dbcve.org
JunosOperating system · Juniper

CVE-2021-0259

HIGH · 7.4 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 →
79/100
Remediation priority · High
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
Due to a vulnerability in DDoS protection in Juniper Networks Junos OS and Junos OS Evolved on QFX5K Series switches in a VXLAN configuration, instability might be experienced in the underlay network as a consequence of exceeding the default ddos-protection aggregate threshold. If an attacker on a client device on the overlay network sends a high volume of specific, legitimate traffic in the overlay network, due to an improperly detected DDoS violation, the leaf might not process certain L2 traffic, sent by spines in the underlay network. Continued receipt and processing of the high volume traffic will sustain the Denial of Service (DoS) condition. This issue affects: Juniper Networks Junos OS on QFX5K Series: 17.3 versions prior to 17.3R3-S11; 17.4 versions prior to 17.4R3-S5; 18.1 versions prior to 18.1R3-S13; 18.2 versions prior to 18.2R2-S8, 18.2R3-S8; 18.3 versions prior to 18.3R3-S5; 18.4 versions prior to 18.4R1-S8, 18.4R2-S6, 18.4R3-S6; 19.1 versions prior to 19.1R3-S4; 19.2 versions prior to 19.2R1-S6, 19.2R3-S2; 19.3 versions prior to 19.3R3-S2; 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; 20.3 versions prior to 20.3R1-S2, 20.3R2. Juniper Networks Junos OS Evolved on QFX5220: All versions prior to 20.3R2-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

A vulnerability in the DDoS protection mechanism on Juniper Networks Junos OS and Junos OS Evolved for QFX5K Series switches in VXLAN configurations causes improper detection of DDoS violations. When an attacker sends high volumes of legitimate-looking traffic across the overlay network, the system incorrectly triggers DDoS protection, causing the leaf switches to drop legitimate L2 traffic from spines in the underlay network, resulting in sustained DoS conditions.

MitigationUpgrade Junos OS to one of the specified fixed versions (17.3R3-S11, 17.4R3-S5, 18.1R3-S13, 18.2R2-S8/18.2R3-S8, 18.3R3-S5, 18.4R1-S8/18.4R2-S6/18.4R3-S6, 19.1R3-S4, 19.2R1-S6/19.2R3-S2, 19.3R3-S2, 19.4R2-S4/19.4R3-S1, 20.1R2, 20.2R2, 20.3R1-S2/20.3R2, or 20.3R2-EVO for Evolved) or consider adjusting DDoS protection thresholds as a temporary mitigation if upgrading is not immediately feasible.

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
Junos Os EvolvedOperating system
Affected:= 18.3= 19.1= 19.2= 19.3= 20.1= 20.2= 20.3

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
Changed
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/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 device model
    Run 'show chassis hardware' or check the device model. This vulnerability only affects QFX5K Series switches.
    Affected if Device is NOT a QFX5K Series switch - likely not affected
  2. Check Junos OS version
    Run 'show version' to obtain the installed Junos version. Compare against the affected ranges: Junos 17.3 through 20.2, or Junos OS Evolved 18.3 through 20.3.
    Affected if Installed version falls within the listed affected versions and no fixed version is applied
  3. Verify VXLAN configuration
    Run 'show configuration protocols vxlan' or 'show configuration protocols evpn' to check if VXLAN overlay is configured.
    Affected if VXLAN overlay network is NOT configured - likely not affected since the flaw requires overlay traffic to trigger
  4. Confirm DDoS protection is active
    Run 'show ddos protection statistics' or check 'show configuration protocols ddos-protection' to verify DDoS protection is enabled on the device.
    Affected if DDoS protection is disabled - the vulnerability cannot be triggered

A defender is affected if they run a QFX5K Series switch with an affected Junos version AND have VXLAN overlay with DDoS protection enabled, as the flaw requires the overlay network and DDoS mechanism to be present.

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

Upgrade Junos OS to one of the specified fixed versions (17.3R3-S11, 17.4R3-S5, 18.1R3-S13, 18.2R2-S8/18.2R3-S8, 18.3R3-S5, 18.4R1-S8/18.4R2-S6/18.4R3-S6, 19.1R3-S4, 19.2R1-S6/19.2R3-S2, 19.3R3-S2, 19.4R2-S4/19.4R3-S1, 20.1R2, 20.2R2, 20.3R1-S2/20.3R2, or 20.3R2-EVO for Evolved) or consider adjusting DDoS protection thresholds as a temporary mitigation if upgrading is not immediately feasible.

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
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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-2021-0259 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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