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JunosOperating system · Juniper

CVE-2021-31376

HIGH · 7.5 CVSS v3.1 Published 2021-10-19
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
84/100
Remediation priority · High
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 Input Validation vulnerability in Packet Forwarding Engine manager (FXPC) process of Juniper Networks Junos OS allows an attacker to cause a Denial of Service (DoS) by sending specific DHCPv6 packets to the device and crashing the FXPC service. Continued receipt and processing of this specific packet will create a sustained Denial of Service (DoS) condition. This issue affects only the following platforms in ACX Series: ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, ACX5096 devices. Other ACX platforms are not affected from this issue. This issue affects Juniper Networks Junos OS on ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, ACX5096: 18.4 version 18.4R3-S7 and later versions prior to 18.4R3-S8. This issue does not affect: Juniper Networks Junos OS 18.4 versions prior to 18.4R3-S7 on ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, ACX5096.

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 input validation vulnerability in the Packet Forwarding Engine manager (FXPC) process of Juniper Junos OS allows remote attackers to cause denial of service by sending specific DHCPv6 packets to affected ACX Series devices (ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, ACX5096). The malicious packets crash the FXPC service, and continued receipt creates a sustained DoS condition.

MitigationUpgrade Junos OS to version 18.4R3-S8 or later on affected ACX Series devices. If immediate upgrade is not possible, consider filtering DHCPv6 traffic at network perimeter to reduce attack surface.

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:= 18.4

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
None
Integrity
None
Availability
High

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

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 model is affected ACX Series
    Run 'show chassis hardware' or 'show version' to identify the exact ACX model (ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, or ACX5096)
    Affected if Device is any of these eight ACX Series models
  2. Check Junos OS version is exactly 18.4
    Run 'show version' and locate the Junos OS version string; look for version 18.4 without any R suffix or with any R1-R7 variant (these are NOT affected, only exact version 18.4)
    Affected if Version string shows exactly 18.4 (no R1-R7 suffix)
  3. Verify DHCPv6 service is configured or active
    Run 'show configuration | match dhcpv6' or 'show system services dhcpv6' to check if DHCPv6 processing is enabled on the device
    Affected if DHCPv6 service is configured and active on the device
  4. Check if FXPC process is running
    Run 'show system processes' or 'request system process' to verify the fxpc (Packet Forwarding Engine manager) process is active
    Affected if FXPC process is running on the device

Device is affected if it is an ACX Series model (ACX500/ACX1000/ACX1100/ACX2100/ACX2200/ACX4000/ACX5048/ACX5096) running exactly Junos OS version 18.4 with DHCPv6 service enabled and FXPC process active.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade Junos OS to version 18.4R3-S8 or later on affected ACX Series devices. If immediate upgrade is not possible, consider filtering DHCPv6 traffic at network perimeter to reduce attack surface.

Recommended fix High confidence

Junos OS 18.4R3-S8 or later 18.4 stable release

  1. 1. Identify the device model - confirm it is one of: ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, or ACX5096
  2. 2. Check current Junos OS version using 'show version' command
  3. 3. Confirm the current version is 18.4R3-S7 or later but prior to 18.4R3-S8 (vulnerable range)
  4. 4. Download Junos OS 18.4R3-S8 or a later stable 18.4 release from the Juniper Networks support portal
  5. 5. Upload the firmware to the device using 'request system software add' or through the web management interface
  6. 6. Reboot the device to apply the new firmware using 'request system reboot'
  7. 7. After reboot, verify the new version is installed using 'show version'
  8. 8. Verify the FXPC process is running normally using 'show system processes extensive | match fxpc'
Caveat Standard Junos upgrade precautions apply - review release notes for any configuration changes, ensure backups are made, and plan for potential maintenance window

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
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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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-2021-31376 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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