ESET Research recommends updating Roundcube Webmail to the latest available version as soon as possible
25 Oct 2023
5 min. read
ESET Research has been closely tracking the cyberespionage operations of Winter Vivern for more than a year and, during our routine monitoring, we found that the group began exploiting a zero-day XSS vulnerability in the Roundcube Webmail server on October 11th, 2023. This is a different vulnerability than CVE-2020-35730, which was also exploited by the group according to our research.
According to ESET telemetry data, the campaign targeted Roundcube Webmail servers belonging to governmental entities and a think tank, all in Europe.
Vulnerability disclosure timeline:
2023-10-12: ESET Research reported the vulnerability to the Roundcube team.
2023-10-14: The Roundcube team responded and acknowledged the vulnerability.
2023-10-14: The Roundcube team patched the vulnerability.
2023-10-16: The Roundcube team released security updates to address the vulnerability (1.6.4, 1.5.5, and 1.4.15).
2023-10-18: ESET CNA issues a CVE for the vulnerability (CVE-2023-5631).
2023-10-25: ESET Research blogpost published.
We would like to thank the Roundcube developers for their quick reply and for patching the vulnerability in such a short time frame.
Winter Vivern profile
Winter Vivern is a cyberespionage group first revealed by DomainTools in 2021. It is thought to have been active since at least 2020 and it targets governments in Europe and Central Asia. To compromise its targets, the group uses malicious documents, phishing websites, and a custom PowerShell backdoor (see the articles from the State Cyber Protection Centre of Ukraine and from SentinelLabs). We believe with low confidence that Winter Vivern is linked to MoustachedBouncer, a sophisticated Belarus-aligned group that we first published about in August, 2023.
Winter Vivern has been targeting Zimbra and Roundcube email servers belonging to governmental entities since at least 2022 – see this article from Proofpoint. In particular, we observed that the group exploited CVE-2020-35730, another XSS vulnerability in Roundcube, in August and September 2023. Note that Sednit (also known as APT28) is exploiting this old XSS vulnerability in Roundcube as well, sometimes against the same targets.
Exploitation of the XSS vulnerability, assigned CVE-2023-5631, can be done remotely by sending a specially crafted email message. In this Winter Vivern campaign, the emails were sent from team.managment@outlook[.]com and had the subject Get started in your Outlook, as shown in Figure 1.
Figure 1. Malicious email message
At first sight, the email doesn’t seem malicious – but if we examine the HTML source code, shown in Figure 2, we can see an SVG tag at the end, which contains a base64-encoded payload.
Figure 2. Email message with a malicious SVG tag
Once we decode the base64-encoded value in the href attribute of the use tag, we have:
<svg id=”x” xmlns=”http://www.w3.org/2000/svg”> <image href=”x” onerror=”eval(atob(‘<base64-encoded payload>’))” /></svg>
Winter Vivern has stepped up its operations by using a zero-day vulnerability in Roundcube. Previously, it was using known vulnerabilities in Roundcube and Zimbra, for which proofs of concept are available online.
Despite the low sophistication of the group’s toolset, it is a threat to governments in Europe because of its persistence, very regular running of phishing campaigns, and because a significant number of internet-facing applications are not regularly updated although they are known to contain vulnerabilities.
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M247 Europe SRL
Winter Vivern C&C server
This table was built using version 13 of the MITRE ATT&CK framework.
Acquire Infrastructure: Domains
Winter Vivern operators bought a domain at Registrar.eu.
Acquire Infrastructure: Server
Winter Vivern operators rented a server at M247.
Develop Capabilities: Exploits
Winter Vivern operators probably developed an exploit for Roundcube.
Exploit Public-Facing Application
Winter Vivern sent an email exploiting CVE‑2023-5631 in Roundcube.
The vulnerability is triggered via a phishing email, which should be opened in the Roundcube webmail by the victim.
Exploitation for Client Execution
Account Discovery: Email Account
Email Collection: Remote Email Collection
Command and Control
Application Layer Protocol: Web Protocols
C&C communications use HTTPs.
Exfiltration Over C2 Channel
Exfiltration is done via HTTPs and to the same C&C server.