Three critical vulnerabilities in the Fortinet FortiSandbox product line — CVE-2026-39813, CVE-2026-39808, and CVE-2026-25089 — all with a CVSS 9.1 score, have drawn the attention of the cybersecurity community after Defused Cyber reported observing exploitation attempts. The vulnerabilities allow an unauthenticated attacker to bypass authentication and execute arbitrary commands at the operating system level. Organizations using FortiSandbox, FortiSandbox Cloud, and FortiSandbox PaaS must immediately verify their patch status.
Technical details of the vulnerabilities
CVE-2026-39813 — authentication bypass via path traversal
This is a path traversal vulnerability in the JRPC API of the FortiSandbox component. According to the NVD, it allows an unauthenticated attacker to bypass the authentication mechanism through specially crafted HTTP requests. The CVSS score is 9.1. Available information indicates that Fortinet released a patch in April 2026.
CVE-2026-39808 — operating system command injection
The second vulnerability is an OS command injection issue which, as stated in the NVD entry, gives an unauthenticated attacker the ability to execute unauthorized code or commands via specially crafted HTTP requests. The CVSS score is 9.1. The fix is likewise reported to have been released in April 2026.
CVE-2026-25089 — command injection across multiple products
The third vulnerability is another OS command injection case, but with a broader scope of affected products. According to the NVD, it affects:
- FortiSandbox
- FortiSandbox Cloud
- FortiSandbox PaaS WEB UI
The vulnerability enables an unauthenticated attacker to execute unauthorized commands via specially crafted HTTP requests. The CVSS score is 9.1. According to the source material, the patch for this vulnerability was released later than the others.
Exploitation status and important caveats
It is important to emphasize that statements about active exploitation of these vulnerabilities are based on a single post by Defused Cyber on social network X. None of the three CVEs is listed in the CISA KEV (Known Exploited Vulnerabilities) catalog. At the time of publication, there is no independent confirmation of in‑the‑wild exploitation. This does not mean exploitation attempts are impossible — the criticality of the vulnerabilities (CVSS 9.1 and no authentication required) makes them attractive targets — but the reliability of these claims should be considered low.
Defused Cyber’s observation that the exploit they found for CVE-2026-25089 was supposedly created using an artificial intelligence model and contains bugs that render it non-functional also warrants attention. As far as is known, no publicly available working exploit for this vulnerability has been disclosed. However, these assertions likewise stem from a single unverified source and require independent confirmation.
Context: Fortinet devices as a priority target
Fortinet products remain among the most frequently attacked targets in enterprise‑grade network equipment. In April 2026, Fortinet reportedly released out‑of‑band patches for the critical CVE-2026-35616 vulnerability (CVSS 9.1) in FortiClient EMS, which, according to the vendor, was being exploited in the wild. The entry for this vulnerability is available in the NVD.
The emergence of several critical vulnerabilities in products from the same vendor over a short period is an alarming signal for organizations that rely on the Fortinet ecosystem for perimeter protection and malware analysis.
Impact assessment
FortiSandbox is a key component of security infrastructure, used for dynamic analysis of suspicious files and detection of advanced threats. Compromise of this device carries a dual risk:
- Direct damage: arbitrary OS‑level command execution gives the attacker full control over the device, the ability to intercept analyzed samples, and access to the internal network.
- Undermining trust in defenses: a compromised sandbox can deliberately allow malicious files through, creating a false sense of security.
Large organizations and managed security service providers (MSSPs) using FortiSandbox, FortiSandbox Cloud, or FortiSandbox PaaS for centralized threat analysis are at greatest risk.
Practical recommendations
- Immediately check the versions of all FortiSandbox, FortiSandbox Cloud, and FortiSandbox PaaS instances in your environment. Refer to official Fortinet (PSIRT) advisories for exact fixed version numbers.
- Apply available patches for CVE-2026-39813, CVE-2026-39808, and CVE-2026-25089 as a priority. All three vulnerabilities require no authentication for exploitation, which significantly lowers the attack threshold.
- Restrict network access to FortiSandbox management interfaces. The JRPC API and WEB UI should not be exposed to the internet. Use network segmentation and access control lists.
- Audit logs for anomalous HTTP requests to the JRPC API and FortiSandbox web interface, especially requests with signs of path traversal or atypical parameters.
- Check FortiClient EMS — if this product is used in your environment, ensure the patch for CVE-2026-35616 is also installed.
Three vulnerabilities with a CVSS 9.1 score that do not require authentication and affect a critical security infrastructure component are grounds for immediate patching, even in the absence of confirmed widespread exploitation. The top priority is to update all FortiSandbox instances to fixed versions and to restrict network access to management interfaces until the update is complete.
