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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.

No manual interaction required

“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.”