CVE-2025-27915
Published: October 7, 2025
Official Description
Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability that exists in the Classic Web Client due to insufficient sanitization of HTML content in ICS files. When a user views an e-mail message containing a malicious ICS entry, its embedded JavaScript executes via an ontoggle event inside a tag. 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.
CISA KEV Advisory
Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability that exists in the Classic Web Client due to insufficient sanitization of HTML content in ICS files. When a user views an e-mail message containing a malicious ICS entry, its embedded JavaScript executes via an ontoggle event inside a tag. 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.
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Risk Analysis
A cross-site scripting vulnerability in Synacor Zimbra Collaboration Suite (ZCS) Classic Web Client allows attackers to run arbitrary JavaScript by embedding malicious ICS entries in emails. This can lead to unauthorized actions like email redirection or data exfiltration. The high EPSS score and presence in CISA's KEV catalog highlight its confirmed exploitation and high likelihood of future attacks.
This vulnerability is actively being exploited in the wild. The attack vector involves a user viewing a malicious email, making it a client-side exploitation scenario.
Users should apply available patches for Synacor Zimbra Collaboration Suite (ZCS) immediately. Exercise caution when opening emails from unknown sources and avoid clicking suspicious links or attachments.
Technical Analysis
CVE-2025-27915 requires local access, meaning attackers must already have a foothold on the target system.
Exploitation requires some privileges, which limits the exposure to scenarios where an attacker has already gained initial access.
CISA has added CVE-2025-27915 to the Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation in the wild. U.S. federal agencies are required to patch this within the mandated timeframe, and all organizations should treat remediation as urgent.
Exploit & PoC Resources
All References (2)
Quick Facts
Recommended Actions
- →Apply vendor patches immediately
- →Monitor CVE-2025-27915 in threat intel feeds
- →Review IDS/IPS signatures for exploitation attempts
- !CISA KEV: Federal agencies must patch per BOD 22-01 timeline
- !Active exploitation confirmed — treat as P1