Three FortiSandbox Bugs Under Active Exploitation, Including a 9.1 Path Traversal

Attackers are hitting Fortinet's FortiSandbox appliance on multiple fronts.

Threat intelligence outfit Defused Cyber said it has logged exploitation of three distinct vulnerabilities in the malware analysis platform over a 24-hour window. The chain of bugs spans path traversal and related API weaknesses, and at least one of the flaws carries a critical CVSS score.

The headline issue is CVE-2026-39813, rated 9.1. It's a path traversal in the FortiSandbox JRPC API that, when abused, can let an attacker reach files outside the intended directory scope. Path traversal flaws in network appliances tend to mean one thing in practice: read access to configuration data, tokens, or credential material that opens the door to deeper compromise.

The two companion CVEs being exploited are CVE-2026-39808 and CVE-2026-25089. Defused Cyber disclosed the activity in a post on X, noting the exploitation window covered the previous day.

FortiSandbox is Fortinet's sandboxing product. It's the box enterprises drop suspicious files into for detonation and verdicting, and it sits inline with email and web traffic on a lot of networks. That position — close to the perimeter, trusted by downstream tooling — is exactly what makes it attractive to attackers looking for a quiet foothold.

Fortinet appliances have been a recurring entry point for both ransomware affiliates and state-aligned operators over the past two years. Initial access brokers have repeatedly listed Fortinet device access on Russian-language forums, and several leak-site victims in 2024 and 2025 were traced back to unpatched FortiOS and FortiManager bugs. A FortiSandbox compromise would fit the same playbook.

Fortinet's PSIRT page is the authoritative source for fix availability and affected versions. Defenders should consult the vendor advisories at fortiguard.fortinet.com/psirt for the three CVEs and apply the relevant builds.

A few practical notes for responders.

Check JRPC API logs for anomalous path parameters and unexpected file reads. Restrict management-plane access to the sandbox to a jump host or bastion, not the broader corporate network. Rotate any credentials, API tokens or service accounts the appliance holds, on the assumption that a successful traversal exposed them. And review outbound connections from the sandbox itself — a detonation box phoning home to infrastructure it didn't analyse is a strong signal.

No victim attribution has been published, and Defused Cyber has not named the actor or actors behind the activity. The firm's post described the exploitation as ongoing.

Threat Vectr has reached out to Fortinet for comment on the scope of exploitation and patch uptake.