CVE-2026-33774: Junos OS: MX Series: Firewall filters on lo0.<non-0> in the default routing instance are not in effect

An Improper Check for Unusual or Exceptional Conditions vulnerability in the packet forwarding engine (pfe) of Juniper Networks Junos OS on MX Series allows an unauthenticated, network-based attacker to bypass the configured firewall filter and access the control-plane of the device.

On MX platforms with

MPC10, MPC11, LC4800 or LC9600

line cards, and MX304, firewall filters applied on a loopback interface lo0.n (where n is a non-0 number) don't get executed when lo0.n is in the global VRF / default routing-instance.

An affected configuration would be:

user@host# show configuration interfaces lo0 | display set set interfaces lo0 unit 1 family inet filter input <filter-name>

where a firewall filter is applied to a non-0 loopback interface, but that loopback interface is not referred to in any routing-instance (RI) configuration, which implies that it's used in the default RI.

The issue can be observed with the CLI command:

user@device> show firewall counter filter <filtername>

not showing any matches.

This issue affects Junos OS on MX Series:

all versions before 23.2R2-S6, 23.4 versions before 23.4R2-S7, 24.2 versions before 24.2R2, 24.4 versions before 24.4R2.