Roundcube Webmail: Insufficient CSS sanitization in HTML e-mail messages
Date: 2026-04
Affected: composer/roundcube/roundcubemail
Attack vector: ssrf
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-35540
CWE: CWE-669
CWE-918
CVSS: 5.4 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N)

An issue was discovered in Roundcube Webmail 1.6.0 before 1.6.14. Insufficient Cascading Style Sheets (CSS) sanitization in HTML e-mail messages may lead to SSRF or Information Disclosure, e.g., if stylesheet links point to local network hosts.


Mitigations:

Architectural layers (MAESTRO):