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JunosOperating system · Juniper

CVE-2021-0289

MEDIUM · 5.3 CVSS v3.1 Published 2021-07-15
Fix available
A fix is available. Upgrade to 15.1 / 18.4 or later.
See remediation →
58/100
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
When user-defined ARP Policer is configured and applied on one or more Aggregated Ethernet (AE) interface units, a Time-of-check Time-of-use (TOCTOU) Race Condition vulnerability between the Device Control Daemon (DCD) and firewall process (dfwd) daemons of Juniper Networks Junos OS allows an attacker to bypass the user-defined ARP Policer. In this particular case the User ARP policer is replaced with default ARP policer. To review the desired ARP Policers and actual state one can run the command "show interfaces <> extensive" and review the output. See further details below. An example output is: show interfaces extensive | match policer Policer: Input: __default_arp_policer__ <<< incorrect if user ARP Policer was applied on an AE interface and the default ARP Policer is displayed Policer: Input: jtac-arp-ae5.317-inet-arp <<< correct if user ARP Policer was applied on an AE interface For all platforms, except SRX Series: This issue affects Juniper Networks Junos OS: All versions 5.6R1 and all later versions prior to 18.4 versions prior to 18.4R2-S9, 18.4R3-S9 with the exception of 15.1 versions 15.1R7-S10 and later versions; 19.4 versions prior to 19.4R3-S3; 20.1 versions prior to 20.1R3; 20.2 versions prior to 20.2R3-S2; 20.3 version 20.3R1 and later versions; 20.4 versions prior to 20.4R3; 21.1 versions prior to 21.1R2; This issue does not affect Juniper Networks Junos OS versions prior to 5.6R1. On SRX Series this issue affects Juniper Networks Junos OS: 18.4 versions prior to 18.4R2-S9, 18.4R3-S9; 19.4 versions prior to 19.4R3-S4; 20.1 versions prior to 20.1R3; 20.2 versions prior to 20.2R3-S2; 20.3 version 20.3R1 and later versions; 20.4 versions prior to 20.4R3; 21.1 versions prior to 21.1R2. This issue does not affect 18.4 versions prior to 18.4R1 on SRX Series. This issue does not affect Junos OS Evolved.

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

A TOCTOU race condition between the Device Control Daemon (DCD) and firewall process (dfwd) in Juniper Junos OS causes user-defined ARP Policers on Aggregated Ethernet (AE) interfaces to be replaced with the default ARP Policer, allowing an attacker to bypass intended rate limiting.

MitigationUpgrade Junos OS to a version beyond the fixed releases (18.4R2-S9, 19.4R3-S3, 20.1R3, 20.2R3-S2, 20.4R3, 21.1R2) and verify ARP policer configuration using 'show interfaces <> extensive' to confirm the correct policer is applied.

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:>= 5.7, < 15.1>= 15.2, < 18.4= 5.6= 18.4= 19.4= 20.1= 20.2= 20.4= 21.1= 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
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:A/AC:H/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. Check Junos OS version
    Run 'show version' or 'request system version' to obtain the installed Junos OS version number
    Affected if The installed version falls within any of the affected ranges: >= 5.7 and < 15.1; >= 15.2 and < 18.4; or versions 5.6, 18.4, 19.4, 20.1, 20.2, 20.4, 21.1, or 20.3
  2. Identify Aggregated Ethernet interfaces
    Run 'show interfaces ae* terse' or 'show interfaces | match ae' to list any active AE interfaces in the configuration
    Affected if Any Aggregated Ethernet (AE) interfaces exist in the environment
  3. Verify ARP policer configuration
    Run 'show configuration firewall' to inspect user-defined ARP policer definitions, then run 'show configuration interfaces ae*' to see if any AE interfaces reference a non-default ARP policer
    Affected if User-defined ARP policers are configured and assigned to AE interfaces
  4. Confirm applied policer on AE interfaces
    Run 'show interfaces <ae-X> extensive | grep policer' for each AE interface to display the actual policer currently applied
    Affected if The output shows the default ARP policer is applied instead of the user-defined one intended for that interface

If Junos version is affected, AE interfaces exist, and user-defined ARP policers are configured but the default policer is applied, the environment is vulnerable to this TOCTOU race condition.

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
Upgrade available Upgrade to 15.1 / 18.4 or later
Fixed in 15.118.4
Interim mitigation

Upgrade Junos OS to a version beyond the fixed releases (18.4R2-S9, 19.4R3-S3, 20.1R3, 20.2R3-S2, 20.4R3, 21.1R2) and verify ARP policer configuration using 'show interfaces <> extensive' to confirm the correct policer is applied.

Fix this in Junos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation6.0 h
  • Testing3.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,690
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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-2021-0289 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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