CVE-2026-21916 is a privilege escalation vulnerability in Juniper Networks Junos OS caused by improper handling of UNIX symbolic links in the CLI. When a user executes a particular 'file link ...' command and another user commits configuration changes, the first user can gain root access. This affects all Junos OS versions before 23.2R2-S7, 23.4 versions before 23.4R2-S6, 24.2 versions before 24.2R2-S3, 24.4 versions before 24.4R2-S2, and 25.2 versions before 25.2R2. Versions 25.4R1 and later are not affected. The vulnerability is rated high severity with CVSS 3.1 vector AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H. Juniper Networks provides patches and manages remediation for their cloud-hosted services.

A local, authenticated attacker with low privileges can exploit this vulnerability to escalate privileges to root, resulting in complete system compromise. This includes full confidentiality, integrity, and availability impact on the affected Junos OS systems. The vulnerability could allow unauthorized root access following specific CLI operations combined with configuration commits.

A patch is available for this vulnerability. Juniper Networks manages remediation for this cloud-hosted service. Users should upgrade to Junos OS version 25.4R1 or later, which is not affected. Check the vendor advisory for the latest patch and update instructions. No additional mitigation steps are indicated beyond applying the official fix.