Russian State-Supported Group LAUNDRY BEAR Exploits Zero-Day CVE-2025-66376 in Zimbra Collaboration Suite
Synacor’s Zimbra Collaboration Suite (ZCS) was compromised through a previously unknown vulnerability that Russian state-supported hackers exploited in zero-day attacks.
The flaw, now tracked as CVE-2025-66376, resides in the stylesheet processing component of the webmail interface and constitutes a stored cross-site scripting issue.
Attackers sent crafted HTML-formatted messages; simply viewing the email in the browser triggered JavaScript execution, enabling theft of historical mailbox contents without any additional user clicks.
On 23 July 2026, authorities from the United States, Europe, Australia and other regions published a coordinated advisory signed by twenty-seven organizations, including the NSA, FBI and CISA.
Researchers attribute the operation to the group tracked as LAUNDRY BEAR, also known as Void Blizzard, CL-STA-1114 and TA488 (formerly UNK_PitStop).
MITRE assigned the vulnerability a CVSS v3.1 base score of 7.2, classifying it as High severity.
The campaign targeted Western government agencies and enterprises with the objective of collecting sensitive information.
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