Detect Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Exploitation in Splunk
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
SPL Detection Query
index=web OR index=proxy OR index=firewall
| where like(uri_path, "%/zimbra/%") OR like(uri_path, "%/service/home/%") OR like(uri_path, "%/service/soap/%")
| eval xss_indicators=mvappend(
if(match(uri_query, "(?i)(<script|javascript:|onerror=|onload=|onfocus=|onmouseover=)"), "script_tag_or_event_handler", null()),
if(match(uri_query, "(?i)(eval\\(|document\\.cookie|window\\.location|String\\.fromCharCode)"), "js_execution_sink", null()),
if(match(request_body, "(?i)(<script|javascript:|onerror=|onload=|eval\\()"), "body_xss_payload", null()),
if(match(uri_path, "(?i)(<script|javascript:|onerror=)"), "path_xss_payload", null())
)
| where mvcount(xss_indicators) > 0
| stats count as attempt_count, earliest(_time) as first_seen, latest(_time) as last_seen, values(uri_path) as uri_paths, values(xss_indicators) as indicators, values(status) as response_codes by src_ip, http_user_agent
| eval risk=case(attempt_count >= 10, "high", attempt_count >= 3, "medium", true(), "low")
| sort - attempt_countDetects XSS exploitation attempts against Zimbra web interfaces by identifying malicious patterns in URI queries, paths, and request bodies. Groups by source IP to distinguish single probes from sustained campaigns.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Automated vulnerability scanners with broad web application testing profiles
- Email content filters that log raw email body content containing HTML entities
- Legitimate HTML-formatted email previews triggering partial pattern matches
- Encoded search queries from Zimbra desktop or mobile clients
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.
Unlock Pro Content
Get the full detection package for CVE-2025-66376 including response playbook, investigation guide, and atomic red team tests.