Detect CVE-2026-35616 — Fortinet FortiClient EMS Improper Access Control Exploitation in Google Chronicle
Detects exploitation attempts targeting CVE-2026-35616, an improper access control vulnerability (CWE-284) in Fortinet FortiClient Enterprise Management Server (EMS). This vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog, indicating active exploitation in the wild. Attackers may leverage this flaw to bypass access controls on the EMS server, potentially enabling unauthorized configuration changes, endpoint agent manipulation, or lateral movement through managed endpoints.
MITRE ATT&CK
YARA-L Detection Query
rule cve_2026_35616_forticlient_ems_access_control {
meta:
author = "df00tech Detection Engineering"
description = "Detects CVE-2026-35616 — Fortinet FortiClient EMS Improper Access Control exploitation"
severity = "CRITICAL"
priority = "HIGH"
reference = "https://fortiguard.fortinet.com/psirt/FG-IR-26-099"
cve = "CVE-2026-35616"
events:
$e.metadata.vendor_name = /(?i)fortinet/
$e.metadata.product_name = /(?i)(forticlient|ems|enterprise management)/
(
$e.metadata.event_type = "NETWORK_CONNECTION" or
$e.metadata.event_type = "USER_LOGIN" or
$e.metadata.event_type = "STATUS_UPDATE"
)
(
$e.security_result.action = "BLOCK" or
$e.security_result.action = "DENY" or
$e.security_result.description = /(?i)(access denied|authorization fail|privilege escalat|unauthorized|bypass|improper access)/
)
$e.principal.ip != ""
match:
$e.principal.ip over 15m
condition:
#e >= 3
}Chronicle YARA-L 2.0 rule detecting repeated access control failures from a single source IP against Fortinet FortiClient EMS infrastructure. Requires 3 or more qualifying events within a 15-minute window, reducing false positive noise from transient failures.
Data Sources
Required Tables
False Positives & Tuning
- Authorised red team engagements targeting EMS infrastructure generating clustered denial events
- EMS load balancer health checks producing repeated blocked connection events
- Endpoint agents in reconnect loops after connectivity disruption triggering threshold within window
- Batch administrative operations from a single management IP causing access evaluation bursts
Other platforms for CVE-2026-35616
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 1FortiClient EMS Unauthenticated API Endpoint Probe
Expected signal: Web server access log entries on EMS showing HTTP requests to API paths from an unknown source IP, with response codes logged. Network flow records show connections from test host to EMS port 8013.
- Test 2FortiClient EMS Privilege Escalation via Access Control Bypass (Simulated)
Expected signal: EMS audit log entries recording the low-privilege user's access attempts to admin-tier endpoints. Windows Security Event Log 4648 if token acquisition involved explicit credential use.
- Test 3FortiClient EMS Management Port Reconnaissance from External Network Segment
Expected signal: Network IDS/IPS alerts for port scan activity targeting EMS management ports. Firewall deny logs if EMS ports are restricted. NetFlow records showing probe connections from test segment to EMS IP.
- Test 4FortiClient Agent Mass Policy Modification via Compromised EMS
Expected signal: EMS audit log entry recording policy creation event with the admin token's associated username. API access log entry for POST to /api/v1/policies. Managed endpoint logs showing receipt of new policy push if EMS propagates it.
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