Security readout for executives and security teams
Plain-English summary
Some Juniper Junos OS devices may silently use a default ARP rate limit instead of the stricter user-defined ARP policer on Aggregated Ethernet interfaces. That can let nearby network attackers overwhelm ARP handling and degrade availability. The issue is operationally important for networks relying on ARP policers for Layer 2 resilience.
Executive priority
Treat this as a moderate availability-risk fix for Juniper environments, especially service-provider, campus, or data-center networks depending on ARP controls. It should be scheduled in normal patch governance, with faster handling for exposed critical aggregation points.
Technical view
CVE-2021-0289 is a TOCTOU race between Junos OS DCD and dfwd processes. When a user-defined ARP policer is applied to AE interface units, the intended policer can be replaced by the default ARP policer. CVSS 3.1 is 6.5, adjacent attack vector, no privileges, and high availability impact.
Likely exposure
Exposure is most likely on Juniper Junos OS devices using Aggregated Ethernet interface units with user-defined ARP policers. Junos OS Evolved is listed as unaffected. Version applicability differs for SRX and non-SRX platforms, so inventory must be checked against Juniper’s advisory.
Exploitation context
The bundle does not show KEV listing or cited active exploitation. The risk is adjacent-network availability impact through bypass of intended ARP policing, not confidentiality or integrity compromise. Evidence is limited to the CVE record and Juniper advisory reference.
Researcher notes
Key triage detail is configuration-dependent exposure: affected Junos OS plus AE units plus user-defined ARP policer. The provided evidence names the incorrect runtime state but does not provide exploit tooling, active exploitation, or independent mitigation beyond vendor guidance.
Mitigation direction
- Upgrade affected Junos OS systems to fixed releases listed by Juniper.
- Prioritize devices using AE interfaces with custom ARP policers.
- Check Juniper JSA11191 for platform-specific SRX and non-SRX guidance.
- Use vendor-approved maintenance procedures for any temporary process restart workaround.
Validation and detection
- Inventory Junos OS version, platform family, and Junos OS Evolved status.
- Identify AE interface units configured with user-defined ARP policers.
- Compare intended ARP policer configuration with reported interface runtime state.
- Confirm the runtime state does not show the default ARP policer unexpectedly.
- Record whether SRX-specific affected version ranges apply.
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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Open ATT&CK lookupCVE-2021-0289 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/JSA11191CVE reference · x_refsource_CONFIRM
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.
Time-of-check Time-of-use (TOCTOU) Race Condition
Time-of-check Time-of-use (TOCTOU) Race Condition represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.