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.
“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.
“This actor has been tenacious in its targeting of American and European officials as well as military and diplomatic personnel in Europe,” a threat researcher from security firm Proofpoint said in March when disclosing the attacks exploiting the Zimbra vulnerability.
Since the tag contained an intentional error, the malicious command was invoked, and the XSS bug ensured that Roundcube executed the resulting JavaScript.
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