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 used

This 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. 

The following software releases have been updated to resolve this specific issue:

Junos OS: 21.2R3-S8, 21.4R3-S7, 22.1R3-S6, 22.2R3-S4, 22.3R3-S3, 22.4R3-S2, 23.2R2-S1, 23.4R1-S2, 23.4R2, 24.2R1, and all subsequent releases.

This issue is being tracked as 1785403 which is visible on the Customer Support website.

Note: Juniper SIRT's policy is not to evaluate releases which are beyond End of Engineering (EOE) or End of Life (EOL).

Modify the IPv6 prefix-length to be 64/96/128 through the command:

[ set services service-set <*> softwire-options dslite-ipv6-prefix-length 64/96/128]

2025-04-09: Initial Publication

2025-04-10: Changed IPv4 to IPv6 in title. 

• KB16613: Overview of the Juniper Networks SIRT Quarterly Security Bulletin Publication Process [juniper.net]
• KB16765: In which releases are vulnerabilities fixed? [juniper.net]
• KB16446: Common Vulnerability Scoring System (CVSS) and Juniper's Security Advisories [juniper.net]
• Report a Security Vulnerability - How to Contact the Juniper Networks Security Incident Response Team