As part of spear-phishing campaigns that began in December 2021, a threat actor, most likely of Chinese origin, is proactively trying to attack a zero-day vulnerability in the Zimbra open-source email infrastructure. 

In a technical report published last week, cybersecurity firm Volexity described the espionage operation, codenamed "EmailThief," stating that successful exploitation of the cross-site scripting (XSS) vulnerability could lead to the execution of arbitrary JavaScript code in the context of the user's Zimbra session. 

The incursions, which commenced on December 14, 2021, were linked to a previously unknown hacker gang that Volexity is investigating under the moniker TEMP HERETIC, with the attacks focused on European government and media organizations. The zero-day vulnerability affects Zimbra's most recent open-source edition, version 8.8.15. 

The assaults are said to have been carried out in two stages, with the first stage targeted at reconnaissance and the distribution of emails to see if a target had received and opened the messages. Multiple waves of email messages were sent out after that to lure users into clicking on a fraudulent link. The attacker used 74 different Outlook.com email identities to send the messages out over two weeks, with the initial recon emails having generic subject lines ranging from invitations to charity auctions and refunds for airline tickets. 

Steven Adair and Thomas Lancaster noted, "For the attack to be successful, the target would have to visit the attacker's link while logged into the Zimbra webmail client from a web browser. The link itself, however, could be launched from an application to include a thick client, such as Thunderbird or Outlook." 

If exploited, the unpatched vulnerability might be used to exfiltrate cookies, providing constant access to a mailbox, sending phishing messages from the hijacked email account to spread the infection, and even facilitating the installation of new malware. 

The researchers stated, "None of the infrastructure identified […] exactly matches infrastructure used by previously classified threat groups."  

"However, based on the targeted organization and specific individuals of the targeted organization, and given the stolen data would have no financial value, it is likely the attacks were undertaken by a Chinese APT actor." 

Further the company recommended, "Users of Zimbra should consider upgrading to version 9.0.0, as there is currently no secure version of 8.8.15."