The flaw, identified as CVE-2025-27915, affected ZCS versions 9.0, 10.0, and 10.1. It stemmed from inadequate sanitization of HTML content in calendar files, allowing cybercriminals to inject arbitrary JavaScript code. Once executed, the code could redirect emails, steal credentials, and access confidential user information. Zimbra patched the issue on January 27 through updates (ZCS 9.0.0 P44, 10.0.13, and 10.1.5), but at that time, the company did not confirm any active attacks.
StrikeReady, a cybersecurity firm specializing in AI-based threat management, detected the campaign while monitoring unusually large .ICS files containing embedded JavaScript. Their investigation revealed that the attacks began in early January, predating the official patch release. In one notable instance, the attackers impersonated the Libyan Navy’s Office of Protocol and sent a malicious email targeting a Brazilian military organization. The attached .ICS file included Base64-obfuscated JavaScript designed to compromise Zimbra Webmail and extract sensitive data.
Analysis of the payload showed that it was programmed to operate stealthily and execute in asynchronous mode. It created hidden fields to capture usernames and passwords, tracked user actions, and automatically logged out inactive users to trigger data theft. The script exploited Zimbra’s SOAP API to search through emails and retrieve messages, which were then sent to the attacker every four hours. It also added a mail filter named “Correo” to forward communications to a ProtonMail address, gathered contacts and distribution lists, and even hid user interface elements to avoid detection. The malware delayed its execution by 60 seconds and only reactivated every three days to reduce suspicion.
StrikeReady could not conclusively link the attack to any known hacking group but noted that similar tactics have been associated with a small number of advanced threat actors, including those linked to Russia and the Belarusian state-sponsored group UNC1151. The firm shared technical indicators and a deobfuscated version of the malicious code to aid other security teams in detection efforts.
Zimbra later confirmed that while the exploit had been used, the scope of the attacks appeared limited. The company urged all users to apply the latest patches, review existing mail filters for unauthorized changes, inspect message stores for Base64-encoded .ICS entries, and monitor network activity for irregular connections. The incident highlights the growing sophistication of targeted attacks and the importance of timely patching and vigilant monitoring to prevent zero-day exploitation.
