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JunosOperating system · Juniper

CVE-2026-33774

MEDIUM · 5.3 CVSS v3.1 Published 2026-04-09
Fix available
A fix is available. Upgrade to 23.2 or later.
See remediation →
62/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Improper check in the packet forwarding engine (PFE) causes firewall filters on non-zero loopback interfaces (lo0.n) to not execute when those interfaces are in the global VRF/default routing-instance, allowing unauthenticated attackers to bypass control-plane filtering on affected MX Series devices with MPC10, MPC11, LC4800, LC9600 line cards or MX304.

MitigationApply the appropriate Junos OS patch (23.2R2-S6 or later, 23.4R2-S7 or later, 24.2R2 or later, or 24.4R2 or later) to affected MX Series devices with the specified line cards. Verify firewall filter counters are incrementing after the upgrade.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
JunosOperating system
Affected:< 23.2= 23.2= 23.4= 24.2= 24.4

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm device is MX Series
    Run 'show chassis hardware' or 'show system information' to verify the device is an MX Series router
    Affected if Device is not an MX Series model
  2. Verify line card type
    Run 'show chassis hardware' and identify if the device has MPC10, MPC11, LC4800, LC9600 line cards or is an MX304
    Affected if Device does not have any of the listed line cards (MPC10, MPC11, LC4800, LC9600) and is not an MX304
  3. Check installed Junos version
    Run 'show version' to identify the exact Junos OS version installed
    Affected if Version is not in the affected range: < 23.2, 23.2, 23.4, 24.2, or 24.4
  4. Identify non-zero loopback interfaces in global VRF
    Run 'show configuration interfaces lo0' and 'show configuration routing-instances' to find lo0.n interfaces configured in the default/global routing instance
    Affected if No lo0.n (lo0.1, lo0.2, etc.) interfaces exist in the global VRF/default routing-instance
  5. Check for firewall filters on loopback interfaces
    Run 'show configuration interfaces lo0.n | find filter' for each lo0.n interface to see if input or output firewall filters are applied
    Affected if No firewall filters are applied to non-zero loopback interfaces in the global VRF

Device is affected if it is an MX Series with MPC10/MPC11/LC4800/LC9600 line cards or MX304, running a vulnerable Junos version, with lo0.n interfaces in the global VRF that have firewall filters applied but those filters are not executing (counter shows no hits).

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 23.2 or later
Fixed in 23.2
Interim mitigation

Apply the appropriate Junos OS patch (23.2R2-S6 or later, 23.4R2-S7 or later, 24.2R2 or later, or 24.4R2 or later) to affected MX Series devices with the specified line cards. Verify firewall filter counters are incrementing after the upgrade.

Recommended fix High confidence

23.4R2-S7, 24.2R2, or 24.4R2 (any fixed release appropriate for deployment timeline)

  1. 1. Identify the current Junos OS version on the MX Series device using 'show version'
  2. 2. Confirm the device has an affected line card (MPC10, MPC11, LC4800, LC9600) or is an MX304 using 'show chassis hardware'
  3. 3. Verify the affected configuration: check if firewall filters are applied to non-0 loopback units (lo0.n where n>0) that are not bound to any routing-instance using 'show configuration interfaces lo0'
  4. 4. Download the appropriate fixed Junos OS release: 23.2R2-S6 or later, 23.4R2-S7 or later, 24.2R2 or later, or 24.4R2 or later
  5. 5. Upload the Junos OS upgrade package to the device using 'file copy' or FTP/SCP
  6. 6. Install the upgrade using 'request system software add <package_name>' with appropriate reboot
  7. 7. After reboot, verify the fix by applying a test firewall filter to lo0.n and confirming counters increment using 'show firewall counter filter <filter_name>'
  8. 8. Alternatively, migrate affected loopback interfaces into explicit routing-instance configurations as a configuration-based workaround
Caveat Standard Junos OS upgrade risks apply - ensure compatibility with other network devices and review release notes for any behavioral changes

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Junos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing12.0 h
  • Review / QA4.0 h
28.0 hours of engineering $4,760
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-33774 in production — separate from our analysis above.

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What this is

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