Detections
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FortiClient EMS 7.4.x Reverse Proxy Header Spoofing Auth Bypass (CVE-2026-35616)

critical Nessus Plugin ID 305155

Synopsis

The remote FortiClient EMS server is affected by an authentication bypass via reverse proxy header spoofing.

Description

The Fortinet FortiClient EMS instance running on the remote host is vulnerable to CVE-2026-35616, a reverse proxy header trust vulnerability. The CertChainAuth middleware blindly trusts the X-SSL-CLIENT-VERIFY and X-SSL-CLIENT-CERT headers without validating that they originated from the legitimate reverse proxy.

An unauthenticated attacker who can reach the EMS HTTPS port directly can inject X-SSL-CLIENT-VERIFY: SUCCESS along with a client certificate to bypass the mutual TLS authentication gate and access CertChainAuth-protected API endpoints.

Solution

Upgrade FortiClient EMS to version 7.4.7 or later, or apply the Fortinet hotfix FG-IR-26-099.
As a workaround, configure the reverse proxy to strip X-SSL-CLIENT-VERIFY and X-SSL-CLIENT-CERT headers from incoming client requests before forwarding to the EMS backend.

See Also

https://fortiguard.fortinet.com/psirt/FG-IR-26-099

Plugin Details

Severity: Critical

ID: 305155

File Name: forticlient-ems-cve_2026_35616-auth_bypass.nbin

Version: 1.2

Type: Remote

Family: Misc.

Published: 4/7/2026

Updated: 4/8/2026

Supported Sensors: Nessus

Risk Information

VPR

Risk Factor: Critical

Score: 9.2

CVSS v2

Risk Factor: High

Base Score: 9

Temporal Score: 7.4

Vector: CVSS2#AV:N/AC:L/Au:N/C:P/I:P/A:C

CVSS Score Source: CVE-2026-35616

CVSS v3

Risk Factor: Critical

Base Score: 9.8

Temporal Score: 9.1

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Temporal Vector: CVSS:3.0/E:F/RL:O/RC:C

Vulnerability Information

CPE: cpe:/a:fortinet:forticlient_ems

Exploit Available: true

Exploit Ease: Exploits are available

Exploited by Nessus: true

Patch Publication Date: 4/2/2026

Vulnerability Publication Date: 4/1/2026

Reference Information

CVE: CVE-2026-35616