“In summary, 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,” ESET researcher Matthieu Faou wrote. “No manual interaction other than viewing the message in a web browser is required.”

The attacks began on October 11, and ESET detected them a day later. ESET reported the zero-day vulnerability to Roundcube developers on the same day, and they issued a patch on October 14. The vulnerability is tracked as CVE-2023-5631 and affects Roundcube versions 1.6.x before 1.6.4, 1.5.x before 1.5.5, and 1.4.x before 1.4.15.