Zimbra Faces Critical XSS Vulnerability

Jul 28, 2023

A new vulnerability has been found in Zimbra Collaboration.

The discovery of the Zimbra XSS vulnerability AKA Cross-Site Scripting (XSS) vulnerability in ZCS version 8.8.15, poses a threat to the confidentiality and integrity of sensitive data.

This vulnerability could severely affect organizations relying on the Zimbra platform for email, calendar, and collaboration services if left unaddressed.

The Zimbra XSS vulnerability allows malicious actors to execute a wide array of actions, such as performing unauthorized activities on behalf of the victim or presenting a counterfeit login screen to collect usernames and passwords.

To exploit the Zimbra XSS vulnerability, attackers need to trick the user into opening a specially crafted email within the ZCS environment. 

What is Zimbra XSS vulnerability?

The Zimbra XSS vulnerability, which was revealed before was successfully tested on ZCS version 8.7.11_GA_1854 (build 20170531151956), and it is reasonable to assume that the Zimbra XSS vulnerability exists in all versions of ZCS from 8.5.0 onwards.

Fortunately, the Zimbra team has been prompt in addressing this serious security concern. The issue has been resolved in Zimbra Collaboration Suite version 8.8.7, which users are strongly encouraged to update immediately to safeguard their systems and data.

Zimbra Collaboration Suite is a widely used enterprise-class solution for cloud-based email, calendar, and collaboration needs.

It boasts a browser-based interface that runs seamlessly across various devices, including smartphones, tablets, and desktops running Windows, Linux, or OS X.

The Zimbra XSS vulnerability stems from how Zimbra handles attachment links within emails. Each attachment generates a link (<a> tag) using the ZmMailMsgView.getAttachmentLinkHtml function, which is responsible for this operation. 

Earlier this year, Zimbra alerted admins about a zero-day vulnerability actively exploited in attacks on its Zimbra Collaboration Suite (ZCS) email servers. The security flaw is a reflected Cross-Site Scripting (XSS) that can compromise data confidentiality and integrity.

Google TAG’s Maddie Stone confirmed the XSS vulnerability’s targeted attack. While official patches are pending, Zimbra offers a manual fix for admins to safeguard their mailbox nodes by sanitizing user-inputted data and preventing XSS flaws.

Admins are requested to apply this measure until the official patch is released for optimal security.

Technical analysis for Zimbra XSS vulnerability

The Zimbra XSS vulnerability arises from the fact that the value for params.href, essential for creating the link, is derived directly from the Content-Location header in the message.

As there is no proper sanitization of this value, an attacker can influence the header’s content and inject arbitrary HTML or JavaScript into the <a> tag.

A proof-of-concept email can be used to demonstrate the severity of the Zimbra XSS vulnerability. An attacker can execute script code when the victim opens the message by sending an email with a specially crafted Content-Location header.

The emergence of this Zimbra XSS vulnerability has raised concerns among users about potential cyber attackers exploiting it.

Organizations are urged to act swiftly in applying the necessary mitigations and upgrades as per the vendor’s instructions.

If mitigations are not available, discontinuing using Zimbra Collaboration Suite until the issue is resolved is a recommended precautionary measure.

The discovery of the Zimbra XSS vulnerability in ZCS version 8.8.15 poses a significant threat to organizations relying on the platform as this vulnerability allows malicious actors to execute unauthorized access to the victim’s system.

However, the Zimbra team promptly addressed this issue in version 8.8.7, urging users to update immediately to safeguard their systems and data. Users should remain vigilant and follow the vendor’s instructions for mitigations and upgrades.

Media Disclaimer: This report is based on internal and external research obtained through various means. The information provided is for reference purposes only, and users bear full responsibility for their reliance on it. The Cyber Express assumes no liability for the accuracy or consequences of using this information.

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