Detect Fortinet Multiple Products Improper Verification of Cryptographic Signature (CVE-2025-59718) in Google Chronicle
Detects exploitation of CVE-2025-59718, an improper verification of cryptographic signature vulnerability (CWE-347) affecting multiple Fortinet products. This vulnerability, listed in CISA's Known Exploited Vulnerabilities catalog, allows attackers to bypass signature validation checks, potentially enabling unsigned firmware/software installation, man-in-the-middle attacks on update channels, or code execution with elevated privileges on affected Fortinet appliances.
MITRE ATT&CK
YARA-L Detection Query
rule cve_2025_59718_fortinet_signature_bypass {
meta:
author = "df00tech Detection Engineering"
description = "Detects CVE-2025-59718 Fortinet cryptographic signature verification bypass"
severity = "CRITICAL"
priority = "HIGH"
reference = "https://fortiguard.fortinet.com/psirt/FG-IR-25-647"
cve = "CVE-2025-59718"
events:
(
$e.metadata.vendor_name = "Fortinet" nocase
or $e.target.application = /fortigate/i
or $e.target.application = /fortios/i
or $e.target.application = /fortimanager/i
)
and (
$e.metadata.description = /signature.*fail/i
or $e.metadata.description = /unsigned.*firmware/i
or $e.metadata.description = /cert.*verif.*fail/i
or $e.metadata.description = /bypass.*sig/i
or (
$e.metadata.event_type = "NETWORK_HTTP"
and $e.network.http.response_code >= 400
and (
$e.target.url = /\/api\/v2\/monitor\/system\/firmware/
or $e.target.url = /\/api\/v2\/cmdb\/system\/auto-update/
)
)
)
condition:
$e
}Chronicle YARA-L rule detecting Fortinet signature verification bypass events associated with CVE-2025-59718, covering both log-based signature failure indicators and API endpoint anomalies related to firmware update paths.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate firmware update API calls that return 4xx errors due to version checks
- Security researchers scanning Fortinet APIs in authorized engagements
- Automated monitoring tools polling Fortinet firmware update endpoints
Other platforms for CVE-2025-59718
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 Fortinet Firmware Signature Bypass via Invalid Update Request
Expected signal: HTTP 400 or 403 response from Fortinet API with error body referencing signature validation failure; corresponding event in Fortinet system logs with type=event subtype=update level=alert
- Test 2Fortinet Management Interface Signature-Related Error Log Injection Test
Expected signal: Three syslog events appearing in SIEM with Fortinet device name, event type 'update' or 'system', and messages containing 'signature verification failed', 'cert verification failed', and 'unsigned firmware'
- Test 3Network-Level Simulation of Fortinet Update Channel Interception
Expected signal: Network flow logs showing Fortinet appliance connecting to update server via proxy; Fortinet logs showing certificate validation errors or signature failures when intercepted responses are presented
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