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 andprefix-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.