MEDIUM Threat Intel
Webmail CSS Injection: Hidden Data Exfiltration Threats
Security researchers warn that Cascading Style Sheets can exfiltrate sensitive data from webmail inboxes if vendors fail to sanitize styles.
- Webmail users face potential data exfiltration risks as attackers leverage advanced Cascading Style Sheets features to bypass traditional email filters.
- Webmail clients that fail to properly sanitize imported styles or attributes are vulnerable to unauthorized data harvesting.
- Defenders must implement strict input sanitization policies and enforce proper Content Security Policies to block malicious style injection.
Runtime Rebel Intel
2 min read · Aug 10, 2026