Fortinet has released out-of-band patches for a critical security flaw impacting FortiClient EMS that it said has been exploited in the wild.
The vulnerability, tracked as CVE-2026-35616 (CVSS score: 9.1), has been described as a pre-authentication API access bypass leading to privilege escalation.
"An improper access control vulnerability [CWE-284] in FortiClient EMS may allow an unauthenticated attacker to execute unauthorized code or commands via crafted requests," Fortinet said in a Saturday advisory.
The issue affects FortiClient EMS versions 7.4.5 through 7.4.6. It's expected to be fully patched in the upcoming version 7.4.7, although the company has released a hotfix to address it.
Simo Kohonen from Defused Cyber and Nguyen Duc Anh have been credited with discovering and reporting the flaw. In a post on X, Defused Cyber said it observed zero-day exploitation of CVE-2026-35616 earlier this week. According to watchTowr, exploitation attempts against CVE-2026-35616 were first recorded against its honeypots on March 31, 2026.
Successful exploitation of the flaw could allow an unauthenticated attacker to sidestep API authentication and authorization protections, and execute malicious code or commands via crafted requests.
"Fortinet has observed this to be exploited in the wild and urges vulnerable customers to install the hotfix for FortiClient EMS 7.4.5 and 7.4.6," the company added.
The development comes merely days after another recently-patched, critical vulnerability in FortiClient EMS (CVE-2026-21643, CVSS score: 9.1) came under active exploitation. It's currently not known if the same threat actor is behind the exploitation of both the flaws, and if they are being weaponized together.
Given the severity of the vulnerabilities, users are advised to update their FortiClient EMS to the latest version as soon as possible.
"The timing of the ramp-up of in-the-wild exploitation of this zero-day is likely not coincidental," watchTowr CEO and founder Benjamin Harris told The Hacker News.
"Attackers have shown repeatedly that holiday weekends are the best time to move. Security teams are at half strength, on-call engineers are distracted, and the window between compromise and detection stretches from hours to days. Easter, like any other holiday, represents opportunity."
"What is disappointing is the bigger picture. This is the second unauthenticated vulnerability in FortiClient EMS in a matter of weeks."
"So, once again, organizations running FortiClient EMS and exposed to the Internet should treat this as an emergency response situation, not something to pick up on Tuesday morning. Apply the hotfix. Attackers already have a head start."
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