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[Zero-Day] Russian Hackers Targeting European Government Email Servers

Executive Summary

A notorious Russian hacking group, Winter Vivern, leveraged a zero-day vulnerability within Roundcube Webmail servers to target European governmental bodies and think tanks.

Roundcube is a freely available, open source webmail solution that’s especially popular with small-to-midsize organisations.

The campaign primarily revolved around phishing emails impersonating the Outlook Team, which, when viewed, initiated the exploit to harvest and exfiltrate emails from the compromised servers.

The timely identification and patching of this vulnerability was a result of collaborative efforts between cybersecurity firm ESET and the developers of Roundcube.

The concerted efforts of Winter Vivern accentuate a rising trend of cyber espionage campaigns aimed at governmental organizations, displaying a sophisticated level of technical prowess and a potential shift in cyberattack strategies.

About the Exploit

The exploit hinged on a stored Cross-Site Scripting (XSS) vulnerability within Roundcube Webmail, labeled CVE-2023-5631.

This flaw was exploited through phishing emails that contained a specially crafted SVG document, which when viewed, enabled the remote injection of a malicious JavaScript code.

The injected code was capable of listing folders and emails within the compromised Roundcube account, and exfiltrating these emails to a command and control server operated by Winter Vivern.

Notably, the exploit required no manual interaction other than viewing the malicious email, exemplifying a higher level of sophistication in exploiting webmail vulnerabilities.

The discovery and reporting of the exploit by ESET Research led to the prompt release of security patches by Roundcube, addressing the identified vulnerabilities across various versions of the webmail solution.

About Winter Vivern

Winter Vivern, also known as TA473, is believed to be a Russian-backed cyber-espionage group that has been active since at least December 2020.

This group has a history of targeting governmental entities globally, with a primary focus on European nations.

The objectives of Winter Vivern closely align with the interests of the governments of Belarus and Russia, and their operations have often served the cyber-espionage goals of these nations.

Winter Vivern’s modus operandi includes the use of malicious documents, phishing campaigns, and exploiting known and zero-day vulnerabilities within webmail solutions like Roundcube and Zimbra to achieve their objectives.


The exploit orchestrated by Winter Vivern underscores the necessity for rigorous cybersecurity measures, especially within governmental and political think tank sectors. It’s imperative to stay updated with the latest security patches and promptly respond to the risks posed by zero-day vulnerabilities.

Additionally, deploying robust endpoint security solutions and educating users on the risks associated with phishing scams provide an added layer of defense against such sophisticated cyber espionage campaigns.

The continuous activities of groups like Winter Vivern highlight the evolving threat landscape and the need for enhanced cybersecurity collaboration and intelligence sharing among organisations and across borders to effectively counter such threats.



Cubic Lighthouse is a cybersecurity publication dedicated to demystifying security, making news actionable, providing deeper thinking about the fundamentals of security, and providing decision-makers and the community at large with the right information to make the right decisions. We will also feature more technical articles and provide personal/family security advice.

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