JULY 16, 2026CVSS 9.8 · CRITICAL · ACTIVELY EXPLOITED5 MIN READ

Fortinet FortiSandbox contains an unauthenticated OS command injection vulnerability in its web interface. Fortinet's CNA record assigns CVSS 9.8, while its PSIRT advisory lists 9.1; NVD has not issued an independent score. Defused reported exploitation attempts in mid-June, and CISA added CVE-2026-25089 to KEV on July 16 with a July 19 deadline for applicable FCEB systems. This is the third FortiSandbox vulnerability exploited in the wild within two months.

The Vulnerability

CVE-2026-25089 (CWE-78: Improper Neutralization of Special Elements used in an OS Command) is an OS command injection in FortiSandbox's web UI. The flaw is reported to affect the "start VNC" feature — an attacker can inject shell metacharacters via JSON payloads in HTTP requests to this endpoint. No authentication is required, no user interaction is needed, and attack complexity is low.

Related FortiSandbox Exploitation Activity

CVE-2026-25089 is the third FortiSandbox vulnerability exploited in the wild in 2026. Two related flaws were patched in April and saw active exploitation by mid-June:

• • CVE-2026-39808 — OS command injection (CNA CVSS 9.8; PSIRT 9.1), unauthenticated RCE. Exploitation observed June 12 by KEVIntel.
• • CVE-2026-39813 — Path traversal leading to authentication bypass (CNA CVSS 9.8; PSIRT 9.1). Exploitation observed June 15 by Defused.

Threat intelligence firm Defused detected exploitation attempts against all three CVEs on honeypots. FortiSandbox is a high-value target because integrated Fortinet products — FortiGate, FortiMail, FortiWeb, FortiProxy — consume its threat verdicts to enforce blocking decisions and trigger automated responses. Because these products depend on FortiSandbox results, responders should review connected workflows for unexpected verdicts, signatures, or configuration changes.

Investigation Workflow

FortiSandbox appliances are typically deployed on internal networks behind firewalls, but their web management interface is often accessible from management VLANs or, in some deployments, from the internet. Identifying exposed instances is the first step.

1. Port Scan: Find FortiSandbox Appliances

FortiSandbox uses standard ports for its web UI and services. The management web interface — where CVE-2026-25089 resides — is the primary target:

• • 443 — HTTPS web UI (management interface — where the vulnerability resides)
• • 80 — HTTP web UI (if HTTPS redirect is not enforced)
• • 22 — SSH administration
• • 514 — OFTP (file/URL/traffic submissions from other Fortinet products)

2. HTTP Headers: Identify FortiSandbox Interfaces

FortiSandbox appliances return identifiable HTTP responses on their management interface:

• • HTML title containing FortiSandbox
• • HTTP headers referencing Fortinet or FortiSandbox
• • Login page with Fortinet branding and FortiSandbox product name
• • Server header may identify the Fortinet web server

3. TLS Inspect: Examine Certificates

Pull the TLS certificate on port 443. Look for:

• • Subject CN or SAN containing FortiSandbox or fortisandbox
• • Default self-signed certificates from Fortinet (common on initial deployments)
• • Organization field referencing Fortinet

4. DNS: Discover FortiSandbox Infrastructure

Query DNS for common FortiSandbox naming patterns: sandbox.*, fortisandbox.*, fsb.*, fsa.*. FortiSandbox appliances are often given descriptive hostnames in internal DNS.

5. CVE Lookup: Track All Three Vulnerabilities

Look up CVE-2026-25089, CVE-2026-39808, and CVE-2026-39813. All three target FortiSandbox and have been exploited in the wild. Check applicability for all three. FortiSandbox 4.4.9+ addresses all three vulnerabilities on the 4.4 branch; 5.0.6+ addresses the applicable vulnerabilities on 5.0. CVE-2026-39808 does not affect 5.0.

Note: The fingerprints above are heuristics — they identify likely FortiSandbox instances but do not confirm patch level. FortiSandbox does not expose its firmware version in HTTP headers. To verify the installed version, log into the management UI and check System > Dashboard.

Cross-Reference with External Data

Remediation

1. Patch immediately. Upgrade FortiSandbox to 4.4.9+ or 5.0.6+. Cloud and PaaS deployments should upgrade to 5.0.6+. The CNA record lists 4.2 as affected but Fortinet's advisory does not provide a 4.2 remediation — contact Fortinet for supported migration or mitigation guidance. If patching is not immediately possible, isolate the management interface from untrusted networks and restrict access to designated administrative hosts.
2. Patch the related CVEs too. CVE-2026-39808 and CVE-2026-39813 were patched in April 2026 but may remain present on systems that have not received the April updates. Verify your FortiSandbox is patched against all three actively exploited vulnerabilities.
3. Restrict management interface access. The web UI (port 443) should never be internet-facing. Restrict it to dedicated management VLANs or jump hosts with MFA. This limits the attack surface for all three CVEs.
4. Audit Fortinet product integrations. FortiSandbox provides threat verdicts to FortiGate, FortiMail, FortiWeb, and FortiProxy. A compromised sandbox could potentially affect downstream verdict accuracy. Review integration logs for unexpected verdict changes or configuration modifications.
5. Conduct compromise hunting. Fortinet has not published validated IOCs for this vulnerability. As general hunting guidance: review web UI access logs for unusual requests to VNC-related endpoints, look for unexpected outbound connections, new user accounts, or modified system configurations.
6. Monitor for follow-on activity. FortiSandbox processes malware samples and has access to files submitted from across the network. A compromised appliance may have exposed sample data, network topology, or integration credentials for other Fortinet products.

Sources