dbcve.org
JunosOperating system · Juniper

CVE-2021-31369

MEDIUM · 5.3 CVSS v3.1 Published 2021-10-19
Fix available
A fix is available. Upgrade to 17.4 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
On MX Series platforms with MS-MPC/MS-MIC, an Allocation of Resources Without Limits or Throttling vulnerability in Juniper Networks Junos OS allows an unauthenticated network attacker to cause a partial Denial of Service (DoS) with a high rate of specific traffic. If a Class of Service (CoS) rule is attached to the service-set and a high rate of specific traffic is processed by this service-set, for some of the other traffic which has services applied and is being processed by this MS-MPC/MS-MIC drops will be observed. Continued receipted of this high rate of specific traffic will create a sustained Denial of Service (DoS) condition. This issue affects: Juniper Networks Junos OS on MX Series with MS-MPC/MS-MIC: All versions prior to 17.4R3-S5; 18.3 versions prior to 18.3R3-S5; 18.4 versions prior to 18.4R3-S9; 19.1 versions prior to 19.1R3-S6; 19.2 versions prior to 19.2R1-S7, 19.2R3-S3; 19.3 versions prior to 19.3R2-S7, 19.3R3-S3; 19.4 versions prior to 19.4R3-S5; 20.1 versions prior to 20.1R2-S2, 20.1R3-S1; 20.2 versions prior to 20.2R3-S2; 20.3 versions prior to 20.3R3; 20.4 versions prior to 20.4R2-S1, 20.4R3; 21.1 versions prior to 21.1R1-S1, 21.1R2.

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

On MX Series with MS-MPC/MS-MIC, Junos OS fails to properly limit or throttle resources when processing high-rate traffic through a service-set with CoS rules attached. An unauthenticated attacker sending high-rate traffic can exhaust resources, causing packet drops on other legitimate traffic flows processed by the MS-MPC/MS-MIC, resulting in partial or sustained DoS.

MitigationUpgrade Junos OS to a fixed version (17.4R3-S5 or later, 18.3R3-S5 or later, etc.). If immediate upgrade is not feasible, consider network-based rate limiting or traffic filtering upstream to reduce high-rate traffic reaching the affected service-set.

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:< 17.4= 17.4= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 20.1= 20.2= 20.3= 20.4

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
Low

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

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 MX Series hardware with MS-MPC/MS-MIC
    Run 'show chassis hardware' and look for MS-MPC or MS-MIC in the hardware inventory
    Affected if The device is an MX Series router with MS-MPC or MS-MIC line cards installed
  2. Check installed Junos OS version
    Run 'show version' and note the Junos OS release version
    Affected if The version matches or falls within: < 17.4, = 17.4, = 18.3, = 18.4, = 19.1, = 19.2, = 19.3, = 19.4, = 20.1, = 20.2, = 20.3, or = 20.4
  3. Verify service-set configuration with CoS rules
    Run 'show configuration | display set | match service-set' to list configured service-sets, then inspect each for CoS rule attachments using 'show configuration service-sets <name>'
    Affected if A service-set exists with CoS rules (classifier, scheduler, or rewrite rules) attached
  4. Confirm service-set is active and processing traffic
    Run 'show services service-set statistics' to see if the service-set is actively processing traffic
    Affected if The service-set shows active traffic statistics indicating it is processing packets through the MS-MPC/MS-MIC

If the device is an MX Series with MS-MPC/MS-MIC, runs an affected Junos version listed above, and has an active service-set with CoS rules, the environment is vulnerable to resource exhaustion from high-rate traffic.

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 17.4 or later
Fixed in 17.4
Interim mitigation

Upgrade Junos OS to a fixed version (17.4R3-S5 or later, 18.3R3-S5 or later, etc.). If immediate upgrade is not feasible, consider network-based rate limiting or traffic filtering upstream to reduce high-rate traffic reaching the affected service-set.

Recommended fix High confidence

17.4R3-S5 (or 18.3R3-S5 or 18.4R3-S9 depending on your current branch)

  1. 1. Backup the current Juniper Junos OS configuration using 'request system snapshot' or via the management interface
  2. 2. Verify the current Junos version using 'show version' command
  3. 3. Download the appropriate fixed Junos software version from the Juniper Networks support portal: 17.4R3-S5 for 17.4, 18.3R3-S5 for 18.3, or 18.4R3-S9 for 18.4
  4. 4. Transfer the Junos software package to the MX Series device using FTP, SCP, or USB
  5. 5. Install the new Junos software package using 'request system software add <package-name>' command
  6. 6. Reboot the device using 'request system reboot' command to complete the installation
  7. 7. After reboot, verify the new Junos version is installed using 'show version'
  8. 8. Verify that services are functioning normally and the CoS/service-set is working as expected
Caveat Standard Junos upgrade risks apply - ensure configuration backup before upgrading, and test in lab environment if possible

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

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,600.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2021-31369 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2021-31369 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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