A now patched security vulnerability in Zimbra Collaboration was exploited as a zero-day earlier this year in cyber attacks targeting the Brazilian military.
Tracked as CVE-2025-27915 (CVSS score: 5.4), the vulnerability is a stored cross-site scripting (XSS) vulnerability in the Classic Web Client that arises as a result of insufficient sanitization of HTML content in ICS calendar files, resulting in arbitrary code execution.
“When a user views an e-mail message containing a malicious ICS entry, its embedded JavaScript executes via an ontoggle event inside a tag,” according to a description of the flaw in the NIST National Vulnerability Database (NVD).
“This allows an attacker to run arbitrary JavaScript within the victim’s session, potentially leading to unauthorized actions such as setting e-mail filters to redirect messages to an attacker-controlled address. As a result, an attacker can perform unauthorized actions on the victim’s account, including e-mail redirection and data exfiltration.”
The vulnerability was addressed by Zimbra as part of versions 9.0.0 Patch 44, 10.0.13, and 10.1.5 released on January 27, 2025. The advisory, however, makes no mention of it having been exploited in real-world attacks.
However, according to a report published by StrikeReady Labs on September 30, 2025, the observed in-the-wild activity involved unknown threat actors spoofing the Libyan Navy’s Office of Protocol to target the Brazilian military using malicious ICS files that exploited the flaw.
The ICS file contained a JavaScript code that’s designed to act as a comprehensive data stealer to siphon credentials, emails, contacts, and shared folders to an external server (“ffrk[.]net”). It also searches for emails in a specific folder, and adds malicious Zimbra email filter rules with the name “Correo” to forward the messages to spam_to_junk@proton.me.
As a way to avoid detection, the script is fashioned such that it hides certain user interface elements and detonates only if more than three days have passed since the last time it was executed.
It’s currently not clear who is behind the attack, but earlier this year, ESET revealed that the Russian threat actor known as APT28 had exploited XSS vulnerabilities in various webmail solutions from Roundcube, Horde, MDaemon, and Zimbra to obtain unauthorized access.
A similar modus operandi has also been adopted by other hacking groups like Winter Vivern and UNC1151 (aka Ghostwriter) to facilitate credential theft.
