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Unified Computing SystemApplication · Cisco

CVE-2021-1387

HIGH · 8.6 CVSS v3.1 Published 2021-02-24
Fix available
A fix is available. Upgrade to 4.0 / 4.1 or later.
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 the network stack of Cisco NX-OS Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability exists because the software improperly releases resources when it processes certain IPv6 packets that are destined to an affected device. An attacker could exploit this vulnerability by sending multiple crafted IPv6 packets to an affected device. A successful exploit could cause the network stack to run out of available buffers, impairing operations of control plane and management plane protocols and resulting in a DoS condition. Manual intervention would be required to restore normal operations on the affected device. For more information about the impact of this vulnerability, see the Details section of this advisory.

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 vulnerability in Cisco NX-OS network stack allows an unauthenticated remote attacker to cause DoS by sending crafted IPv6 packets. The software improperly releases resources when processing these packets, causing the network stack to exhaust available buffers and impair control plane and management plane protocols.

MitigationApply Cisco's available software patches for NX-OS; alternatively, filter or block untrusted IPv6 traffic at network perimeter until patching is feasible.

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
Unified Computing SystemApplication
Affected:>= 4.0, < 4.0\(4k\)>= 4.1, < 4.1\(1e\)
Nx OsOperating system
Affected:= 7.0\(0\)n1\(1\)= 7.0\(1\)n1\(1\)= 7.0\(2\)n1\(1\)= 7.0\(3\)n1\(1\)= 7.0\(4\)n1\(1\)= 7.0\(4\)n1\(1a\)= 7.0\(5\)n1\(1\)= 7.0\(5\)n1\(1a\)= 7.0\(6\)n1\(1\)= 7.0\(6\)n1\(2s\)= 7.0\(6\)n1\(3s\)= 7.0\(6\)n1\(4s\)

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 Cisco product and software version
    Run 'show version' on the device CLI to obtain the installed NX-OS or UCS software version
    Affected if The version matches one of the affected ranges: Cisco UCS >= 4.0 to < 4.0(4k), >= 4.1 to < 4.1(1e), or NX-OS 7.0(0)n1(1) through 7.0(6)n1(4s)
  2. Confirm IPv6 is configured on the device
    Run 'show ipv6 interface' or 'show running-config | include ipv6' to check if IPv6 is enabled
    Affected if IPv6 interfaces are active or IPv6 is configured in the running configuration - the vulnerability is exploited via crafted IPv6 packets
  3. Verify control plane and management plane protocols
    Check for any degradation in control plane protocols (e.g., BGP, OSPF, HSRP) or management access (SSH, SNMP) after receiving IPv6 traffic
    Affected if Device shows signs of resource exhaustion, buffer saturation, or protocol instability specifically after processing IPv6 traffic

A Cisco NX-OS or UCS device is affected if it runs a version in the listed vulnerable ranges AND has IPv6 enabled or configured, making it susceptible to the resource exhaustion DoS via crafted IPv6 packets.

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 4.0 / 4.1 or later
Fixed in 4.04.1
Interim mitigation

Apply Cisco's available software patches for NX-OS; alternatively, filter or block untrusted IPv6 traffic at network perimeter until patching is feasible.

Fix this in Unified Computing System Scoped from the published advisory
  • Consultation6.0 h
  • Implementation12.0 h
  • Testing6.0 h
  • Review / QA4.0 h
28.0 hours of engineering $4,980
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Scan for this in your stack

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dbcve dependency scanner

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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-2021-1387 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