CVE-2025-27915 - Stored XSS Vulnerability in Zimbra Collaboration Suite Leads to Email Redirection and Data Exfiltration

An issue, classified as CVE-2025-27915, has been discovered in Zimbra Collaboration Suite (ZCS) versions 9., 10., and 10.1. The vulnerability, a stored cross-site scripting (XSS) flaw, is specifically found in ZCS's Classic Web Client, where HTML content in ICS files isn't adequately sanitized. As a result, an attacker can execute arbitrary JavaScript code within the victim’s session, leading to unauthorized actions on the victim's account, including email redirection and data exfiltration.

Exploit Details

When a user views an email message containing a malicious ICS entry, the JavaScript embedded within it executes via an ontoggle event inside a <details> tag. Here's an example:

<details ontoggle=alert(1)>
  <summary>Click me to execute the JavaScript code</summary>
  This is a malicious ICS entry.</details>

To orchestrate this exploit, the attacker would first craft an ICS file with the malicious payload embedded in an event description, like this:

BEGIN:VCALENDAR
VERSION:2.
BEGIN:VEVENT
SUMMARY:Malicious Meeting
DESCRIPTION: <details ontoggle="javascript_code">Malicious Content</details>
DTSTART:2022-10-01T10:00:00Z
DTEND:2022-10-01T12:00:00Z
END:VEVENT
END:VCALENDAR

Next, the attacker sends an email to the victim with this malicious ICS file attached. When the victim opens the email via the ZCS Classic Web Client, the JavaScript payload will execute within the victim's session, enabling the attacker to perform malicious actions on the victim's account.

The original reference to this vulnerability can be found here: Zimbra Security Advisory

Impact

Users of Zimbra Collaboration Suite (ZCS) 9., 10., and 10.1 are at risk of attackers gaining unauthorized access to their email accounts, potentially exfiltrating sensitive information or redirecting incoming emails to an attacker-controlled address. Businesses and organizations that rely on the Classic Web Client for communication may encounter significant disruptions to their workflows, with potentially massive reputational harm and financial loss.

Mitigation

To protect against this stored XSS vulnerability, it's essential to properly sanitize user-generated data and prevent malicious payloads from executing. Employ strong input-validation methods, such as white-listing and output-encoding techniques, and ensure prompt application of security patches provided by Zimbra. Regularly review Zimbra's security advisories and updates to stay up-to-date with potential vulnerabilities and fixes.

In addition to applying the necessary updates and patches, users should consider employing additional security measures, such as using the ZCS Modern Web Client, which isn't impacted by this vulnerability. Furthermore, actively promoting security best practices and awareness among users can go a long way in protecting against a wide range of threats.

In conclusion, CVE-2025-27915 represents a severe stored XSS vulnerability in Zimbra Collaboration Suite (ZCS) that can result in unauthorized email redirection and data exfiltration. Users of affected ZCS versions should take the necessary precautions to safeguard their accounts and work with their organization's security team to stay current with critical security updates.

Timeline

Published on: 03/12/2025 15:15:39 UTC
Last modified on: 04/02/2025 20:38:25 UTC