Junos OS mgd Incorrect Sync DoS (<=23.4R2S4, 24.2R2S1, 24.4R1S3)
CVE-2026-21919 Published on April 9, 2026
Junos OS and Junos OS Evolved: A high frequency of connecting and disconnecting NETCONF sessions causes management unavailability
An Incorrect Synchronization vulnerability in the management daemon (mgd) of Juniper Networks Junos OS and Junos OS Evolved allows a network-based attacker with low privileges to cause a complete Denial-of-Service (DoS) of the management plane.
When NETCONF sessions are quickly established and disconnected, a locking issue causes mgd processes to hang in an unusable state. When the maximum number of mgd processes has been reached, no new logins are possible. This leads to the inability to manage the device and requires a power-cycle to recover.
This issue can be monitored by checking for mgd processes in lockf state in the output of 'show system processes extensive':
user@host> show system processes extensive | match mgd
<pid> root 20 0 501M 4640K lockf 1 0:01 0.00% mgd
If the system still can be accessed (either via the CLI or as root, which might still be possible as last resort as this won't invoke mgd), mgd processes in this state can be killed with 'request system process terminate <PID>' from the CLI or with 'kill -9 <PID>' from the shell.
This issue affects:
Junos OS:
* 23.4 versions before 23.4R2-S4,
* 24.2 versions before 24.2R2-S1,
* 24.4 versions before 24.4R1-S3, 24.4R2;
This issue does not affect Junos OS versions before 23.4R1;
Junos OS Evolved:
* 23.4 versions before 23.4R2-S5-EVO,
* 24.2 versions before 24.2R2-S1-EVO,
* 24.4 versions before 24.4R1-S3-EVO, 24.4R2-EVO.
This issue does not affect Junos OS Evolved versions before 23.4R1-EVO;
Vulnerability Analysis
CVE-2026-21919 can be exploited with network access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Type
Incorrect Synchronization
The software utilizes a shared resource in a concurrent manner, but it does not correctly synchronize access to the resource. If access to a shared resource is not correctly synchronized, then the resource may not be in a state that is expected by the software. This might lead to unexpected or insecure behaviors, especially if an attacker can influence the shared resource.
Products Associated with CVE-2026-21919
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Affected Versions
Juniper Networks Junos OS:- Version 23.4 and below 23.4R2-S4 is affected.
- Version 24.2 and below 24.2R2-S1 is affected.
- Version 24.4 and below 24.4R1-S3, 24.4R2 is affected.
- Version all version prior to and below 23.4R1 is unaffected.
- Version 23.4 and below 23.4R2-S5-EVO is affected.
- Version 24.2 and below 24.2R2-S1-EVO is affected.
- Version 24.4 and below 24.4R1-S3-EVO, 24.4R2-EVO is affected.
- Version all version prior to and below 23.4R1-EVO is unaffected.