CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability

CVE-2025-27915 is a cross-site scripting vulnerability in Synacor Zimbra Collaboration Suite (ZCS). It affects the Classic Web Client and stems from insufficient sanitization of HTML content inside ICS files. When a user opens an email that contains a crafted ICS entry, attacker-controlled JavaScript can run in the victim’s authenticated session. That access can let an attacker perform unauthorized account actions such as creating mail filters that redirect messages, which in turn enables data exfiltration. Organizations that run Zimbra for email and calendaring should treat this as a high-priority issue for any users of the Classic Web Client.

How it works

The weakness is classified as CWE-79 (cross-site scripting). According to the CISA summary, the Classic Web Client fails to properly sanitize HTML content carried in ICS calendar attachments. An attacker embeds malicious markup that includes an ontoggle event handler inside a tag. When the recipient views the message, the browser executes the embedded JavaScript in the context of the logged-in Zimbra session. Because the script runs with the victim’s privileges, it can issue requests that the user is authorized to make—most notably the creation or modification of email filters that silently forward mail to an attacker-controlled address. The same capability can be used for other unauthorized actions or for harvesting data visible to the session. Exact payload construction and any additional vectors must be confirmed against the vendor advisory; no further exploit mechanics are provided in the public summary.

Am I affected? How to find it in your systems

Zimbra Collaboration Suite is commonly deployed as an on-premises or hosted email, calendar, and collaboration platform. The vulnerability is specific to the Classic Web Client interface. Inventory steps include:

• Identify every Zimbra server or cluster in the environment (mailstores, proxies, and related nodes).
• Determine whether users still access the Classic Web Client rather than the modern client; check client-selection policies, bookmarks, and authentication logs.
• Review installed Zimbra package versions and configuration against the vendor advisory to confirm whether the affected code path is present. Specific version ranges are not listed here and must be verified with the vendor.
• Search mail logs and web-access logs for messages that contain ICS attachments and for subsequent filter-creation or filter-modification events that occur shortly after a user opens such a message.
• Look for anomalous filter rules that forward mail to external addresses, especially rules created outside normal administrative workflows.

Telemetry that shows unexpected JavaScript errors or unusual session activity after ICS rendering can also serve as an indicator, but absence of such signals does not prove the environment is clean.

How to remediate

Apply the vendor-supplied update or mitigation instructions for CVE-2025-27915 as soon as they are available. CISA’s required action is to follow the vendor’s guidance, apply applicable BOD 22-01 controls for any cloud-hosted instances, or discontinue use of the product if mitigations cannot be obtained. After patching:

• Force a full restart of Zimbra services so the updated client code is loaded.
• Re-validate that the Classic Web Client no longer renders unsanitized ICS content; retest with a controlled sample if possible.
• Audit existing mail filters for any unauthorized redirect rules and remove them.
• Review and tighten HTML/ICS sanitization settings if the product exposes such controls.
• Confirm that only necessary users retain access to the Classic Web Client; migrate remaining users to the modern client where feasible.

Document the change window and retain evidence of the applied fix for compliance and incident-response purposes.

If you can't patch immediately

Until the vendor update can be installed, reduce exposure with the following compensating controls:

• Segment Zimbra servers so that only authenticated internal networks or VPN users can reach the web client ports.
• If a web application firewall is in place, enable or create rules that inspect and block suspicious ICS or HTML content containing event-handler attributes such as ontoggle; treat these as virtual patches and tune them carefully to avoid false positives.
• Disable or restrict the Classic Web Client for all users who do not strictly require it; force the modern client via configuration or proxy rules.
• Block or quarantine inbound messages that contain ICS attachments at the mail gateway until the patch is applied, or strip HTML from those attachments if the gateway supports content rewriting.
• Increase monitoring of filter-creation APIs, unexpected outbound mail-forwarding rules, and session anomalies; alert on any new filter that targets external domains.
• Limit administrative and high-privilege accounts from using the Classic Web Client.

These measures lower risk but do not eliminate the underlying flaw; schedule the official patch without delay.

If your data may have been exposed

Actively exploited vulnerabilities of this type can lead to account takeover and subsequent data breaches through mail redirection or session abuse. If you suspect compromise, immediately revoke active sessions, reset credentials for affected users, and examine mail filters and forwarding rules for unauthorized changes. You can also run a free exposure scan of your email address against known breach data to determine whether credentials or personal information have already appeared in public breach corpora.