Critical Zero-Day Vulnerability in FortiMail Allows Unauthenticated File Writes
Fortinet has disclosed a critical zero-day vulnerability in its FortiMail email security product that is already being actively exploited in the wild.
The company published a security advisory on October 1, 2026, detailing the flaw tracked as CVE-2026-104286. The issue resides in the graphical user interface component and results from inadequate neutralization of path traversal sequences and NULL bytes or NULL characters.
Successful exploitation allows an unauthenticated attacker to craft malicious HTTP requests and write arbitrary files to the underlying system. The vulnerability carries a CVSS v3.1 base score of 9.8 and is rated Critical, the highest severity level.
Although Fortinet discovered the flaw internally, the company confirmed reports of real-world exploitation. It plans to release fixed versions including FortiMail 8.0.2, FortiMail 7.6.7, and FortiMail 7.4.9. Organizations running the FortiMail 7.2 branch are instructed to migrate to the 7.4 release or newer.
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