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JunosOperating system · Juniper

CVE-2021-31365

MEDIUM · 6.5 CVSS v3.1 Published 2021-10-19
Fix available
A fix is available. Upgrade to 18.1 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 Uncontrolled Resource Consumption vulnerability in Juniper Networks Junos OS on EX2300, EX3400 and EX4300 Series platforms allows an adjacent attacker sending a stream of layer 2 frames will trigger an Aggregated Ethernet (AE) interface to go down and thereby causing a Denial of Service (DoS). By continuously sending a stream of specific layer 2 frames an attacker will sustain the Denial of Service (DoS) condition. This issue affects: Juniper Networks Junos OS EX4300 Series All versions prior to 15.1R7-S7; 16.1 versions prior to 16.1R7-S8; 17.1 versions prior to 17.1R2-S12; 17.2 versions prior to 17.2R3-S4; 17.3 versions prior to 17.3R3-S8; 17.4 versions prior to 17.4R2-S10, 17.4R3-S2; 18.1 versions prior to 18.1R3-S10; 18.2 versions prior to 18.2R2-S7, 18.2R3-S3; 18.3 versions prior to 18.3R2-S4, 18.3R3-S2; 18.4 versions prior to 18.4R1-S7, 18.4R2-S4, 18.4R3-S1; 19.1 versions prior to 19.1R1-S5, 19.1R2-S1, 19.1R3; 19.2 versions prior to 19.2R1-S5, 19.2R2; 19.3 versions prior to 19.3R2-S2, 19.3R3; 19.4 versions prior to 19.4R1-S2, 19.4R2. Juniper Networks Junos OS EX3400 and EX4300-MP Series All versions prior to 18.1R3-S12; 18.2 versions prior to 18.2R3-S7; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R2-S9, 18.4R3-S7; 19.1 versions prior to 19.1R2-S3, 19.1R3-S4; 19.2 versions prior to 19.2R3-S1; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R3-S1; 20.1 versions prior to 20.1R3; 20.2 versions prior to 20.2R3; 20.3 versions prior to 20.3R2. Juniper Networks Junos OS EX2300 Series All versions prior to 18.3R3-S5; 18.4 versions prior to 18.4R2-S9, 18.4R3-S9; 19.1 versions prior to 19.1R2-S3, 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-S1; 20.4 versions prior to 20.4R2-S1, 20.4R3; 21.1 versions prior to 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

This is an uncontrolled resource consumption vulnerability in Juniper Junos OS on EX2300, EX3400, and EX4300 series switches. An adjacent attacker with layer 2 network access can send a stream of specifically crafted layer 2 frames that triggers an Aggregated Ethernet (AE) interface to go down, causing denial of service. The attack can be sustained indefinitely by continuously sending the malicious frames.

MitigationUpgrade Junos OS to the fixed versions specified in the advisory (15.1R7-S7, 16.1R7-S8, 17.1R2-S12, 17.2R3-S4, 17.3R3-S8, 17.4R2-S10, 18.1R3-S10, 18.2R3-S7, 18.3R3-S4, 18.4R3-S7, 19.4R3-S1, 20.3R2, 20.4R3, 21.1R2 or later depending on platform). During the upgrade window, monitor AE bundle status and consider network segmentation to limit adjacent attacker access.

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= 18.1= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 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
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 device label to confirm it is an EX2300, EX3400, or EX4300 series switch
    Affected if The device is not one of these three model series, then it is not affected by this CVE
  2. Check the Junos OS version
    Run 'show version' and note the firmware version number
    Affected if The version is 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, 20.2, or 20.3, or any version below 18.1, indicating the device falls within the affected version range
  3. Verify if Aggregated Ethernet (AE) interfaces exist
    Run 'show interfaces ae* terse' or 'show configuration interfaces ae' to list any configured AE bundle interfaces
    Affected if No AE interfaces are configured, the specific exploit path does not apply to this device
  4. Confirm layer 2 network accessibility
    Assess whether untrusted adjacent devices can send layer 2 frames to the switch, typically via access ports or trunk ports without proper network segmentation
    Affected if Adjacent attackers with layer 2 access can reach the switch, creating the conditions for exploitation

If the device is an EX2300, EX3400, or EX4300 running a Junos version in the 18.1 through 20.3 range with configured AE interfaces and accessible layer 2 network segments, the environment is affected by this CVE.

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 18.1 or later
Fixed in 18.1
Interim mitigation

Upgrade Junos OS to the fixed versions specified in the advisory (15.1R7-S7, 16.1R7-S8, 17.1R2-S12, 17.2R3-S4, 17.3R3-S8, 17.4R2-S10, 18.1R3-S10, 18.2R3-S7, 18.3R3-S4, 18.4R3-S7, 19.4R3-S1, 20.3R2, 20.4R3, 21.1R2 or later depending on platform). During the upgrade window, monitor AE bundle status and consider network segmentation to limit adjacent attacker access.

Recommended fix High confidence

For EX4300: 19.4R2 or later; For EX3400: 20.3R2 or later; For EX2300: 21.1R2 or later

  1. 1. Identify the exact EX Series model (EX2300, EX3400, or EX4300) in your environment using 'show chassis hardware' command
  2. 2. Check current Junos OS version using 'show version' command
  3. 3. For EX4300 Series: Upgrade to Junos 19.4R2 or later, or 20.1R3/20.2R3/20.3R2 depending on your support window
  4. 4. For EX3400 Series: Upgrade to Junos 20.3R2 or later (or the latest 19.4R3-S1 if staying on 19.x branch)
  5. 5. For EX2300 Series: Upgrade to Junos 21.1R2 or later (or the latest 20.4R3 if staying on 20.x branch)
  6. 6. Download the appropriate Junos software from the Juniper Networks support portal
  7. 7. Copy the software to the device using 'file copy' or FTP/SCP to /var/tmp/
  8. 8. Install the software using 'request system software add <package> reboot'
Caveat Upgrading Junos OS may introduce compatibility issues with existing configurations or connected devices; review release notes for behavioral changes before upgrading

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
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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-2021-31365 in production — separate from our analysis above.

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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
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