Detect Fortinet Multiple Products Authentication Bypass via Alternate Path or Channel (CVE-2026-24858) in Google Chronicle
Detects exploitation of CVE-2026-24858, an authentication bypass vulnerability (CWE-288) affecting multiple Fortinet products. Attackers abuse an alternate authentication path or channel — specifically SSO abuse on FortiOS — to bypass normal authentication controls and gain unauthorized access. This vulnerability is listed on CISA's Known Exploited Vulnerabilities catalog, indicating active exploitation in the wild.
MITRE ATT&CK
YARA-L Detection Query
rule fortinet_auth_bypass_cve_2026_24858 {
meta:
author = "df00tech Detection Engineering"
description = "Detects Fortinet authentication bypass via alternate path/SSO abuse (CVE-2026-24858)"
severity = "CRITICAL"
priority = "HIGH"
reference = "https://fortiguard.fortinet.com/psirt/FG-IR-26-060"
cve = "CVE-2026-24858"
events:
$e.metadata.event_type = "NETWORK_CONNECTION"
(
$e.network.application_protocol = "HTTPS" or
$e.network.application_protocol = "HTTP"
)
(
$e.target.asset.hostname /(?i)forti/ or
$e.observer.hostname /(?i)forti/ or
$e.target.application /(?i)fortinet/
)
(
$e.metadata.description /(?i)(sso|saml|alternate.path|auth.bypass|unauthenticated)/ or
$e.network.http.request_url /(?i)(sso|saml|bypass|alternate)/
)
$e.security_result.action = "ALLOW"
condition:
$e
}Chronicle YARA-L rule detecting network authentication events on Fortinet assets with indicators of SSO abuse or alternate authentication channel exploitation matching CVE-2026-24858 exploitation patterns.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate SSO redirect flows through Fortinet SSL VPN portals
- SAML-based authentication for FortiAuthenticator integrations
- Security assessments generating matching URL patterns
- API calls to FortiManager or FortiAnalyzer using OAuth tokens
Other platforms for CVE-2026-24858
Testing Methodology
Validate this detection against 3 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 1Simulate SSO Authentication Request to Fortinet Management Interface
Expected signal: HTTP request to /remote/logincheck with SSO/SAML parameters should appear in FortiGate authentication logs and in web proxy or network monitoring logs. Source IP, timestamp, and HTTP response code should be captured.
- Test 2Enumerate Fortinet Management Interface SSO Endpoints
Expected signal: Series of HTTPS connections to Fortinet management interface on TCP 443 from a single source IP targeting multiple URL paths in rapid succession. Should appear in web access logs and network flow data.
- Test 3Create Unauthorized Admin Account Post-Authentication-Bypass
Expected signal: FortiOS admin audit log should record a new admin account creation event with the account name 'svc_backup_acct', profile 'super_admin', and the API source IP. Event type=event, subtype=system, action=add should appear in FortiAnalyzer.
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