CVE-2026-21901 is a NULL Pointer Dereference vulnerability in the management daemon (mgd) of Juniper Networks Junos OS and Junos OS Evolved. A local attacker with high privileges can exploit this by modifying a specific 'system services ssh' configuration parameter, causing the mgd process to dereference a null pointer and crash. This results in a Denial of Service condition due to the mgd process restarting repeatedly. The vulnerability affects Junos OS versions from 22.3 before 22.3R3-S5, from 22.4 before 22.4R3-S10, from 23.2 before 23.2R2-S7, and from 23.4 before 23.4R2-S8. For Junos OS Evolved, affected versions are from 22.3R1-EVO before 23.2R2-S7-EVO and from 23.4 before 23.4R2-S8-EVO. Versions prior to 22.3R1 (Junos OS) and 22.3R1-EVO (Junos OS Evolved) are not affected.

The vulnerability allows a local high-privileged user to cause the mgd process to crash and restart by triggering a NULL pointer dereference via SSH configuration changes. This leads to a Denial of Service (DoS) condition, impacting availability of the management daemon. There is no impact on confidentiality or integrity according to the CVSS vector. No known exploits in the wild have been reported.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or temporary workaround is indicated in the provided data. Until a patch is available, restrict local high-privileged user access to configuration commands related to 'system services ssh' to prevent exploitation.