Fortinet says attackers are exploiting CVE-2026-104286, a FortiMail path traversal and NULL-byte vulnerability. Crafted HTTP or HTTPS requests to the publicly reachable FortiMail GUI may let an unauthenticated attacker write arbitrary files to the appliance; code execution is a possible consequence, not a guaranteed result. Administrators should check the installed version and apply the update Fortinet specifies for that branch.
What CVE-2026-104286 does
The vulnerability affects Fortinet FortiMail, a secure email gateway. MS-ISAC describes the flaw as involving path traversal and improper handling of a NULL byte. A crafted HTTP or HTTPS request to the publicly reachable GUI may allow an unauthenticated remote attacker to write arbitrary files on the underlying system. The resulting file write could potentially enable arbitrary code execution, but the advisory does not say that code execution occurs in every attack.
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MS-ISAC reproduces and attributes this description to Fortinet: “An Improper Limitation of a Pathname to a Restricted Directory (‘Path Traversal’) [CWE-22] and Improper Neutralization of NULL Byte or NULL Character [CWE-158] vulnerability may allow an unauthenticated attacker to write arbitrary files on the underlying system via crafted HTTP or HTTPS requests.” Read the MS-ISAC advisory.
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Exploitation status
Fortinet reports that CVE-2026-104286 is being exploited in the wild. The Canadian Centre for Cyber Security’s advisory AV26-989, dated October 1, 2026, says CISA added the CVE to its Known Exploited Vulnerabilities catalog on that date. This establishes active exploitation, but does not show that any particular FortiMail appliance has been compromised or quantify the number of affected organizations.
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See the Canadian Centre advisory AV26-989.
Which FortiMail versions are affected
The Canadian Centre advisory and MS-ISAC identify these affected releases and branch thresholds:
| FortiMail branch | Affected versions | Advisory threshold |
|---|---|---|
| 8.0 | 8.0.0–8.0.1 | Before 8.0.2 |
| 7.6 | 7.6.0–7.6.6 | Before 7.6.7 |
| 7.4 | 7.4.0–7.4.8 | Before 7.4.9 |
| 7.2 | 7.2.0–7.2.9, according to MS-ISAC | The Canadian Centre says systems on 7.2 should upgrade to branch 7.4 or later |
These are the ranges in advisories dated October 1 and 2, 2026. Check Fortinet’s current branch-specific guidance before choosing a target release; the cited snapshots do not establish whether later updates or guidance have since changed.
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What administrators should do
- Inventory FortiMail appliances. Record each appliance’s installed branch and full version, including systems managed by other teams or hosted in separate environments.
- Compare each version with the affected ranges. Use the table above to identify versions flagged by the October advisories, then confirm the current affected and fixed releases in Fortinet’s guidance.
- Apply the applicable Fortinet update promptly. MS-ISAC recommends applying Fortinet updates immediately after appropriate testing; the Canadian Centre encourages applying the necessary updates. Follow your change controls while treating an affected release as urgent.
- Review exposure and decide whether to investigate. Determine whether the FortiMail GUI was publicly reachable, and assess whether incident-response review is warranted, particularly if the appliance ran an affected release. The advisories establish a serious potential impact and active exploitation, but do not provide a case-specific forensic checklist or confirm compromise of any individual system.
The Canadian Centre advisory links to Fortinet’s PSIRT notice, but the advisory details cited here do not establish a workaround or a fixed-release status beyond the version guidance above. Consult Fortinet directly for the latest instructions rather than assuming that a particular mitigation or upgrade target remains current.
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