Junos OS (SRX firewalls)
CVE-2026-21901: Configuration of a specific SSH option results in mgd crash
A NULL Pointer Dereference vulnerability in the management daemon (mgd) of Juniper Networks Junos OS and Junos OS Evolved allows a local, high-privileged attacker setting or deactivating a specific SSH configuration parameter to create a Denial of Service (DoS).
Published
Automatic summary from the vendor's CVE record. We haven't written a full analysis of this vulnerability yet.
Affected and fixed versions
| Product / branch | Fixed in |
|---|---|
| Junos OS 22.3 | 22.3R3-S5 or later |
| Junos OS 22.4 | 22.4R3-S10 or later |
| Junos OS 23.2 | 23.2R2-S7 or later |
| Junos OS 23.4 | 23.4R2-S8 or later |
| Junos OS 0 | Not affected |
| Junos OS Evolved 23.2 | 23.2R2-S7-EVO or later |
| Junos OS Evolved 23.4 | 23.4R2-S8-EVO or later |
| Junos OS Evolved 0 | Not affected |
Always confirm against the vendor advisory, which lists every fixed hotfix.
What it is
A NULL Pointer Dereference vulnerability in the management daemon (mgd) of Juniper Networks Junos OS and Junos OS Evolved allows a local, high-privileged attacker setting or deactivating a specific SSH configuration parameter to create a Denial of Service (DoS).
A local high-privileged user configuring or deactivating a specific 'system services ssh' configuration parameter can exploit a null pointer dereference in one of the functions used by SSH. The function attempts to dereference a null pointer when accessing certain configuration data, resulting in an mgd process crash and restart. Continued execution of these configuration commands will create a sustained Denial of Service (DoS) condition.
This issue affects: Junos OS:
- from 22.3 before 22.3R3-S5;
- from 22.4 before 22.4R3-S10;
- from 23.2 before 23.2R2-S7;
- from 23.4 before 23.4R2-S8.
This issue does not affect Junos OS before 22.3R1.
Junos OS Evolved: * from 22.3R1-EVO before 23.2R2-S7-EVO; * from 23.4 before 23.4R2-S8-EVO.
This issue does not affect Junos OS Evolved before 22.3R1-EVO.
Workarounds
There are no direct workarounds for this issue.
One of the following mitigations will prevent malicious exploitation:
- Use access lists or firewall filters to limit access to the CLI only from trusted hosts and administrators.
- Utilize command authorization to limit the ability to enter config mode to trusted administrators.
Exploitation
Juniper SIRT is not aware of any malicious exploitation of this vulnerability.
Sources
KEV status, EPSS score and vendor data refreshed automatically, last on 13 Oct 2026.