The previously unknown vulnerability resulted from a critical cross-site scripting error in Roundcube, a server application used by more than 1,000 webmail services and millions of their end users. Members of a pro-Russia and Belarus hacking group tracked as Winter Vivern used the XSS bug to inject JavaScript into the Roundcube server application. The injection was triggered simply by viewing a malicious email, which caused the server to send emails from selected targets to a server controlled by the threat actor. //

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.