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Nx OsOperating system · Cisco

CVE-2021-1523

HIGH · 8.6 CVSS v3.1 Published 2021-08-25
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
95/100
Remediation priority · Urgent
Remotely reachable 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
A vulnerability in Cisco Nexus 9000 Series Fabric Switches in Application Centric Infrastructure (ACI) Mode could allow an unauthenticated, remote attacker to cause a queue wedge on a leaf switch, which could result in critical control plane traffic to the device being dropped. This could result in one or more leaf switches being removed from the fabric. This vulnerability is due to mishandling of ingress TCP traffic to a specific port. An attacker could exploit this vulnerability by sending a stream of TCP packets to a specific port on a Switched Virtual Interface (SVI) configured on the device. A successful exploit could allow the attacker to cause a specific packet queue to queue network buffers but never process them, leading to an eventual queue wedge. This could cause control plane traffic to be dropped, resulting in a denial of service (DoS) condition where the leaf switches are unavailable. Note: This vulnerability requires a manual intervention to power-cycle the device to recover.

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 vulnerability in Cisco Nexus 9000 Series Fabric Switches in ACI Mode allows an unauthenticated remote attacker to cause a queue wedge by sending TCP packets to a specific port on a Switched Virtual Interface (SVI). The mishandling of ingress TCP traffic causes a packet queue to accumulate buffers without processing them, leading to control plane traffic being dropped and leaf switches being removed from the fabric.

MitigationApply Cisco-provided patches or firmware updates for this vulnerability; until then, consider implementing ACLs or network segmentation to restrict unauthorized access to SVI interfaces.

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
Nx OsOperating system
Affected:= 13.2\(3n\)= 14.2\(4i\)

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
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Changed
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/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 device model and mode
    Run 'show version' or 'show module' to confirm the hardware is a Cisco Nexus 9000 Series Fabric Switch. Then check if the switch is running in ACI mode using 'show system mode' or 'show aci mode'
    Affected if The device is a Nexus 9000 Series Fabric Switch running in ACI mode and has SVIs configured
  2. Determine the installed NX-OS version
    Execute 'show version' and locate the NX-OS software version string (typically displayed as 'system: version X.X(Xn)' or similar)
    Affected if The installed version is exactly 13.2(3n) or exactly 14.2(4i)
  3. Verify if Switched Virtual Interfaces are configured
    Run 'show ip interface brief' or 'show interface vlan' to list all active SVIs on the device
    Affected if Any SVI interfaces are present and active on the switch

The environment is affected if the device is a Cisco Nexus 9000 Series Fabric Switch in ACI Mode running NX-OS version 13.2(3n) or 14.2(4i) with Switched Virtual Interfaces configured.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply Cisco-provided patches or firmware updates for this vulnerability; until then, consider implementing ACLs or network segmentation to restrict unauthorized access to SVI interfaces.

Recommended fix High confidence

Contact Cisco for specific fixed release (advisory provides fixed software releases for versions 13.2 and 14.2)

  1. Contact Cisco TAC or access the official Cisco Security Advisory (cisco.com) to obtain the fixed software release for your Nx OS version
  2. Download the recommended fixed software release from the Cisco Software Download center
  3. Schedule a maintenance window as the advisory notes manual power-cycle is required for recovery
  4. Follow Cisco's standard upgrade procedure for Nexus 9000 Series switches in ACI mode
  5. After upgrade, verify the switch is properly rejoined to the fabric and SVI interfaces are functioning correctly
Caveat Review Cisco's release notes for any compatibility or feature changes between your current version and the fixed release

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Nx Os Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,130
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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-2021-1523 in production — separate from our analysis above.

No notes yet

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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