CVE-2026-21919: A high frequency of connecting and disconnecting NETCONF sessions causes management unavailability

What it is

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;

Am I affected?

To be exposed to this issue the device needs to be configured for NETCONF as follows:

[ system services netconf ... ]

Workarounds

Use access lists or firewall filters to limit access to the device only from trusted hosts and administrators.

To further reduce the risk of exploitation you can set values as low as needed for your normal operations for:

[ system services netconf ssh connection-limit <max_connections> ] [ system services netconf ssh rate-limit <connections_per_minute> ]

Exploitation

Juniper SIRT is not aware of any malicious exploitation of this vulnerability.