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JunosOperating system · Juniper

CVE-2021-0237

MEDIUM · 6.5 CVSS v3.1 Published 2021-04-22
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
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
On Juniper Networks EX4300-MP Series, EX4600 Series, EX4650 Series, QFX5K Series deployed as a Virtual Chassis with a specific Layer 2 circuit configuration, Packet Forwarding Engine manager (FXPC) process may crash and restart upon receipt of specific layer 2 frames. Continued receipt and processing of this packet will create a sustained Denial of Service (DoS) condition. This issue affects: Juniper Networks Junos OS on EX4300-MP Series, EX4600 Series, EX4650 Series, QFX5K Series 15.1 versions prior to 15.1R7-S9; 17.3 versions prior to 17.3R3-S11; 17.4 versions prior to 17.4R2-S13, 17.4R3-S4, 17.4R3-S5; 18.2 versions prior to 18.2R3-S8; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R2-S7, 18.4R3-S6; 19.1 versions prior to 19.1R3-S4; 19.2 versions prior to 19.2R1-S6, 19.2R3-S1; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S4, 19.4R3-S1; 20.1 versions prior to 20.1R2; 20.2 versions prior to 20.2R2, 20.2R3; 20.3 versions prior to 20.3R1-S2, 20.3R2;

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 Juniper EX4300-MP, EX4600, EX4650, and QFX5K Series switches configured as Virtual Chassis with Layer 2 circuit, the Packet Forwarding Engine manager (FXPC) process crashes and restarts when receiving specific crafted Layer 2 frames, creating a sustained DoS condition.

MitigationApply the appropriate Junos OS version per Juniper's advisory (15.1R7-S9 and later, 17.3R3-S11 and later, etc.) after confirming Virtual Chassis with L2 circuit configuration exists. No workarounds documented; upgrade is required.

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:= 15.1= 17.3= 17.4= 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
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 switch model
    Run 'show chassis hardware' or check the physical device labeling to confirm it is an EX4300-MP, EX4600, EX4650, or QFX5K Series switch
    Affected if The device is NOT one of these models - the CVE does not apply
  2. Check the Junos OS version
    Run 'show version' to display the installed Junos version
    Affected if The version matches 15.1X64, 17.3, 17.4, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, or 20.2 (specific builds matter; compare against Juniper's advisory for exact affected builds) - the device is potentially affected if other conditions also apply
  3. Verify Virtual Chassis is configured
    Run 'show virtual-chassis' to confirm the device is part of a Virtual Chassis configuration
    Affected if Virtual Chassis is NOT configured - the CVE does not apply
  4. Confirm Layer 2 circuit is enabled
    Run 'show configuration protocols l2-circuit' or review the active configuration for 'protocols l2-circuit' statements
    Affected if Layer 2 circuit (l2-circuit) is NOT configured under the protocols stanza - the CVE does not apply

The device is affected only if it is an EX4300-MP, EX4600, EX4650, or QFX5K Series switch running a vulnerable Junos version AND configured as Virtual Chassis WITH Layer 2 circuit 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.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the appropriate Junos OS version per Juniper's advisory (15.1R7-S9 and later, 17.3R3-S11 and later, etc.) after confirming Virtual Chassis with L2 circuit configuration exists. No workarounds documented; upgrade is required.

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-2021-0237 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