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JunosOperating system · Juniper

CVE-2021-31362

MEDIUM · 6.5 CVSS v3.1 Published 2021-10-19
Fix available
A fix is available. Upgrade to after 20.3 or later.
See remediation →
70/100
Remediation priority · Elevated
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
A Protection Mechanism Failure vulnerability in RPD (routing protocol daemon) of Juniper Networks Junos OS and Junos OS Evolved allows an adjacent unauthenticated attacker to cause established IS-IS adjacencies to go down by sending a spoofed hello PDU leading to a Denial of Service (DoS) condition. Continued receipted of these spoofed PDUs will create a sustained Denial of Service (DoS) condition. This issue affects: Juniper Networks Junos OS All versions prior to 18.2R3-S8; 18.3 versions prior to 18.3R3-S5; 18.4 versions prior to 18.4R3-S9; 19.1 versions prior to 19.1R3-S7; 19.2 versions prior to 19.2R1-S7, 19.2R3-S3; 19.3 versions prior to 19.3R2-S6, 19.3R3-S2; 19.4 versions prior to 19.4R3-S3; 20.1 versions prior to 20.1R3; 20.2 versions prior to 20.2R3; 20.3 versions prior to 20.3R3; 20.4 versions prior to 20.4R2. Juniper Networks Junos OS Evolved All versions prior to 20.4R2-EVO; 21.1 versions prior to 21.1R2-EVO.

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 a Protection Mechanism Failure vulnerability in the RPD (routing protocol daemon) of Juniper Networks Junos OS and Junos OS Evolved. An adjacent unauthenticated attacker can cause established IS-IS (Intermediate System to Intermediate System) adjacencies to go down by sending spoofed hello PDUs, leading to a Denial of Service. Continued receipt of these spoofed PDUs creates a sustained DoS condition.

MitigationUpgrade Junos OS or Junos OS Evolved to a fixed version as specified in the Juniper advisory. Consider network access controls to limit adjacent attackers from reaching the IS-IS interface.

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:<= 18.1= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 20.1= 20.2= 20.3= 20.4
Junos Os EvolvedOperating system
Affected:<= 20.3= 20.4= 21.1

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
Adjacent
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:A/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. Identify the Junos product variant
    Run 'show system information' or check the device model to confirm if the device runs Junos OS or Junos OS Evolved
    Affected if The device does not run Junos OS or Junos OS Evolved (not affected)
  2. Check the Junos OS version
    Run 'show version' to obtain the installed Junos OS version number
    Affected if The version matches one of the affected versions: 18.1 or earlier, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, 20.2, 20.3, or 20.4
  3. Check the Junos OS Evolved version
    Run 'show version' on Evolved devices to obtain the installed version
    Affected if The version matches one of the affected Evolved versions: 20.3 or earlier, 20.4, or 21.1
  4. Verify IS-IS protocol is configured
    Run 'show protocols isis' or 'show configuration protocols isis' to check if IS-IS is enabled on the device
    Affected if IS-IS protocol is not configured (the vulnerability only affects devices with IS-IS enabled)

The device is affected if it runs Junos OS or Junos OS Evolved with an affected version AND has IS-IS protocol configured and enabled.

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 a release after 20.3
Interim mitigation

Upgrade Junos OS or Junos OS Evolved to a fixed version as specified in the Juniper advisory. Consider network access controls to limit adjacent attackers from reaching the IS-IS interface.

Recommended fix High confidence

Junos OS: 20.4R2 or later (alternatives: 18.2R3-S8, 18.3R3-S5, 18.4R3-S9, 19.1R3-S7, 19.2R3-S3, 19.3R3-S2, 19.4R3-S3, 20.1R3, 20.2R3, 20.3R3) | Junos OS Evolved: 20.4R2-EVO or 21.1R2-EVO

  1. 1. Identify the current Junos OS or Junos OS Evolved version running on the affected device using the command: show version
  2. 2. Determine the appropriate upgrade path based on your current branch (18.x, 19.x, 20.x, or 21.x)
  3. 3. Download the corresponding fixed release from the Juniper Networks support portal
  4. 4. Upload the upgrade package to the device using: file copy or FTP/SCP to /var/tmp/
  5. 5. For Junos OS: Initiate the upgrade with: request system software add /var/tmp/<package-name> reboot
  6. 6. For Junos OS Evolved: Use: request system software evolve add /var/tmp/<package-name> reboot
  7. 7. After reboot, verify the new version and confirm IS-IS adjacencies are stable using: show isis adjacency
  8. 8. Monitor the device to ensure the DoS condition is resolved
Caveat Standard Junos upgrade best practices apply - review release notes for behavioral changes and test in a lab environment before deploying to production

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 / QA2.0 h
20.0 hours of engineering $3,500
Get the upgrade done

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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-2021-31362 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data