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JunosOperating system · Juniper

CVE-2026-21910

MEDIUM · 6.5 CVSS v3.1 Published 2026-01-15
Fix available
A fix is available. Upgrade to 21.4 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
An Improper Check for Unusual or Exceptional Conditions vulnerability in the packet forwarding engine (PFE) of Juniper Networks Junos OS on EX4k Series and QFX5k Series platforms allows an unauthenticated network-adjacent attacker flapping an interface to cause traffic between VXLAN Network Identifiers (VNIs) to drop, leading to a Denial of Service (DoS). On all EX4k and QFX5k platforms, a link flap in an EVPN-VXLAN configuration Link Aggregation Group (LAG) results in Inter-VNI traffic dropping when there are multiple load-balanced next-hop routes for the same destination. This issue is only applicable to systems that support EVPN-VXLAN Virtual Port-Link Aggregation Groups (VPLAG), such as the QFX5110, QFX5120, QFX5200, EX4100, EX4300, EX4400, and EX4650. Service can only be restored by restarting the affected FPC via the 'request chassis fpc restart slot <slot-number>' command. This issue affects Junos OS on EX4k and QFX5k Series:  * all versions before 21.4R3-S12,  * all versions of 22.2 * from 22.4 before 22.4R3-S8,  * from 23.2 before 23.2R2-S5,  * from 23.4 before 23.4R2-S5,  * from 24.2 before 24.2R2-S3, * from 24.4 before 24.4R2.

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

An improper check for unusual conditions in the Packet Forwarding Engine (PFE) causes Inter-VNI traffic to drop when a link flap occurs in an EVPN-VXLAN LAG configuration with multiple load-balanced next-hop routes, resulting in a DoS that requires FPC restart to recover.

MitigationUpgrade Junos OS to a fixed version (21.4R3-S12 or later, 22.4R3-S8 or later, 23.2R2-S5 or later, 23.4R2-S5 or later, 24.2R2-S3 or later, or 24.4R2 or later) on affected EX4k and QFX5k Series platforms.

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:< 21.4= 21.4= 22.2= 22.4= 23.2= 23.4= 24.2= 24.4

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
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. Confirm Junos OS version
    Run 'show version' on the device and compare the installed version against the affected ranges: versions < 21.4, 21.4, 22.2, 22.4, 23.2, 23.4, 24.2, or 24.4.
    Affected if The device runs any of the listed affected versions.
  2. Verify EVPN-VXLAN configuration exists
    Run 'show configuration | match evpn' or 'show configuration protocols evpn' to check if EVPN-VXLAN is configured.
    Affected if EVPN-VXLAN is configured on the device.
  3. Confirm LAG is configured in EVPN-VXLAN setup
    Run 'show configuration interface' and look for ae* (aggregated Ethernet) interfaces participating in the EVPN-VXLAN configuration, or check 'show configuration protocols evpn' for interface bindings.
    Affected if Aggregated Ethernet (LAG) interfaces are used in the EVPN-VXLAN configuration.
  4. Check for multiple load-balanced next-hop routes
    Run 'show route forwarding-table' or 'show route table <vrf> extensive' to inspect next-hop entries. Look for multiple ECMP (equal-cost multi-path) or load-balanced next hops for Inter-VNI routes.
    Affected if Inter-VNI routes have multiple load-balanced or ECMP next hops configured.

A device is affected if it runs a vulnerable Junos version AND has an EVPN-VXLAN LAG configuration with multiple load-balanced next-hop routes, where a link flap could trigger Inter-VNI traffic loss requiring FPC restart to recover.

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 21.4 or later
Fixed in 21.4
Interim mitigation

Upgrade Junos OS to a fixed version (21.4R3-S12 or later, 22.4R3-S8 or later, 23.2R2-S5 or later, 23.4R2-S5 or later, 24.2R2-S3 or later, or 24.4R2 or later) on affected EX4k and QFX5k Series platforms.

Recommended fix High confidence

21.4R3-S12 or later; 22.4R3-S8 or later; 23.2R2-S5 or later; 23.4R2-S5 or later; 24.2R2-S3 or later; or 24.4R2 or later - select the earliest fixed release in your major version branch

  1. 1. Identify the current Junos OS version on the device using 'show version'
  2. 2. Identify the EX4k or QFX5k platform model and FPC slot numbers using 'show chassis hardware'
  3. 3. Plan a maintenance window as the upgrade will require a system reboot
  4. 4. Download the appropriate fixed Junos OS release from the Juniper Networks support portal (kb.juniper.net) matching your platform
  5. 5. Upload the Junos OS upgrade package to the device using 'file copy' or FTP/SCP to /var/tmp/
  6. 6. Verify the integrity of the package using 'file checksum md5' against the known hash from Juniper
  7. 7. Initiate the upgrade using 'request system software add /var/tmp/<package-name> reboot'
  8. 8. After the device reboots, verify the new version is installed using 'show version'
Caveat Upgrading Junos OS may introduce changes to CLI commands or behavior; review release notes for your target version for any breaking changes before upgrading; ensure compatibility with other network devices in EVPN-VXLAN fabric

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

Fix this in Junos Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,750
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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-2026-21910 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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