Detect Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Exploitation in Elastic Security
Detects exploitation attempts targeting CVE-2025-66376, a cross-site scripting vulnerability in Synacor Zimbra Collaboration Suite (ZCS). This KEV-listed vulnerability allows attackers to inject malicious scripts into the Zimbra web interface, potentially leading to session hijacking, credential theft, or further compromise of email infrastructure. Active exploitation has been confirmed by CISA.
MITRE ATT&CK
Elastic Detection Query
sequence by source.ip with maxspan=10m
[network where event.category == "network" and
(
url.path like~ "*/zimbra/*" or
url.path like~ "*/service/home/*" or
url.path like~ "*/service/soap/*"
) and
(
url.query like~ "*<script*" or
url.query like~ "*javascript:*" or
url.query like~ "*onerror=*" or
url.query like~ "*onload=*" or
url.query like~ "*onfocus=*" or
url.query like~ "*eval(*" or
url.query like~ "*document.cookie*" or
http.request.body.content like~ "*<script*" or
http.request.body.content like~ "*javascript:*" or
http.request.body.content like~ "*onerror=*"
)
] with runs=1EQL sequence detecting XSS payload injection into Zimbra HTTP request parameters and body content. Uses sequence correlation to group rapid repeated attempts from the same source IP.
Data Sources
Required Tables
False Positives & Tuning
- Vulnerability scanners running automated web application assessments
- Legitimate SOAP API calls with XML content triggering angle-bracket detection
- Email gateway logs forwarding raw HTML email payloads to Elastic
- Pen testing tools such as Burp Suite or OWASP ZAP during authorized engagements
Other platforms for CVE-2025-66376
Testing Methodology
Validate this detection against 4 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Basic Reflected XSS Probe Against Zimbra Login
Expected signal: HTTP request to /zimbra/h/search with '<script>' in query parameter visible in web server access logs and WAF logs. Response code 200 or 302 depending on authentication state.
- Test 2Event Handler XSS Payload via Zimbra Mail Search
Expected signal: HTTP GET request with URL-encoded onerror= payload in query string visible in Zimbra access logs. Source IP and full URI captured in web server logs.
- Test 3SVG-Based XSS Payload Injection via Zimbra SOAP API
Expected signal: HTTP POST to /service/soap/ with SVG onload payload in request body. Body content inspection required — perimeter WAF or SIEM HTTP body logging must be enabled to capture this variant.
- Test 4JavaScript URI Scheme XSS in Zimbra Contact Import
Expected signal: HTTP POST to /service/home/ with multipart form data containing vCard. File upload logged in Zimbra access and mailbox logs. The javascript: URI in vCard URL field tests stored XSS if imported contact is rendered without sanitization.
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