Zimbra Classic Web Client Vulnerability Allows Attackers to Execute Arbitrary JavaScript

Zimbra has released critical security patches addressing a severe stored cross-site scripting vulnerability in its Classic Web Client that could allow attackers to execute malicious JavaScript code on user systems.

he company has issued fixes across multiple supported versions, including patches for both the latest releases and legacy systems, urging immediate deployment to prevent potential security breaches.

A critical security vulnerability identified as CVE-2025-27915 has been discovered in the Zimbra Classic Web Client, representing a stored cross-site scripting (XSS) attack vector that poses significant risks to organizations using the collaboration platform.

This vulnerability enables malicious actors to inject and execute arbitrary JavaScript code within the context of legitimate user sessions, potentially compromising sensitive data and system integrity.

The stored XSS vulnerability differs from reflected XSS attacks in that the malicious payload persists within the application’s data storage, making it particularly dangerous as it can affect multiple users who access the compromised content.

When exploited, attackers could potentially steal session cookies, capture user credentials, perform unauthorized actions on behalf of legitimate users, or redirect users to malicious websites.

Zimbra has addressed this critical vulnerability by implementing enhanced input sanitization mechanisms and strengthening overall security controls within the Classic Web Client.

The company has assigned this vulnerability a pending CVSS score determination, though the classification as “critical” suggests a high-severity rating that warrants immediate attention from system administrators.

DoS Vulnerability

Alongside the XSS vulnerability, Zimbra has resolved a separate denial of service (DoS) vulnerability affecting the administrative console.

This vulnerability could potentially allow attackers to disrupt service availability, causing significant operational impacts for organizations relying on Zimbra’s collaboration services.

The DoS vulnerability in the admin console represents a threat to system availability, one of the three pillars of information security alongside confidentiality and integrity.

While specific technical details about the attack vector have not been disclosed, DoS vulnerabilities typically exploit resource exhaustion or application logic vulnerabilities to render services unavailable to legitimate users.

The simultaneous release of patches for both vulnerabilities demonstrates Zimbra’s commitment to comprehensive security maintenance, addressing threats that could impact different aspects of system security and operational continuity.

Immediate Action

Zimbra has released patches across multiple supported versions to address these security vulnerabilities.

The fixes are available in Zimbra Collaboration Suite versions 9.0.0 Patch 46, 10.0.15, and 10.1.9. Organizations are strongly advised to implement these updates immediately to protect against potential exploitation.

System administrators can obtain the latest patches through standard update mechanisms using yum update or apt update commands, depending on their Linux distribution.

The company emphasizes that while only supported versions are explicitly referenced in the security advisory, older unsupported versions likely contain the same vulnerabilities.

Organizations running legacy Zimbra installations should prioritize upgrading to supported versions as soon as possible to maintain adequate security posture.

Given the critical nature of the stored XSS vulnerability and its potential for widespread impact, Zimbra strongly recommends that all customers deploy these security patches without delay to prevent potential security incidents and maintain the integrity of their collaboration environments.

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