advanced cybersecurity measures to prevent such sophisticated threats.

CSS: A New Weapon in Email-Based Attacks

CSS, primarily intended for defining the visual layout of web content, has now been weaponized by threat actors to facilitate various malicious campaigns. Talos researcher Omid Mirzaei revealed that attackers are leveraging CSS to track users’ preferences and actions, bypassing traditional spam detection mechanisms.

While JavaScript and other dynamic content are typically restricted in email clients, CSS remains permissible, providing a loophole for threat actors. Malicious actors exploit this by embedding hidden CSS properties within emails, allowing them to:

• Evade Spam Filters: By adding hidden or irrelevant content using CSS, attackers can confuse spam detection engines, increasing the chances of malicious emails landing in a user’s inbox.

• Track User Behavior: Embedded CSS properties enable cybercriminals to track email recipients' actions, such as opening, viewing, or printing the email.

• Conduct Fingerprinting Attacks: CSS properties can be used to fingerprint recipients by collecting information about their device, browser, or email client.

How Attackers Use CSS to Bypass Spam Filters

Talos’ research builds on its previous findings regarding a surge in email-based threats leveraging hidden text salting in the second half of 2024. This tactic involves using legitimate HTML and CSS features to insert hidden comments and irrelevant content, which remain invisible to recipients but successfully bypass spam detection systems.

One of the most notable techniques involves using CSS properties like:

• text-indent – To shift content away from the visible area.
• opacity – To make content completely transparent and invisible.

Deceptive Content Concealment Using CSS

Malicious actors manipulate email content by using CSS to obfuscate malicious URLs or phishing links. These hidden elements remain undetected by security parsers but can trick unsuspecting users into interacting with the malicious content.

Furthermore, CSS enables attackers to create visually appealing emails that appear legitimate, increasing the likelihood that victims will click on phishing links or download malicious attachments.

CSS-Enabled User Tracking and Fingerprinting

Beyond bypassing spam filters, CSS also offers attackers the ability to monitor user activity and conduct fingerprinting attacks through spam emails. Fingerprinting involves gathering data about the recipient’s system, environment, and preferences, which allows threat actors to customize future attacks and improve their success rate.

The @media CSS at-rule is one such property that attackers leverage to detect and record information about recipients, including:

• Screen Size and Resolution: Attackers can identify the screen size and resolution, allowing them to optimize phishing pages for a more convincing appearance.

• Color Scheme and Font Preferences: By detecting the recipient's color and font preferences, attackers can design phishing pages that mimic legitimate websites.

• Language Settings: Language preferences can be exploited to craft personalized phishing emails in the recipient’s native language.

This sophisticated fingerprinting capability makes CSS a potent tool in the arsenal of cybercriminals, enabling them to profile targets and execute more effective attacks.

Potential Risks and Implications of CSS Exploitation

The abuse of CSS in email-based attacks poses significant risks to both individuals and organizations, including:

• Data Privacy Compromise: Tracking recipients' actions and preferences raises serious privacy concerns, enabling attackers to gather sensitive information.

• Phishing and Credential Theft: By crafting visually appealing and personalized phishing emails, attackers can lure victims into providing login credentials or financial information.

• Malware Distribution: Malicious attachments or links embedded in spam emails can compromise systems and lead to widespread malware infections.

How to Mitigate CSS-Based Email Threats

To counter the growing threat of CSS exploitation in emails, organizations and individuals must adopt proactive security measures, including:

1. Advanced Email Filtering and Detection

Implementing advanced spam filtering solutions that detect hidden text salting, obfuscation, and suspicious CSS properties can significantly reduce the risk of malicious emails reaching the inbox. Security gateways should be configured to identify abnormal CSS usage in email content.

2. Email Privacy Proxies

Using email privacy proxies can prevent direct communication between email clients and external content, reducing the likelihood of tracking and fingerprinting. Proxies can block requests that attempt to gather sensitive information.

3. Content Disarm and Reconstruction (CDR)

CDR technology neutralizes potential threats by removing executable content from incoming emails while preserving the original message structure. This approach ensures that hidden CSS elements are sanitized before reaching the recipient.

4. User Education and Awareness

Training employees and users to recognize phishing attempts and suspicious email content remains critical. Awareness campaigns should emphasize the dangers of interacting with unsolicited emails and encourage verifying sender identities before clicking on any links.

Conclusion: A Growing Threat in Email Security

As cybercriminals continuously evolve their techniques, the use of CSS to evade spam filters and track email recipients highlights the need for vigilance and advanced security protocols. Organizations must stay ahead of these threats by enhancing their email security infrastructure and educating users about the risks associated with phishing and email-based attacks.

By implementing robust filtering mechanisms, using email privacy proxies, and maintaining constant awareness of evolving cyber threats, organizations can mitigate the risks posed by CSS exploitation and protect their sensitive information.