https://arstechnica.com/security/2023/10/pro-russia-hackers-target-inboxes-with-0-day-in-webmail-app-used-by-millions/

A relentless team of pro-Russia hackers has been exploiting a zero-day vulnerability in widely used webmail software in attacks targeting governmental entities and a think tank, all in Europe, researchers from security firm ESET said on Wednesday.

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.