CVE-2026-25089: Fortinet FortiSandbox — Unauthenticated OS Command Injection

Background

Fortinet FortiSandbox is a threat analysis and sandboxing platform used by enterprise security operations teams to detonate suspicious files and URLs in an isolated environment and identify malicious behaviour. It integrates with other Fortinet products and third-party security controls to provide automated malware analysis as part of a detection pipeline. FortiSandbox is deployed as an on-premises appliance, a cloud-hosted service (FortiSandbox Cloud), and a platform-as-a-service offering (FortiSandbox PaaS).

CVE-2026-25089 is a critical unauthenticated OS command injection vulnerability (CWE-78) in FortiSandbox’s management interface. An attacker with network access to the management interface can send a specially crafted HTTP request to execute arbitrary commands on the host operating system without any credentials. CISA added it to the Known Exploited Vulnerabilities catalogue on 16 July 2026, confirming active exploitation. A companion vulnerability, CVE-2026-39808 (PSIRT FG-IR-26-100), affects a different code path in the same product and was added to CISA KEV simultaneously.

Technical Mechanism

OS command injection (CWE-78) occurs when attacker-controlled input is incorporated into a system shell command string without adequate sanitisation. When a web application passes unsanitised HTTP request data to a shell execution function — system(), exec(), popen(), or equivalents — an attacker can inject shell metacharacters to execute arbitrary commands alongside or instead of the intended command.

FortiSandbox’s management interface processes HTTP requests containing administrative parameters for file submission, analysis configuration, reporting, and system management functions. CVE-2026-25089 affects a code path that is reachable before authentication is checked. Request data reaching the vulnerable handler is incorporated into a shell command string without sanitisation.

An attacker crafts an HTTP request embedding shell metacharacters (;, &&, |, `, $(...)) in the affected parameter. The FortiSandbox host operating system executes the injected commands. Because FortiSandbox requires elevated OS privileges to perform sandbox analysis operations — including file execution, network monitoring, and process inspection — the injected commands execute with root or high-privilege access.

Fortinet’s PSIRT advisory FG-IR-26-141 confirms the attack vector: “unauthenticated attacker to execute unauthorized commands via specifically crafted HTTP requests.” This describes a pre-authentication injection against a web-facing management endpoint.

Real-World Exploitation Evidence

CISA added CVE-2026-25089 to the Known Exploited Vulnerabilities catalogue on 16 July 2026. The federal remediation deadline of 19 July 2026 — three days after the KEV addition — is consistent with CISA’s treatment of vulnerabilities under active high-urgency exploitation.

Fortinet management interface vulnerabilities have sustained high exploitation rates from multiple threat actor categories in recent years:

• Nation-state actors affiliated with China, including groups tracked as Volt Typhoon and UNC3886, have specifically targeted Fortinet management interfaces in campaigns against critical infrastructure
• CISA and the FBI have issued prior advisories identifying FortiOS and FortiGate vulnerabilities among the most consistently exploited products for initial access into enterprise and government networks
• Ransomware operators including BlackCat/ALPHV, LockBit, and successor groups have used Fortinet VPN and firewall vulnerabilities extensively as initial access vectors

CVE-2026-39808, a separate command injection vulnerability in FortiSandbox (FG-IR-26-100) affecting a different code path, was added to CISA KEV on the same date. Simultaneous KEV addition for two distinct injection flaws in the same product indicates FortiSandbox was under active exploitation across multiple vulnerable endpoints.

Impact Assessment

Successful exploitation of CVE-2026-25089 provides unauthenticated root-level command execution on the FortiSandbox host. The consequences extend beyond the compromised appliance:

Access to security intelligence: FortiSandbox processes malware samples submitted by the security team. An attacker with host access can access all files submitted for analysis — including malware samples, suspicious documents, and potentially sensitive files submitted for detonation. This counterintelligence value is distinct from standard infrastructure compromise.

Evasion intelligence: An attacker who can read FortiSandbox’s detection rules and analysis configuration gains insight into what techniques the security team’s tooling can detect, enabling them to tailor future attacks to evade detection.

Security network foothold: FortiSandbox is typically deployed inside the security operations network segment. Root access on the appliance provides a pivot point into this high-trust network zone, with access to SIEM integrations, ticketing systems, and other security infrastructure.

Cloud and PaaS exposure: The vulnerability explicitly extends to FortiSandbox Cloud and FortiSandbox PaaS. For cloud-deployed instances, patching responsibility and timeline differ from on-premises; customers should confirm with Fortinet whether cloud instances were updated automatically.

Affected Versions

Consult the Fortinet PSIRT advisory at https://fortiguard.fortinet.com/psirt/FG-IR-26-141 for the specific version ranges. Fortinet’s FortiSandbox uses a major.minor.patch versioning scheme; the advisory specifies affected minor releases and the minimum patched version per branch.

Note that CVE-2026-39808 (FG-IR-26-100) is a separate vulnerability in the same product requiring a separate patch. Both CVEs must be remediated independently.

Remediation Steps

1. Apply the patch specified in Fortinet PSIRT advisory FG-IR-26-141 via the Fortinet Support portal (support.fortinet.com). Federal agencies must meet the BOD 26-04 deadline of 19 July 2026.

2. Confirm cloud and PaaS instance status. Contact Fortinet support to verify whether FortiSandbox Cloud and FortiSandbox PaaS instances have been patched automatically or require customer-initiated action.

3. Apply the patch for CVE-2026-39808 simultaneously. FG-IR-26-100 covers the companion vulnerability in a different code path. Patching only one CVE leaves the other exploitable.

4. If immediate patching is not possible, restrict network access to the FortiSandbox management interface:

   • Apply firewall rules limiting management interface access to specific administrative source IPs
   • Disable any remote administration access not strictly required
   • Place the management interface behind a VPN or authenticated jump host

5. Monitor for post-exploitation indicators during and after remediation (see Detection Guidance).

No workaround exists that removes the OS command injection attack surface without restricting management interface access. The patch is required for full remediation.

Detection Guidance

Network monitoring: HTTP requests to the FortiSandbox management interface from unexpected source IP addresses should be investigated. Management interfaces should have a narrow, well-defined source IP whitelist; traffic from outside this list warrants immediate review.

Request content analysis: HTTP requests to the management interface containing shell metacharacters in parameter values (;, &&, |, `, $(...)) indicate active exploitation attempts.

Process monitoring: Unexpected shell processes (sh, bash, ash) spawned as child processes of the FortiSandbox web server process indicate successful injection. Examine process trees on FortiSandbox appliances for unusual execution chains.

Outbound connections: Exploitation typically results in a reverse shell or download cradle. Monitor for outbound connections from the FortiSandbox management host to external IPs on non-standard ports, particularly correlated with inbound HTTP requests to the management interface.

System logs: Review FortiSandbox system logs under /var/log/ for command execution events at anomalous times or by the web server user account.

FortiAnalyzer/FortiSIEM: Create correlation rules alerting on the combination of an unauthenticated inbound request to the management interface followed by an outbound connection from the FortiSandbox host within a short time window. This pattern is a high-confidence exploitation indicator.

Timeline

References

• Fortinet PSIRT Advisory FG-IR-26-141
• CISA Known Exploited Vulnerabilities Catalog
• NVD — CVE-2026-25089
• CISA BOD 26-04 — Prioritizing Security Updates Based on Risk
• Fortinet Support Portal