Description

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.

Classification

CVE ID: CVE-2025-21594

CVSS Base Severity: HIGH

CVSS Base Score: 7.5

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected Products

Vendor: Juniper Networks

Product: Junos OS

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.05% (probability of being exploited)

EPSS Percentile: 16.32% (scored less or equal to compared to others)

EPSS Date: 2025-04-20 (when was this score calculated)

References

https://nvd.nist.gov/vuln/detail/CVE-2025-21594
https://supportportal.juniper.net/JSA96449

Timeline