Full Report
Malicious actors are exploiting Cascading Style Sheets (CSS), which are used to style and format the layout of web pages, to bypass spam filters and track users' actions. That's according to new findings from Cisco Talos, which said such malicious activities can compromise a victim's security and privacy. "The features available in CSS allow attackers and spammers to track users' actions and
Analysis Summary
# Tool/Technique: CSS Abuse for Spam Evasion and User Tracking
## Overview
Cybercriminals are leveraging Cascading Style Sheets (CSS) features within emails to evade spam filters and to track user actions and environmental details (fingerprinting) of the recipient's email client or webmail environment. This abuse relies on embedding malicious formatting while keeping content visually hidden from the end-user.
## Technical Details
- Type: Technique (Abuse of legitimate web technology)
- Platform: Email clients and webmail services supporting HTML and CSS in emails.
- Capabilities: Spam filter evasion, content concealment, user behavior tracking, and environment fingerprinting.
- First Seen: Findings built upon previous work identifying hidden text salting spikes in the second half of 2024; latest analysis published around March 2025 (based on article date).
## MITRE ATT&CK Mapping
Since this technique focuses on email delivery/evasion and gathering information about the client environment upon opening the email, the primary mapping is related to defense evasion and discovery.
- **TA0005 - Defense Evasion**
- T1027 - Obfuscated Files or Information
- T1027.008 - Embedded Content
**(Note:** While not strictly file obfuscation, concealing content execution/display via CSS fits the objective of hiding malicious intent from security tools.)
- **TA0007 - Discovery**
- T1592 - Gather Victim Identity Information (Implied through fingerprinting email/client details)
- T1082 - System Information Discovery (By detecting screen size, color scheme, etc., via media queries)
## Functionality
### Core Capabilities
- **Spam Filter Evasion:** Using legitimate HTML and CSS features like comments and irrelevant content (hidden text salting) to confuse email parsers and detection engines.
- **Content Concealment:** Employing CSS properties like `text-indent` and `opacity` to render unwanted or malicious content completely invisible to the user in the email body.
- **Phishing Redirection:** The hidden content can sometimes include mechanisms intended to redirect the recipient to a phishing page.
### Advanced Features
- **User/Environment Fingerprinting:** Utilizing the CSS **`@media`** at-rule to query and determine attributes of the user's environment, such as:
- Font and color scheme preferences.
- Client language.
- Screen size and resolution.
- Color depth.
- **Action Tracking:** Monitoring user actions, including whether an email was viewed or printed, via CSS rules compatible with the rendering environment.
## Indicators of Compromise
*Note: As this is a technique utilizing visual obfuscation rather than specific malware binaries, hard IoCs like hashes are not applicable. IoCs focus on the CSS properties used.*
- File Hashes: N/A (Technique-based)
- File Names: N/A (Technique-based)
- Registry Keys: N/A
- Network Indicators: Redirection links to phishing pages (precise IPs/domains not provided in the text, but are the outcome).
- Behavioral Indicators:
- Presence of CSS rules designed to set `text-indent` to extreme negative values or large positive values.
- Use of the `@media` at-rule in email HTML to query system/client attributes.
- High volume of invisible or null-content elements within the email payload.
## Associated Threat Actors
- Threat actors and spammers exploiting email vectors (Research findings noted by Cisco Talos).
## Detection Methods
- Signature-based detection: Creating signatures against known obfuscation patterns, particularly for hidden text salting.
- Behavioral detection: Monitoring email rendering engines for abnormal query patterns initiated by CSS (e.g., frequent checks via `@media` rules).
- YARA rules: Potentially applicable for scanning incoming email content for specific obfuscating CSS structures.
## Mitigation Strategies
- Implement advanced filtering mechanisms designed to specifically detect hidden text salting and content concealment techniques within HTML emails.
- Utilize email privacy proxies to sandbox or strip potentially interactive or fingerprinting elements before delivery.
- Security posture should treat CSS/HTML in emails as potentially untrusted content, limiting the execution scope allowed by the email client.
## Related Tools/Techniques
- Hidden text salting (A related technique used for evasion discussed previously by Talos).
- Email fingerprinting techniques utilizing HTML features (though CSS is highlighted here for dynamic environment discovery).
- Standard phishing techniques leveraging email redirects.