ClickCease Zimbra Zero-Day Exploit Unveiled

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Zimbra Zero-Day Exploit Unveiled

Wajahat Raja

December 1, 2023 - TuxCare expert team

Cyber threats in business email systems have become extremely common in this digital world. Recently, a critical zero-day vulnerability in the widely used Zimbra Collaboration email software has been exploited by multiple threat actors, posing significant risks to email data, user credentials, and authentication tokens. This flaw, identified as CVE-2023-37580 with a CVSS score of 6.1, is a reflected cross-site scripting (XSS) vulnerability affecting versions predating 8.8.15 Patch 41. Despite Zimbra’s swift response with patches released on July 25, 2023, real-world attacks continued to unfold. This blog details the Zimbra zero-day exploit as well as measures taken to address the issue.

Exploitation Dynamics

According to findings from the Google Threat Analysis Group (TAG), four distinct groups seized upon this vulnerability in live attacks. Astonishingly, most of these exploits occurred after Zimbra had already addressed the issue and made the fix public on GitHub. This showcases the urgency for organizations to promptly apply
Zimbra security updates.

Zimbra Zero-Day Exploit Overview

CVE-2023-37580 flaw allowed attackers to execute malicious scripts on victims’ web browsers by tricking them into clicking on a specially crafted URL. This, in turn, initiated a cross-site scripting request to Zimbra, reflecting the attack back to the user. The simplicity of this method underscores the critical need for vigilance against email security vulnerabilities.

Campaign Timelines and Targets

The TAG report highlighted multiple campaign waves commencing on
June 29, 2023, two weeks prior to Zimbra’s advisory. Of the four cybersecurity threats in Zimbra, three campaigns occurred before the patch release, with the fourth emerging a month after the fixes were made public.

  1. Government Targeting in Greece

    The first campaign, an
    email system vulnerability, specifically targeted a government organization in Greece. Exploit URLs were delivered through emails, triggering an email-stealing malware upon click. This operation, identified as TEMP_HERETIC by Volexity, utilized a Zimbra zero-day flaw alongside other tactics.
  2. Winter Vivern’s Activity

    The second actor, known as Winter Vivern, focused on government organizations in Moldova and Tunisia shortly after the vulnerability patch was available on GitHub. Winter Vivern has previously been
    associated with exploiting Roundcube vulnerabilities and Zimbra Collaboration, amplifying the severity of their activities.
  3. Phishing in Vietnam

    TAG detected a third, unidentified group leveraging the vulnerability before the patch release. This group aimed to phish credentials from a government organization in Vietnam, utilizing an exploit URL that directed users to a phishing page. Stolen credentials were then posted to a compromised official government domain.
  4. Targeting Pakistan

    The fourth campaign targeted a government organization in Pakistan on August 25, resulting in the exfiltration of Zimbra authentication tokens to a remote domain named “ntcpk[.]org.” This underscores the global reach and impact of such exploits and highlights the importance of enhanced cybersecurity measures that help mitigate any
    email software security risks.

The Urgency of Timely Fixes

The discoveries by TAG emphasize the critical importance of organizations promptly applying fixes to their mail servers in
protecting against zero-day flaws. The fact that three out of the four campaigns occurred after the vulnerability became public underscores the need for swift action. The report also sheds light on a concerning trend where threat actors are actively monitoring open-source repositories to exploit vulnerabilities opportunistically.

Zimbra Email Security Measures and Future Preparedness

Google recommends a thorough audit of mail servers, especially focusing on XSS vulnerabilities, given the recurring pattern of exploitation in this regard. The evolving landscape of
zero-day exploits in email softwares calls for organizations to not only stay vigilant but also proactively assess and fortify their security measures to mitigate potential risks.



The recent exploitation of the Zimbra Collaboration email software vulnerability serves as a stark reminder of the persistent and evolving nature of cyber threats. As threat actors become increasingly sophisticated, it is imperative for organizations to adopt a proactive approach to cybersecurity. Prioritizing robust cybersecurity measures, such as timely patching and vigilant monitoring, is paramount for organizations to effectively mitigate the risks associated with zero-day exploitation prevention, ensuring the resilience of critical communication systems against emerging vulnerabilities. 

The sources for this piece include articles in The Hacker News and Vulners

Zimbra Zero-Day Exploit Unveiled
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Zimbra Zero-Day Exploit Unveiled
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