CVE-2025-27915

CVE-2025-27915 is a stored cross-site scripting (XSS) flaw in the Classic Web Client of Synacor Zimbra Collaboration Suite (ZCS) 9.0, 10.0, and 10.1, caused by insufficient sanitization of HTML content in ICS calendar files. It is triggered when a user views an email message containing a malicious ICS entry, at which point embedded JavaScript executes in the victim's session via an ontoggle event handler inside a tag. A successful attacker can run arbitrary JavaScript in the victim's session and perform unauthorized actions on the account, notably creating email filters that silently redirect messages to attacker-controlled addresses, enabling data exfiltration. Any organization running the affected ZCS versions whose users read mail through the Classic Web Client is exposed, since delivery of a single crafted email can compromise a session. Exploitation is confirmed in the wild: the flaw was added to CISA KEV on 2025-10-07, and public reporting describes active zero-day attacks, including targeting of the Brazilian military via malicious ICS files.

What to do: Upgrade affected ZCS 9.0, 10.0, and 10.1 deployments to the latest patched builds per Zimbra's security advisory and apply any vendor-recommended mitigations (federal agencies must act per CISA KEV/BOD 22-01 requirements). Hunt for suspicious ICS-based emails and calendar entries, and review user mail filter rules for unauthorized forwarding or redirection to attacker-controlled addresses. Consider restricting or disabling Classic Web Client access until systems are patched.