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Winter Vivern’s Roundcube Zero-Day Exploits

Wajahat Raja

November 7, 2023 - TuxCare expert team

In a recent cybersecurity development, an elusive threat actor named Winter Vivern aimed its sights at the popular Roundcube webmail software, successfully exploiting a zero-day vulnerability on October 11th. This breach allowed unauthorized access to sensitive email messages, causing alarm in the online security community. This blog delves into the details of the Roundcube zero-day exploits, offering insights into Winter Vivern’s activities, the new vulnerability (CVE-2023-5631), their attack sequence, as well as the persistent threat they pose to European governments.


Unveiling Roundcube Zero-day Exploits


Latest news on Roundcube exploits has revealed that Winter Vivern, identified as a Russian hacking group, has been active since 2020. Their nefarious operations primarily target governments in Central Asia and Europe. Known for launching phishing campaigns, employing customer PowerShell backdoors, and utilizing various malicious codes and documents, Winter Vivern has become a formidable adversary in the cyber realm. 

Interestingly, reports suggest a connection between Winter Vivern and MoustachedBouncer, a group based in Belarus. Recent months have witnessed an escalation in their attacks on Ukraine, Poland, and multiple government entities across Europe and India.


Winter Vivern’s Past Encounters

Zero-day exploits in Roundcube
aren’t Winter Vivern’s first interaction with the webmail software. They previously exploited a different flaw, CVE-2020-35730, making them the second nation-state group, after APT28, to target this open-source platform.

The New Vulnerability: CVE-2023-5631

The latest
Roundcube security issues hinged on a specific vulnerability known as CVE-2023-5631, which has a CVSS score of 5.4. This flaw allowed for stored cross-site scripting, enabling remote attackers to inject arbitrary JavaScript code into the software. Fortunately, a patch was swiftly released on October 14, 2023, to address this issue and enhance Roundcube’s security.


Winter Vivern’s Attack Sequence

The attack orchestrated by Winter Vivern typically starts with a phishing message containing a Base64-encoded payload hidden within the HTML source code. Upon decoding, this payload launches a JavaScript injection from a remote server, exploiting the XSS vulnerability. ESET, a prominent cybersecurity research group, explained,
“By sending a specially crafted email message, attackers are able to load arbitrary JavaScript code in the context of the Roundcube user’s browser window. No manual intervention other than viewing the message in a web browser is required” The second-stage JavaScript, known as checkupdate.js, acts as a loader, facilitating the execution of a final JavaScript payload. This payload allows the threat actor to extract email messages to a command-and-control (C2) server, thereby compromising sensitive data.


Roundcube Email Vulnerabilities: A Persistent Threat

Despite Winter Vivern’s use of relatively unsophisticated tools, they remain a significant threat to European governments. Their persistence in conducting phishing campaigns targeting vulnerable internet-facing applications, often left unpatched, creates ample opportunities for exploitation. As the security landscape continues to evolve, vigilance and timely updates are essential to mitigate the risks posed by threat actors like Winter Vivern.



The Winter Vivern attack on Roundcube’s zero-day flaw serves as a stark reminder of the ever-present cybersecurity challenges that organizations face today. It underscores the importance of
proactive security measures, timely patching, and continuous vigilance in preventing Roundcube zero-day attacks. As the digital world advances, so do the capabilities of cybercriminals, making it crucial for businesses and governments to stay one step ahead in the ongoing battle for online security.

The sources for this piece include articles in The Hacker News and Tech Times


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Winter Vivern’s Roundcube Zero-Day Exploits
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