Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability (CVE-2025-27915)
Severity: medium · Patch status: unknown · Since 2025-10-07 · CVE-2025-27915
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 added CVE-2025-27915 to its Known Exploited Vulnerabilities catalog on Oct 7, 2025, which means attackers are exploiting it in the wild.
What to do
- Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
- US federal agencies must remediate by Oct 28, 2025 (BOD 22-01); treat that as the outer limit.
Sources
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