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Fortinet reports active exploitation of CVE-2026-104286, a path traversal and NULL-byte flaw in FortiMail. Crafted HTTP or HTTPS requests to the publicly reachable FortiMail GUI may allow an unauthenticated attacker to write arbitrary files to the underlying system. Code execution is a possible consequence of that file-write capability, not a guaranteed result of every attack. Administrators should check their FortiMail version and apply the relevant Fortinet update promptly.
What CVE-2026-104286 does
The vulnerability affects FortiMail, Fortinet’s secure email gateway. MS-ISAC describes a path traversal (CWE-22) and NULL-byte or NULL-character neutralization flaw (CWE-158) in the publicly reachable GUI. An attacker may send crafted HTTP or HTTPS requests without authenticating and write arbitrary files to the underlying system. MS-ISAC reproduces this impact from Fortinet’s advisory; the direct Fortinet page was not independently accessible for this advisory snapshot. MS-ISAC advisory.
Arbitrary file writing can potentially lead to code execution, but the advisory does not say that successful file writing or code execution occurs in every attempt. Nor does active exploitation establish that a particular FortiMail appliance has been compromised.
Is the flaw being exploited?
Yes. Fortinet reports that CVE-2026-104286 is exploited in the wild. The Canadian Centre for Cyber Security’s advisory AV26-989, dated October 1, 2026, says CISA added the vulnerability to its Known Exploited Vulnerabilities catalog that day. Canadian Centre advisory AV26-989.
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The consulted advisories do not provide an incident count, exploitation volume, or estimate of affected organizations. Treat the confirmed exploitation as a reason to prioritize checking and updating systems, not as evidence that every exposed appliance was breached.
Which FortiMail versions are affected?
The Canadian Centre advisory’s October 1, 2026 version thresholds and the ranges listed by MS-ISAC identify these affected releases:
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| FortiMail branch | Affected versions | Advisory guidance |
|---|---|---|
| 8.0 | 8.0.0–8.0.1 | Use the applicable Fortinet update; the Canadian advisory lists versions before 8.0.2 as affected. |
| 7.6 | 7.6.0–7.6.6 | Use the applicable Fortinet update; the Canadian advisory lists versions before 7.6.7 as affected. |
| 7.4 | 7.4.0–7.4.8 | Use the applicable Fortinet update; the Canadian advisory lists versions before 7.4.9 as affected. |
| 7.2 | 7.2.0–7.2.9 | The Canadian advisory says to upgrade to branch 7.4 or above; confirm the current supported target and update instructions with Fortinet. |
These thresholds reflect the cited advisory snapshots, not a substitute for checking current vendor guidance. The direct Fortinet PSIRT page was linked from the Canadian advisory but could not be retrieved for this snapshot, so confirm the fixed release and upgrade path for your appliance and branch with Fortinet before applying a change.
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What FortiMail administrators should do
- Inventory your FortiMail deployments. Record each appliance’s installed branch and full version, including systems that may be managed separately or used for a secondary site.
- Compare each version with the affected ranges. Prioritize any appliance running an affected release, and verify the applicable current fixed version and upgrade path against Fortinet’s guidance.
- Apply the appropriate Fortinet update promptly. MS-ISAC recommends applying Fortinet updates immediately after appropriate testing; the Canadian Centre also encourages applying necessary updates. Follow your organization’s change-control process without delaying an urgent security update unnecessarily.
- Review GUI exposure. Determine whether the FortiMail GUI was reachable from the public internet during the period the appliance ran a vulnerable version. Public reachability increases the relevance of an investigation, but does not by itself prove compromise.
- Decide whether incident-response review is warranted. Given confirmed exploitation and the potential for arbitrary file writes, involve your security or incident-response team if an affected appliance was publicly reachable or if you find suspicious activity. The cited advisories establish the vulnerability and exploitation status, but do not provide a case-specific forensic checklist.
What is known—and what is not
- Established: Fortinet reports exploitation in the wild; the vulnerability can permit unauthenticated arbitrary file writing through crafted HTTP or HTTPS requests to the GUI.
- Potential, not guaranteed: Arbitrary code execution may follow from the file-write capability.
- Not established by the cited advisories: Whether a particular appliance was compromised, how many organizations were affected, or how much exploitation has occurred.
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