CVE-2026-33774 Junos OS: MX Series: Firewall filters on lo0.<non-0> in the default routing instance are not in effect
Attackers can bypass firewall filters on specific loopback interfaces in the default routing instance, gaining unauthorized access to the control plane.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-33774?
CVE-2026-33774 is a medium-severity vulnerability in Juniper Junos, published on 9 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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 <filter_name>
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.
Weakness: CWE-754
Which versions are affected by CVE-2026-33774?
It affects 1 product.
Products include Juniper Junos.
How do I fix CVE-2026-33774?
No fix has been published for CVE-2026-33774 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-33774 being exploited?
There is no known exploitation of CVE-2026-33774 in the wild and no public exploit code.
Other vulnerabilities in Juniper Junos
- CVE-2023-36845 Juniper Junos OS EX Series and SRX Series PHP External Variable Modification
- CVE-2020-1631 Juniper Junos OS Path Traversal
- CVE-2025-21590 Juniper Junos OS Improper Isolation or Compartmentalization
- CVE-2023-36851 Juniper Junos OS SRX Series Missing Authentication for Critical Function
- CVE-2023-36844 Juniper Junos OS EX Series PHP External Variable Modification
Sources
Also known as GHSA-3p5v-chx4-4483.