Detect Fortinet Multiple Products Improper Verification of Cryptographic Signature (CVE-2025-59718) in Splunk
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
SPL Detection Query
index=network OR index=security sourcetype IN ("fortigate_log", "fortinet:fortigate:firewall", "fortinet:syslog", "syslog")
(vendor="Fortinet" OR source="fortigate" OR host IN (fortinet_hosts))
(
(message IN ("*signature*bypass*", "*invalid signature*", "*cert*verification*failed*", "*unsigned firmware*", "*signature check failed*"))
OR (subtype="update" message="*error*")
OR (type="event" subtype="system" action="upgrade" status!="success")
)
| eval risk_score=case(
match(message, "(?i)bypass"), 90,
match(message, "(?i)unsigned"), 85,
match(message, "(?i)signature.*fail"), 80,
true(), 60
)
| eval event_category=case(
match(subtype, "update|upgrade|firmware"), "firmware_update",
match(message, "(?i)cert"), "certificate_validation",
true(), "signature_verification"
)
| stats count AS event_count, min(_time) AS first_seen, max(_time) AS last_seen, values(message) AS messages, max(risk_score) AS max_risk by host, src_ip, event_category
| where event_count > 0
| eval severity=if(max_risk >= 80, "critical", "high")
| sort -max_riskSearches Fortinet device logs for signature verification failures, unsigned firmware installation attempts, and certificate validation errors that may indicate CVE-2025-59718 exploitation.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate failed firmware upgrades due to network interruptions or version incompatibility
- Self-signed certificates used in test/staging Fortinet environments
- Routine certificate renewal processes generating transient validation errors
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
Unlock Pro Content
Get the full detection package for CVE-2025-59718 including response playbook, investigation guide, and atomic red team tests.