Exploitation of SVG-Based XSS in RoundCube Webmail Instances
Vulnerability Overview
Recent threat intelligence indicates active exploitation of a cross-site scripting (XSS) vulnerability in RoundCube Webmail. The flaw originates from the application’s failure to properly sanitize <animate> tags within Scalable Vector Graphics (SVG) documents. When a user views a specially crafted email containing these malicious SVG elements, the payload executes within the context of the user’s browser session.
Technical Analysis and TTPs
The attack vector utilizes the XML-based nature of SVG files. Attackers bypass existing filters by embedding JavaScript within the values or to attributes of the <animate> tag. Because the webmail client’s sanitization engine failed to account for these specific attributes, the browser interprets the data as executable code rather than static graphical instructions.
Key technical observations include:
- Persistence Mechanism: Successful execution allows for the theft of session cookies, enabling unauthorized account access without requiring credentials.
- Delivery Method: The exploit is typically delivered via spear-phishing emails containing inline SVG images or attachments.
- Impact: Beyond session hijacking, attackers can use the XSS to perform actions on behalf of the user, such as modifying mail filters, exfiltrating sensitive communications, or conducting internal phishing campaigns.
Mitigation and Defense
RoundCube released a patch in late 2024 (and subsequent updates into 2025) to address the sanitization logic within the program’s core distribution. Organizations must ensure they are running the latest stable release (1.6.7 or 1.5.7 and higher).
When assessing the external attack surface, organizations should utilize Pocket Pentest to discover unpatched webmail servers vulnerable to this class of input sanitization failure. Furthermore, security teams should implement Content Security Policy (CSP) headers to restrict the execution of inline scripts and untrusted remote resources, providing a layer of defense-in-depth against XSS-based session exploitation.