Zimbra Classic Web Client Vulnerability Let Attackers Execute Arbitrary JavaScript
A critical security vulnerability has been discovered in Zimbra Classic Web Client that enables attackers to execute arbitrary JavaScript code through stored cross-site scripting (XSS) attacks.
The vulnerability, designated as CVE-2025-27915, poses significant risks to organizations using affected Zimbra installations, prompting immediate patch deployment recommendations from security experts.
Summary
1. Zimbra Classic Web Client has a stored cross-site scripting flaw (CVE-2025-27915) allowing arbitrary JavaScript execution.
2. Attackers can steal credentials, hijack sessions, and perform unauthorized actions through persistent malicious script injection.
3. Security updates available for versions 9.0.0 Patch 46, 10.0.15, and 10.1.9 with enhanced input sanitization.Critical Stored XSS Vulnerability
The vulnerability represents a stored cross-site scripting (XSS) flaw that allows malicious actors to inject and execute arbitrary JavaScript code within the Zimbra Classic Web Client environment.
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Unlike reflected XSS attacks that require user interaction, stored XSS vulnerabilities are particularly dangerous because the malicious payload persists on the server and executes automatically when users access the compromised content.
The vulnerability stems from inadequate input sanitization mechanisms within the Classic Web Client interface, enabling attackers to bypass existing security controls and embed malicious scripts directly into the application.
Once successfully exploited, attackers can potentially steal session cookies, harvest sensitive user credentials, manipulate email content, or perform unauthorized actions on behalf of legitimate users.
The arbitrary JavaScript execution capability provides attackers with extensive control over the client-side environment, making this a high-priority security concern.
Security researchers emphasize that this vulnerability affects the core functionality of Zimbra’s web-based email client, potentially exposing millions of users worldwide to sophisticated phishing campaigns and data exfiltration attempts.
| Risk Factors | Details |
| Affected Products | Zimbra Classic Web Client versions prior to 9.0.0 Patch 46, 10.0.15, and 10.1.9 |
| Impact | Stored Cross-Site Scripting (XSS) allowing arbitrary JavaScript execution |
| Exploit Prerequisites | Access to Zimbra Classic Web Client interface, ability to inject malicious content through inadequately sanitized input fields |
| CVSS 3.1 Score | TBD (likely receive a high to critical severity rating) |
Patch Available
Zimbra has responded swiftly to address this critical security flaw by releasing comprehensive patches across multiple product versions.
The security update specifically targets input validation routines and implements enhanced content security policies (CSP) to prevent malicious script injection.
The following Zimbra versions have received critical security patches: Zimbra 9.0.0 Patch 46, Zimbra 10.0.15, and Zimbra 10.1.9.
These patch releases include strengthened input sanitization mechanisms that properly encode user-supplied data before rendering it within the web client interface.
Additionally, the patches implement improved output encoding functions and enhanced HTML parsing algorithms to neutralize potentially dangerous script elements.
The patch deployment process involves updating core Zimbra components, including the jetty web server configuration, servlet filters, and JavaScript libraries responsible for handling user input processing within the Classic Web Client framework.
Zimbra administrators are strongly urged to implement these security patches immediately to protect their organizations from potential exploitation attempts.
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