A Russian state-sponsored threat actor has exploited a high-severity XSS vulnerability in Zimbra Collaboration in attacks against Ukraine, security researchers warn.
The flaw, tracked as CVE-2025-66376 (CVSS score of 7.2) and affecting the collaboration software suite’s Classic UI, was addressed in November 2025, in Zimbra versions 10.1.13 and 10.0.18.
The stored XSS bug could allow attackers to abuse Cascading Style Sheets (CSS) @import directives in email HTML, Zimbra notes in its advisory.
Insufficient sanitization of CSS content within HTML email messages could allow attackers to reference external resources or to inject inline scripts that would be executed when the recipient opens the message in a browser.
The successful exploitation of the bug for remote code execution (RCE) allows threat actors to compromise the recipient’s email account and the Zimbra environment.
On Wednesday, the US cybersecurity agency CISA added CVE-2025-66376 to its Known Exploited Vulnerabilities (KEV) catalog, urging federal agencies to patch it within two weeks, as mandated by Binding Operational Directive (BOD) 22-01.
While CISA has not shared details on the observed attacks, Seqrite Labs reports that Russian state-sponsored threat actors have been exploiting it in attacks against Ukraine.
As part of the observed attacks, JavaScript code embedded in the email body would detonate when the message was opened, to steal information from the victims’ mailboxes.
“The script executes silently in the browser and begins harvesting credentials, session tokens, backup 2FA codes, browser-saved passwords, and the contents of the victim’s mailbox going back 90 days with all the data exfiltrated over both DNS and HTTPS,” Seqrite Labs explains.
A critical national infrastructure entity responsible for maritime and hydrographic support of shipping received the phishing email on January 22. The email came from a likely compromised account belonging to a student of Ukraine’s National Academy of Internal Affairs (NAVS).
Seqrite Labs, which named the campaign Operation GhostMail, believes that APT28, a highly sophisticated Russian APT also tracked as Forest Blizzard, Fancy Bear, GruesomeLarch, and Sofacy, is responsible for the attacks.
Users are advised to update their Zimbra deployments as soon as possible, as vulnerabilities in the collaboration software suite are often targeted by threat actors.
In January, a local file inclusion (LFI) issue in the appliance’s webmail UI was flagged as exploited in highly targeted, intelligence-driven campaigns.
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