CVE-2024-27443 Overview
CVE-2024-27443 is a Cross-Site Scripting (XSS) vulnerability in Zimbra Collaboration Suite (ZCS) versions 9.0 and 10.0. The flaw resides in the CalendarInvite feature of the classic webmail user interface. Improper input validation in the handling of the calendar header allows an attacker to embed a malicious JavaScript payload inside an email message. When a victim opens the message in the Zimbra webmail classic interface, the payload executes in the context of the victim's session. The vulnerability is tracked under CWE-79 and is listed in the CISA Known Exploited Vulnerabilities Catalog.
Critical Impact
Attackers can execute arbitrary JavaScript in a victim's authenticated webmail session, enabling session theft, mailbox exfiltration, and account takeover. Active exploitation has been documented by ESET Research in Operation RoundPress.
Affected Products
- Zimbra Collaboration Suite 9.0.0 through patch level P38
- Zimbra Collaboration Suite 10.0.x prior to 10.0.7
- Zimbra webmail classic user interface (CalendarInvite feature)
Discovery Timeline
- 2024-08-12 - CVE-2024-27443 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2024-27443
Vulnerability Analysis
The vulnerability is a stored/reflected Cross-Site Scripting (XSS) issue rooted in improper input validation of calendar header content. When Zimbra's classic webmail parses an inbound email containing a calendar invitation, it renders header fields without adequately sanitizing embedded HTML or JavaScript. An attacker sends an email with a crafted calendar header containing an XSS payload. Once the victim views the message in the classic UI, the browser interprets the payload as executable script within the Zimbra origin.
Execution in the webmail context grants the attacker access to session cookies, DOM contents, mailbox data, and any Zimbra API accessible to the authenticated user. The attack requires no privileges but does require user interaction (opening the crafted message).
Root Cause
The root cause is missing or insufficient output encoding when rendering calendar header values in the classic webmail interface. Zimbra treats attacker-controlled header data as trusted markup rather than as text. This class of defect is cataloged as CWE-79: Improper Neutralization of Input During Web Page Generation.
Attack Vector
Exploitation is remote and unauthenticated on the attacker side. An adversary emails a crafted iCalendar-style message to any Zimbra user. Reported activity in Operation RoundPress chained similar Zimbra XSS flaws to inject JavaScript that exfiltrated mailbox contents, credentials, and two-factor authentication data to attacker-controlled infrastructure. No exploitation code snippets are reproduced here; refer to the linked vendor and researcher advisories for technical detail.
Detection Methods for CVE-2024-27443
Indicators of Compromise
- Inbound emails containing calendar invitations with unusual <script> tags, event handlers (onerror, onload), or encoded JavaScript inside header fields
- Outbound HTTP or HTTPS requests from the Zimbra web client to unfamiliar external domains shortly after a user opens an email
- Unexpected creation of mail filters, forwarding rules, or delegate access on user accounts
- Anomalous access to /service/soap or /h/ endpoints from a single session in rapid succession
Detection Strategies
- Inspect message store and mail transfer logs for calendar (text/calendar) parts containing HTML tags or JavaScript keywords in header lines
- Correlate webmail access logs with subsequent outbound network connections to non-corporate domains from user browsers
- Hunt for browser process activity on endpoints that renders Zimbra pages and initiates data uploads to third-party hosts
Monitoring Recommendations
- Enable and centralize Zimbra mailbox.log, audit.log, and Nginx proxy access logs
- Alert on session anomalies such as concurrent logins from geographically distant IPs
- Monitor for new zimbraPrefMailForwardingAddress values set through the SOAP API
How to Mitigate CVE-2024-27443
Immediate Actions Required
- Upgrade Zimbra Collaboration 10.0.x to version 10.0.7 or later, per the Zimbra 10.0.7 Security Fixes
- Upgrade Zimbra Collaboration 9.0.0 to patch P39 or later, per the Zimbra 9.0.0/P39 Security Fixes
- Force password resets and revoke active sessions for accounts that opened suspicious calendar invitations
- Review mail forwarding rules and delegate permissions for signs of tampering
Patch Information
Zimbra addressed the CalendarInvite input validation flaw in Zimbra Collaboration 10.0.7 and 9.0.0 Patch 39. Both releases apply proper output encoding to calendar header values rendered by the classic webmail UI. As this CVE is on the CISA KEV list, federal agencies and regulated organizations should treat patching as time-bound.
Workarounds
- Direct users to the modern Zimbra webmail UI until the classic interface is patched, since the flaw is specific to the classic client
- Deploy a mail gateway rule that strips or quarantines inbound text/calendar MIME parts containing HTML script tags or JavaScript event handlers
- Restrict outbound network access from user browsers to known-good domains to limit exfiltration if a payload executes
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
