# CVE-2026-33774: Junos OS: MX Series: Firewall filters on lo0. 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. - **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/AU:Y/R:U/RE:L`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** No - **Published:** 9 Apr 2026 - **Updated:** 10 Apr 2026 - **Also known as:** GHSA-3p5v-chx4-4483 ## 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 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 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 (Improper Check for Unusual or Exceptional Conditions) ## Which versions are affected by CVE-2026-33774? It affects 1 product. Products: 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](https://insights.sbomify.com/CVE-2023-36845/): Juniper Junos OS EX Series and SRX Series PHP External Variable Modification (critical 9.8, exploited in the wild) - [CVE-2020-1631](https://insights.sbomify.com/CVE-2020-1631/): Juniper Junos OS Path Traversal (high 8.8, exploited in the wild) - [CVE-2025-21590](https://insights.sbomify.com/CVE-2025-21590/): Juniper Junos OS Improper Isolation or Compartmentalization (medium 6.7, exploited in the wild) - [CVE-2023-36851](https://insights.sbomify.com/CVE-2023-36851/): Juniper Junos OS SRX Series Missing Authentication for Critical Function (medium 5.3, exploited in the wild) - [CVE-2023-36844](https://insights.sbomify.com/CVE-2023-36844/): Juniper Junos OS EX Series PHP External Variable Modification (medium 5.3, exploited in the wild) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-33774 and fix it if it is affected. What it is: 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. Exploitation: There is no known exploitation of CVE-2026-33774 in the wild and no public exploit code. Affected product: Juniper Junos. No package-level version data is available, so check the vendor advisory linked from the reference page. Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-33774/ (Markdown: https://insights.sbomify.com/CVE-2026-33774/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33774 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33774 - OSV: https://osv.dev/vulnerability/CVE-2026-33774 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-33774/ Summaries are generated and may contain mistakes; the official description above is the record.