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JunosOperating system · Juniper

CVE-2021-31351

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 Check for Unusual or Exceptional Conditions in packet processing on the MS-MPC/MS-MIC utilized by Juniper Networks Junos OS allows a malicious attacker to send a specific packet, triggering the MS-MPC/MS-MIC to reset, causing a Denial of Service (DoS). Continued receipt and processing of this packet will create a sustained Denial of Service (DoS) condition. This issue only affects specific versions of Juniper Networks Junos OS on MX Series: 17.3R3-S11; 17.4R2-S13; 17.4R3 prior to 17.4R3-S5; 18.1R3-S12; 18.2R2-S8, 18.2R3-S7, 18.2R3-S8; 18.3R3-S4; 18.4R3-S7; 19.1R3-S4, 19.1R3-S5; 19.2R1-S6; 19.3R3-S2; 19.4R2-S4, 19.4R2-S5; 19.4R3-S2; 20.1R2-S1; 20.2R2-S2, 20.2R2-S3, 20.2R3; 20.3R2, 20.3R2-S1; 20.4R1, 20.4R1-S1, 20.4R2; 21.1R1; This issue does not affect any version of Juniper Networks Junos OS prior to 15.1X49-D240;

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

This is an improper check for unusual or exceptional conditions vulnerability in packet processing on the MS-MPC/MS-MIC modules used by Juniper Networks Junos OS on MX Series routers. An unauthenticated remote attacker can send a specific crafted packet that triggers a reset of the MS-MPC/MS-MIC component, causing denial of service. Repeated receipt of this packet creates a sustained DoS condition.

MitigationUpgrade Junos OS to a fixed version as specified in the Juniper advisory (versions 17.4R3-S5 and later, 18.2R3-S9 and later, etc.) or apply relevant security patches. Consider implementing packet filtering or rate limiting on the MS-MPC/MS-MIC interfaces as a temporary mitigation while planning 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:= 17.3= 17.4= 18.1= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 20.1= 20.2

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 is an MX Series router
    Run 'show chassis hardware' or check the device model via 'show version' to verify the hardware is from the MX Series family
    Affected if The device is not an MX Series router, the vulnerability does not apply
  2. Identify the Junos OS version
    Run 'show version' or 'show version detail' to retrieve the installed Junos OS version number
    Affected if The version matches one of the affected releases: 17.3, 17.4, 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, or 20.2
  3. Verify MS-MPC or MS-MIC module is present
    Run 'show chassis hardware' and look for MS-MPC or MS-MIC in the hardware inventory, or run 'show ms-MPC status' if available
    Affected if No MS-MPC or MS-MIC module is installed, the vulnerability cannot be triggered
  4. Check module is active and processing traffic
    Run 'show services/ms-MPC interface' or 'show interfaces ms-*' to confirm the multiservices module is online and operational
    Affected if The MS-MPC/MS-MIC module is physically present but not online or not configured for packet processing, exploitation is not possible

A user is affected if they are running an MX Series router with an online MS-MPC or MS-MIC module on any of the listed vulnerable Junos versions.

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 a fixed version as specified in the Juniper advisory (versions 17.4R3-S5 and later, 18.2R3-S9 and later, etc.) or apply relevant security patches. Consider implementing packet filtering or rate limiting on the MS-MPC/MS-MIC interfaces as a temporary mitigation while planning the upgrade.

Recommended fix High confidence

Upgrade to 21.1R2 or later (or the latest available LTS release). For specific branch fixes: 17.4R3-S5+, 18.1R3-S13+, 18.2R3-S9+, 18.3R3-S5+, 18.4R3-S8+, 19.1R3-S6+, 19.2R1-S7+, 19.3R3-S3+, 19.4R2-S6+, 19.4R3-S3+, 20.1R2-S2+, 20.2R3-S1+, 20.3R3+, 20.4R3+

  1. 1. Identify the current Junos OS version on the affected MX Series device using 'show version'
  2. 2. Check if the MS-MPC/MS-MIC service is enabled and in use
  3. 3. Plan maintenance window for the upgrade as this will require a system reboot
  4. 4. Download the appropriate fixed Junos OS version from the Juniper Networks support portal
  5. 5. Upload the Junos OS upgrade package to the device's /var/tmp directory
  6. 6. Run 'request system software add /var/tmp/<junos-file>.tgz reboot' to upgrade and reboot
  7. 7. After reboot, verify the new Junos version with 'show version'
  8. 8. Confirm MS-MPC/MS-MIC is operational with 'show services ms-pcc interface'
Caveat Standard Junos upgrade - review release notes for any configuration changes between versions. May require 30-60 minute maintenance window for reboot.

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

Fix this in Junos Scoped from the published advisory
  • Consultation6.0 h
  • Implementation8.0 h
  • Testing8.0 h
  • Review / QA4.0 h
26.0 hours of engineering $4,560
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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-2021-31351 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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