Hackers Exploit Roundcube Webmail XSS Vulnerability to Steal Login Credentials

Unknown threat actors have been observed attempting to exploit a now-patched security flaw in the open-source Roundcube webmail software as part of a phishing attack designed to steal user credentials.

Russian cybersecurity company Positive Technologies said it discovered last month that an email was sent to an unspecified governmental organization located in one of the Commonwealth of Independent States (CIS) countries. However, it bears noting that the message was originally sent in June 2024.

"The email appeared to be a message without text, containing only an attached document," it said in an analysis published earlier this week.

"However, the email client didn't show the attachment. The body of the email contained distinctive tags with the statement eval(atob(...)), which decode and execute JavaScript code."

The attack chain, per Positive Technologies, is an attempt to exploit CVE-2024-37383 (CVSS score: 6.1), a stored cross-site scripting (XSS) vulnerability via SVG animate attributes that allows for execution of arbitrary JavaScript in the context of the victim's web browser.

Put differently, a remote attacker could load arbitrary JavaScript code and access sensitive information simply by tricking an email recipient into opening a specially-crafted message. The issue has since been resolved in versions 1.5.7 and 1.6.7 as of May 2024.

"By inserting JavaScript code as the value for 'href,' we can execute it on the Roundcube page whenever a Roundcube client opens a malicious email," Positive Technologies noted.

The JavaScript payload, in this case, saves the empty Microsoft Word attachment ("Road map.docx"), and then proceeds to obtain messages from the mail server using the ManageSieve plugin. It also displays a login form in the HTML page displayed to the user in a bid to deceive victims into providing their Roundcube credentials.

In the final stage, the captured username and password information is exfiltrated to a remote server ("libcdn[.]org") hosted on Cloudflare.

It's currently not clear who is behind the exploitation activity, although prior flaws discovered in Roundcube have been abused by multiple hacking groups such as APT28, Winter Vivern, and TAG-70.

"While Roundcube webmail may not be the most widely used email client, it remains a target for hackers due to its prevalent use by government agencies," the company said. "Attacks on this software can result in significant damage, allowing cybercriminals to steal sensitive information."