CVE-2022-24682: Synacor Zimbra Collaborate Suite (ZCS) Cross-Site Scripting Vulnerability

CVE-2022-24682 is a cross-site scripting vulnerability in the Calendar feature of Synacor Zimbra Collaboration Suite (ZCS). It allows an attacker to execute arbitrary code in the context of a victim’s session. Because Zimbra is widely used for enterprise email and calendaring, successful abuse can lead to account takeover, data theft, and further lateral movement. CISA notes known ransomware use of this vulnerability, so organizations running ZCS should treat it as a priority.

Public detail is limited to the Calendar component and the stated weakness classes; exact affected builds, attack prerequisites, and scoring must be confirmed against the vendor advisory.

How it works

The flaw is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation) and CWE-116 (Improper Encoding or Escaping of Output). In essence, user-controlled or attacker-supplied content that reaches the Calendar feature is not properly sanitized or encoded before being rendered in a victim’s browser.

An attacker who can introduce malicious script into Calendar data—through an invitation, shared event, or other Calendar input path—can cause that script to execute when a legitimate user views the item. Because the script runs with the privileges of the authenticated Zimbra session, the attacker can perform actions as the victim, steal session tokens, or pivot to other parts of the collaboration suite. No further exploit mechanics are provided in the public summary; defenders should treat any untrusted Calendar content as a potential vector until the vendor patch is applied.

Am I affected? How to find it in your systems

Zimbra Collaboration Suite typically runs as an on-premises or self-hosted mail and calendaring platform, often exposed to the internet for webmail and mobile access. Inventory every server or virtual machine that hosts ZCS, including any secondary or lab instances.

• Identify running Zimbra packages or containers and record the installed version string.
• Compare that version against the fixed releases listed in the vendor advisory for CVE-2022-24682; do not rely on informal version ranges.
• Check whether the Calendar service is enabled and reachable by internal or external users.
• Review web-server and application logs for anomalous Calendar requests, unexpected script payloads in event fields, or sudden spikes in Calendar-related errors around the time of suspected activity.
• If you forward Zimbra logs to a SIEM, search for indicators of XSS delivery (encoded script tags, unusual calendar object sizes) and for post-exploitation signs such as new admin accounts or bulk mailbox access.

Any system still running an unpatched build that includes the vulnerable Calendar code should be considered exposed until verified otherwise.

How to remediate

The primary remediation is to apply the updates supplied by Synacor/Zimbra exactly as described in the vendor advisory. CISA’s required action is simply “Apply updates per vendor instructions.”

• Schedule and test the vendor-supplied patch or upgrade package in a non-production environment first, then roll it out to all production ZCS nodes.
• After patching, restart the affected services and confirm the new version string is reported by the administration console or package manager.
• As hardening for this class of flaw, ensure that any custom themes, zimlets, or third-party Calendar integrations also sanitize output; remove unused Calendar sharing features if they are not required.
• Enforce least-privilege mailbox and admin roles so that a compromised user session has limited blast radius.
• Re-enable or tighten Content-Security-Policy headers and other browser-side controls that Zimbra supports, once the patch is confirmed stable.

If you can't patch immediately

When immediate patching is not feasible, apply compensating controls to reduce the attack surface until the update can be installed.

• Segment Zimbra servers so that only necessary clients and mail relays can reach the Calendar and webmail ports; block direct internet access if business needs allow.
• Place a web application firewall (WAF) or reverse-proxy filter in front of the Zimbra web interface and enable rules that detect and block common XSS patterns in Calendar-related parameters and request bodies. Treat this as virtual patching only—validate that legitimate Calendar traffic is not broken.
• Temporarily disable or restrict Calendar sharing, external invitations, and any features that accept rich HTML input if operational impact is acceptable.
• Increase monitoring: alert on new or unusual Calendar objects containing script-like content, on authentication anomalies, and on administrative actions performed outside normal change windows.
• Force re-authentication and review active sessions for high-value accounts until the patch is deployed.

If your data may have been exposed

Actively exploited vulnerabilities, especially those with documented ransomware use, frequently precede broader compromise. If you have evidence of exploitation or cannot rule it out, assume credentials and mailbox data may have been accessed. Rotate passwords and app tokens for affected accounts, review mail-forwarding rules and delegation settings, and examine backups for integrity before restoration. You can also run a free exposure scan of your email addresses against known breach data to determine whether those addresses already appear in public breach corpora.