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Nx OsOperating system · Cisco

CVE-2021-1368

HIGH · 8.8 CVSS v3.1 Published 2021-02-24
Fix available
A fix is available. Upgrade to 4.0 / 4.1 or later.
See remediation →
93/100
Remediation priority · Urgent
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 Unidirectional Link Detection (UDLD) feature of Cisco FXOS Software and Cisco NX-OS Software could allow an unauthenticated, adjacent attacker to execute arbitrary code with administrative privileges or cause a denial of service (DoS) condition on an affected device. This vulnerability is due to insufficient input validation. An attacker could exploit this vulnerability by sending crafted Cisco UDLD protocol packets to a directly connected, affected device. A successful exploit could allow the attacker to execute arbitrary code with administrative privileges or cause the Cisco UDLD process to crash and restart multiple times, causing the affected device to reload and resulting in a DoS condition. Note: The UDLD feature is disabled by default, and the conditions to exploit this vulnerability are strict. The attacker needs full control of a directly connected device. That device must be connected over a port channel that has UDLD enabled. To trigger arbitrary code execution, both the UDLD-enabled port channel and specific system conditions must exist. In the absence of either the UDLD-enabled port channel or the system conditions, attempts to exploit this vulnerability will result in a DoS condition. It is possible, but highly unlikely, that an attacker could control the necessary conditions for exploitation. The CVSS score reflects this possibility. However, given the complexity of exploitation, Cisco has assigned a Medium Security Impact Rating (SIR) to this vulnerability.

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's Unidirectional Link Detection (UDLD) protocol allows an adjacent attacker with full control of a directly connected device to execute arbitrary code with administrative privileges or cause a DoS. The flaw is due to insufficient input validation when processing crafted UDLD protocol packets on an affected device with UDLD enabled over a port channel.

MitigationEnsure UDLD remains disabled if not required; apply Cisco vendor patches for FXOS and NX-OS when available; verify that only trusted devices are directly connected to network infrastructure.

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:= 8.4\(3.108\)= 8.4\(3.117\)= 7.0\(3\)i5\(2\)= 7.0\(3\)i7\(9\)= 9.3\(5\)= 7.3\(8\)n1\(1\)= 7.3\(9\)n1\(0.823\)
Unified Computing SystemApplication
Affected:< 4.0\(4i\)>= 4.1, < 4.1\(2c\)
Firepower Extensible Operating SystemOperating system
Affected:= r231

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
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 NX-OS version
    Run 'show version' or check the system banner to determine the exact NX-OS version installed
    Affected if The version matches any of: 8.4(3), 8.4(3), 7.0(3)i5(2), 7.0(3)i7(9), 9.3(5), 7.3(8)n1(1), 7.3(9)n1(0.823)
  2. Identify the FXOS version
    Run 'show version' or check the Firepower chassis manager to determine the FXOS version
    Affected if The version equals r231
  3. Identify the UCS version
    Run 'show version' in UCS Manager or check the firmware associated with the fabric interconnects
    Affected if The version is before 4.0(4i) or between 4.1 and 4.1(2c) exclusive
  4. Check if UDLD is enabled globally
    Run 'show udld global' or 'show running-config | include udld' to check UDLD status
    Affected if UDLD is enabled (global state shows 'UDLD is enabled' or configuration lines exist)
  5. Check if UDLD is configured on any port channel
    Run 'show udld port-channel' or 'show running-config | include channel-group' to list port channel interfaces with UDLD
    Affected if UDLD is enabled on any port-channel interface (port-channel with UDLD mode not 'disabled')

The device is affected if it runs a vulnerable NX-OS, FXOS, or UCS version AND has UDLD enabled on a port channel interface.

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

Ensure UDLD remains disabled if not required; apply Cisco vendor patches for FXOS and NX-OS when available; verify that only trusted devices are directly connected to network infrastructure.

Fix this in Nx Os Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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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-2021-1368 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