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 <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.
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- EPSS 0.3% (30-day exploitation probability)+3
- Technical severity (medium)+3
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Juniper Networks Junos, Juniper Networks Junos OS).
- The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
- No fix documented in the sources: reduce exposure and monitor the vendor advisory.
- Search available logs for exploit probes matching the vulnerability class (logic error), errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| Juniper Networks | Junos |
version 23.2-r1version 24.4-r1-s3version 24.4-r1-s2version 24.4-r1version 24.2-r1-s2version 24.2-r1-s1version 24.2-r1version 23.4-r2-s6 and 18 moreversion 23.4-r2-s5version 23.4-r2-s4version 23.4-r2-s3version 23.4-r2-s2version 23.4-r2-s1version 23.4-r1-s2< 23.2version 23.2-r1-s1version 23.2-r1-s2version 23.2-r2version 23.2-r2-s1version 23.2-r2-s2version 23.2-r2-s3version 23.2-r2-s4version 23.2-r2-s5version 23.4-r1version 23.4-r1-s1version 23.4-r2 |
Vulnerable | NIST National Vulnerability Database |
| Juniper Networks | Junos OS | < 23.2R2-S6≥ 23.4 and < 23.4R2-S7≥ 24.2 and < 24.2R2≥ 24.4 and < 24.4R2 | Vulnerable | CVE Program (cve.org) e CISA ADP |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 6.9 | CVSS 4.0 | Medium | — | — | sirt@juniper.net via NIST National Vulnerability Database |
| 6.9 | CVSS 4.0 | Medium | — | — | juniper via CVE Program (cve.org) e CISA ADP |
| 6.5 | CVSS 3.1 | Medium | 3.9 | 2.5 | sirt@juniper.net via NIST National Vulnerability Database |
| 6.5 | CVSS 3.1 | Medium | — | — | juniper via CVE Program (cve.org) e CISA ADP |
| 5.3 | CVSS 3.1 | Medium | 3.9 | 1.4 | nvd@nist.gov via NIST National Vulnerability Database |
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N (score shown at the top: nvd@nist.gov) Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
CWE-754 · Improper Check for Unusual or Exceptional Conditions
EPSS over time FIRST
Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.29%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
Timeline
What happened to this CVE, newest first.
- 2026
- Affected products added · juniper networks/junos os
- SSVC assessment updated
- Record published
- CVSS changed · 5.3
- Source status changed · → Analyzed
- Record created in this database
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 1× Official description updated · Oct 13, 2026 – Oct 13, 2026
References
1 on the record
Elsewhere on this site
Related CVEs (same products)
- High CVE-2026-102077 Junos OS: MX Series with SPC3: In a source NAT scenario malformed packets will lead to resource leaks and eventually traffic stops 1d
- Critical CVE-2026-101982 Junos OS and Junos OS Evolved: A EVPN peer causes an rpd crash or root-level compromise when EVPN-VPWS is used 1d
- Critical CVE-2026-101983 Junos OS and Junos OS Evolved: An attacker causes remote code execution when J-Web virtual-domain is configured 1d
- Medium CVE-2026-101964 Junos OS and Junos OS Evolved: An attacker can cause an rpd crash via a crafted LDP message 1d
- Medium CVE-2026-101975 Junos OS and Junos OS Evolved: Improper OSPF LSA validation causes an rpd crash when prefix-SID best-route installation is used 1d
- Medium CVE-2026-101972 Junos OS and Junos OS Evolved: Malformed OSPF packets causes routing disruption when OSPF is enabled 1d
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No fixed version, patch or mitigation documented.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2026-33774 is "Junos OS: MX Series: Firewall filters on lo0.<non-0> in the default routing instance are not in effect", rated Medium. It affects Junos (Juniper Networks); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 5.3 describes technical severity. EPSS 0.29% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
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