CVE-2024-50599: Zimbra Collaboration Suite XSS Vulnerability

CVE-2024-50599 Overview

CVE-2024-50599 is a reflected Cross-Site Scripting (XSS) vulnerability affecting Synacor Zimbra Collaboration Suite (ZCS) 8.8.15. The flaw resides in one of the webmail calendar endpoints and stems from improper handling of user-supplied input. An attacker can craft a malicious URL that reflects attacker-controlled script content back in the HTML response. When a victim clicks the link while authenticated to the webmail interface, the script executes in the browser under the Zimbra origin.

Critical Impact
Successful exploitation allows attackers to execute arbitrary JavaScript in a victim's authenticated webmail session, enabling session theft, mailbox content exfiltration, and phishing against Zimbra users [CWE-79].

Affected Products

Synacor Zimbra Collaboration Suite 8.8.15 (base release)
Zimbra Collaboration Suite 8.8.15 patch levels P1 through P45
Fixed in Zimbra Collaboration Suite 8.8.15 P46

Discovery Timeline

2024-11-07 - CVE-2024-50599 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-50599

Vulnerability Analysis

The vulnerability is a classic reflected XSS flaw in a webmail calendar endpoint of Zimbra Collaboration Suite 8.8.15. The affected endpoint accepts user-supplied parameters from the HTTP request and embeds them directly into the server's HTML response without proper output encoding or input sanitization. Because the reflection occurs in an HTML context, attacker-supplied markup, including <script> tags or event-handler attributes, is parsed and executed by the victim's browser.

Exploitation requires user interaction, typically clicking a crafted link. The impact extends beyond the vulnerable component because scripts run in the Zimbra webmail origin and can access session cookies, perform actions through the Zimbra SOAP API, read mailbox contents, and pivot to additional web resources accessible to the user.

Root Cause

The root cause is missing or insufficient output encoding of request parameters that are reflected into the calendar endpoint's HTML response. Zimbra's templating logic did not neutralize HTML metacharacters such as <, >, and ", allowing attacker-controlled strings to break out of their intended data context and become executable script.

Attack Vector

The attack vector is network-based and requires victim interaction. An attacker constructs a URL to the vulnerable calendar endpoint containing a malicious payload in a reflected parameter. The attacker then delivers the link through phishing email, instant message, or a referring web page. When a Zimbra user who is authenticated to webmail follows the link, the server reflects the payload into the response and the browser executes it under the Zimbra domain, granting the attacker access to session state and mailbox data.

No verified public proof-of-concept has been published. See the Zimbra 8.8.15 Security Fixes advisory for vendor technical details.

Detection Methods for CVE-2024-50599

Indicators of Compromise

Zimbra webmail access log entries containing URL parameters with HTML or JavaScript metacharacters such as <script, onerror=, onload=, javascript:, or URL-encoded equivalents (%3Cscript, %22).
Unusually long or heavily URL-encoded query strings targeting calendar endpoints under /h/, /zimbra/, or /service/ paths.
HTTP Referer headers pointing to external domains preceding requests to Zimbra calendar URLs.
Unexpected outbound requests from Zimbra user browsers to attacker-controlled domains shortly after calendar endpoint access.

Detection Strategies

Deploy web application firewall (WAF) rules that inspect query parameters sent to Zimbra webmail endpoints for script tags, event handlers, and encoded variants.
Correlate Zimbra access_log and mailbox.log entries with user mailbox activity to spot session anomalies following suspicious calendar URL access.
Hunt for phishing emails delivered to Zimbra users containing links to internal Zimbra hosts with unusually structured query strings.

Monitoring Recommendations

Forward Zimbra webmail HTTP access logs to a centralized logging platform and alert on patterns matching reflected XSS signatures.
Monitor authenticated session token reuse from multiple IP addresses or user agents, which can indicate cookie theft following successful XSS.
Track browser-side telemetry via Content Security Policy (CSP) violation reports if a reporting endpoint is configured.

How to Mitigate CVE-2024-50599

Immediate Actions Required

Upgrade Zimbra Collaboration Suite 8.8.15 to patch level P46 or later, which contains the vendor fix for CVE-2024-50599.
Inventory all Zimbra 8.8.15 instances, including internet-facing webmail servers, and prioritize patching those exposed to untrusted networks.
Reset active user sessions after patching to invalidate any tokens that may have been captured via prior exploitation attempts.

Patch Information

Synacor addressed CVE-2024-50599 in Zimbra Collaboration Suite 8.8.15 Patch 46. Full patch details are available in the Zimbra 8.8.15 P46 Security Fixes advisory and the general Zimbra Security Advisories page.

Workarounds

Deploy WAF rules in front of Zimbra webmail that block requests containing HTML or JavaScript metacharacters in calendar endpoint parameters until patching is complete.
Enforce a strict Content Security Policy (CSP) header on Zimbra responses to restrict inline script execution and limit the impact of reflected XSS.
Educate Zimbra users to avoid clicking Zimbra webmail links received from untrusted senders and to validate that URLs point to legitimate internal hosts.
Restrict external access to the Zimbra webmail interface via VPN or IP allowlisting where business requirements permit.

bash

# Example: upgrade a Zimbra 8.8.15 server to Patch 46 (run as the zimbra user context per vendor guidance)
sudo su - zimbra -c "zmcontrol stop"
# Follow the official vendor upgrade procedure documented at:
# https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P46
sudo su - zimbra -c "zmcontrol start"
sudo su - zimbra -c "zmcontrol -v"