Security readout for executives and security teams
Plain-English summary
This is a denial-of-service flaw in Juniper Junos OS Evolved. Specific legitimate Layer 2 traffic can crash and restart a forwarding component, disrupting packet forwarding. The impact is availability only, but repeated traffic could sustain an outage on affected network devices.
Executive priority
Treat as a moderate operational risk for affected Juniper network infrastructure. Prioritize remediation where outages would affect customer traffic, data centers, or shared Layer 2 access networks. No confidentiality or integrity impact is described in the supplied sources.
Technical view
CVE-2021-0239 affects Junos OS Evolved before 20.4R1-EVO. A stream of specific genuine Layer 2 frames can crash Evo-aftmand, the AFT manager for route, CoS, and firewall operations in the PFE, causing repeated PFE disruption and DoS. Standard Junos OS is stated as unaffected.
Likely exposure
Exposure is limited to Juniper Networks devices running Junos OS Evolved versions earlier than 20.4R1-EVO. The CVSS vector indicates adjacent-network access is required, so risk is highest on shared or untrusted Layer 2 segments.
Exploitation context
The provided bundle does not show CISA KEV listing or other evidence of active exploitation. The attack condition is adjacent network reachability and repeated delivery of specific legitimate Layer 2 frames, resulting in availability impact rather than data compromise.
Researcher notes
The source bundle is specific but limited. It names the crashing process, affected product line, fixed threshold, and non-impact to Junos OS. It does not provide packet details, workarounds, exploit prevalence, or broader affected platform mapping.
Mitigation direction
- Upgrade affected Junos OS Evolved systems to 20.4R1-EVO or later.
- Review Juniper advisory JSA11134 for exact fixed-release and operational guidance.
- Prioritize edge, shared-access, and untrusted Layer 2 environments.
- Confirm standard Junos OS systems are not incorrectly included in remediation scope.
Validation and detection
- Inventory Juniper devices running Junos OS Evolved and record versions.
- Identify systems below 20.4R1-EVO as affected.
- Check device logs for Evo-aftmand crashes, restarts, or PFE instability.
- Map affected interfaces connected to adjacent or shared Layer 2 networks.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
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ATT&CK lookup starting points
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CWE-754: Exact CWE lookup
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Open ATT&CK lookupCVE-2021-0239 mapping review
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Open ATT&CK lookup- Severity
- Medium
- CVSS
- 6.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H2.83.6Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
6.5MediumVector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://kb.juniper.net/JSA11134CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Check for Unusual or Exceptional Conditions
Improper Check for Unusual or Exceptional Conditions represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.