dbcve.org
Junos Os EvolvedOperating system · Juniper

CVE-2021-0239

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
In Juniper Networks Junos OS Evolved, receipt of a stream of specific genuine Layer 2 frames may cause the Advanced Forwarding Toolkit (AFT) manager process (Evo-aftmand), responsible for handling Route, Class-of-Service (CoS), Firewall operations within the packet forwarding engine (PFE) to crash and restart, leading to a Denial of Service (DoS) condition. By continuously sending this specific stream of genuine Layer 2 frames, an attacker can repeatedly crash the PFE, causing a sustained Denial of Service (DoS). This issue affects Juniper Networks Junos OS Evolved: All versions prior to 20.4R1-EVO. This issue does not affect Junos OS versions.

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

In Juniper Networks Junos OS Evolved, the Advanced Forwarding Toolkit (AFT) manager process (Evo-aftmand) contains a flaw in handling specific legitimate Layer 2 frames, causing it to crash and restart. Since this process controls Route, CoS, and Firewall operations in the packet forwarding engine (PFE), its crash disrupts packet forwarding. An attacker can trigger repeated crashes by continuously sending the specific L2 frame sequence, causing sustained DoS.

MitigationUpgrade Junos OS Evolved to version 20.4R1-EVO or later. As an interim measure, consider network segmentation or access controls to limit exposure to untrusted L2 traffic sources.

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
Junos Os EvolvedOperating system
Affected:= 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
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 Junos OS Evolved version
    Run 'show version' or 'show system information' on the device to retrieve the installed Junos OS Evolved version
    Affected if The version listed is exactly 20.4 (any sub-variant) as this is the affected version range
  2. Verify the Evo-aftmand process is running
    Run 'show system processes process-name Evo-aftmand' or check process status via 'ps aux | grep aftmand'
    Affected if The AFT manager process is active and the system is accepting Layer 2 traffic
  3. Check for recent Evo-aftmand crash events
    Review system logs using 'show log messages' or 'show system core-dumps' for evidence of Evo-aftmand process restarts or crashes
    Affected if Logs show repeated Evo-aftmand crash events or unexpected process restarts
  4. Assess Layer 2 network exposure
    Evaluate whether untrusted or external Layer 2 traffic sources can reach the affected device. Check interface configurations and any applied Layer 2 filtering or port security settings.
    Affected if The device is directly exposed to untrusted Layer 2 traffic sources without network segmentation or access controls
  5. Confirm the AFT feature is enabled
    Run 'show configuration | match aftp' or check forwarding options to verify the Advanced Forwarding Toolkit manager is actively handling packet forwarding
    Affected if AFT manager is enabled and actively processing Layer 2 frames

A system is affected if it runs Junos OS Evolved version 20.4, has the AFT manager process active, and is exposed to Layer 2 traffic that can trigger the specific frame handling flaw causing Evo-aftmand crashes.

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 Evolved to version 20.4R1-EVO or later. As an interim measure, consider network segmentation or access controls to limit exposure to untrusted L2 traffic sources.

Fix this in Junos Os Evolved Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing12.0 h
  • Review / QA4.0 h
28.0 hours of engineering $4,760
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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-2021-0239 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
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data