CVE-2025-21594

An Improper Check for Unusual or Exceptional Conditions vulnerability in the pfe (packet forwarding engine) of Juniper Networks Junos OS on MX Series causes a port within a pool to be blocked leading to Denial of Service (DoS).

In a DS-Lite (Dual-Stack Lite) and NAT (Network Address Translation) scenario, when crafted IPv6 traffic is received and prefix-length is set to 56, the ports assigned to the user will not be freed.  Eventually, users cannot establish new connections. Affected FPC/PIC need to be manually restarted to recover.
Following is the command to identify the issue: 


    user@host> show services nat source port-block 
    Host_IP                       External_IP                    Port_Block      Ports_Used/        Block_State/
                                                               Range             Ports_Total        Left_Time(s)
    2001::                        x.x.x.x                     58880-59391     256/256*1         Active/-        >>>>>>>>port still usedThis issue affects Junos OS on MX Series: 

  *  from 21.2 before 21.2R3-S8, 
  *  from 21.4 before 21.4R3-S7, 
  *  from 22.1 before 22.1R3-S6, 
  *  from 22.2 before 22.2R3-S4, 
  *  from 22.3 before 22.3R3-S3, 
  *  from 22.4 before 22.4R3-S2, 
  *  from 23.2 before 23.2R2-S1, 
  *  from 23.4 before 23.4R1-S2, 23.4R2.


This issue does not affect versions before 20.2R1.