Detect Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Exploitation in Google Chronicle
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
YARA-L Detection Query
rule zimbra_xss_cve_2025_66376 {
meta:
author = "df00tech Detection Engineering"
description = "Detects XSS exploitation attempts targeting Zimbra Collaboration Suite (CVE-2025-66376)"
severity = "HIGH"
priority = "HIGH"
reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-66376"
yara_version = "YL2.0"
rule_version = "1.0"
events:
$req.metadata.event_type = "NETWORK_HTTP"
(
re.regex($req.target.url, `(?i)/zimbra/`) or
re.regex($req.target.url, `(?i)/service/home/`) or
re.regex($req.target.url, `(?i)/service/soap/`)
)
(
re.regex($req.target.url, `(?i)(<script|javascript:|onerror=|onload=|onfocus=|onmouseover=)`) or
re.regex($req.network.http.request_body, `(?i)(<script|javascript:|onerror=|onload=|eval\()`) or
re.regex($req.target.url, `(?i)(eval\(|document\.cookie|window\.location|String\.fromCharCode)`)
)
condition:
$req
}Chronicle YARA-L 2.0 rule detecting HTTP requests to Zimbra paths containing XSS payload patterns in URL parameters or request bodies. Matches single events to ensure no exploitation attempt is missed.
Data Sources
Required Tables
False Positives & Tuning
- Automated security scanning tools probing all web endpoints for XSS vulnerabilities
- Legitimate Zimbra SOAP API calls with XML payloads containing encoded angle brackets
- Email security gateway log forwarding of raw HTML email body content
- URL-encoded search strings from Zimbra desktop clients with special characters
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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