CVE-2026-102077: MX Series with SPC3: In a source NAT scenario malformed packets will lead to resource leaks and eventually traffic stops

In short

The Packet Forwarding Engine in Junos OS on MX Series with SPC3 fails to properly handle specific malformed IPv6 packets when source NAT is configured. Instead of dropping these packets, the system creates sessions and allocates NAT resources, allowing an unauthenticated, network-based attacker to exhaust these resources and cause a Denial-of-Service that impacts all traffic through the device.

Written by a language model from the vendor's description. Versions, fixes and exploitation status on this page come straight from the vendor and CISA.

The vendor's description

An Improper Check for Unusual or Exceptional Conditions vulnerability in the Packet Forwarding Engine (PFE) of Juniper Networks Junos OS on MX Series with SPC3 allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS).

On MX Series with SPC3 line-card and source NAT configuration, when a specific malformed IPv6 packet is received, it is not dropped but instead a session is created and NAT resources are allocated. Continued receipt of such packets will eventually lead to session or resource exhaustion, which will impact all traffic through the device. To recover the device needs to be rebooted. To monitor session usage, use the following:

show services sessions summary

To monitor source NAT resource usage, use the following:

show services nat pool

This issue affects Junos OS on MX Series with SPC3:

• All versions before 23.2R2-S4;
• 23.4 versions before 23.4R2-S5;
• 24.2 versions before 24.2R2.

Am I affected?

To be exposed to this issue, you need to have source NAT configured similar to the following:

[ services nat source rule-set <rule set name> rule <name> then source-nat ... ]

Workarounds

There are no known workarounds for this issue.

Exploitation

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