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CVE Record

CVE-2021-34714: Multiple Cisco Operating Systems Unidirectional Link Detection Denial of Service Vulnerability

A vulnerability in the Unidirectional Link Detection (UDLD) feature of Cisco FXOS Software, Cisco IOS Software, Cisco IOS XE Software, Cisco IOS XR Software, and Cisco NX-OS Software could allow an unauthenticated, adjacent attacker to cause an affected device to reload. This vulnerability is due to improper input validation of the UDLD packets. An attacker could exploit this vulnerability by sending specifically crafted UDLD packets to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a denial of service (DoS) condition. Note: The UDLD feature is disabled by default, and the conditions to exploit this vulnerability are strict. An attacker must have full control of a directly connected device. On Cisco IOS XR devices, the impact is limited to the reload of the UDLD process.

HighCVSS 7.4Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This flaw can let someone with control of a directly connected network device crash certain Cisco network operating systems when UDLD is enabled. The practical risk is availability loss, not data theft. Exposure is narrower because UDLD is off by default and exploitation requires adjacent network position.

Executive priority

Treat as a targeted network availability risk. Prioritize critical switching, routing, and security infrastructure where UDLD is enabled, especially on links to less trusted equipment. The narrow attack precondition lowers broad internet risk but not operational impact.

Technical view

CVE-2021-34714 is improper input validation in Cisco UDLD packet handling across FXOS, IOS, IOS XE, IOS XR, and NX-OS. A crafted UDLD packet from an adjacent attacker can trigger reload. IOS XR impact is limited to the UDLD process.

Likely exposure

Organizations are most exposed on Cisco devices using UDLD on links where the directly connected neighbor is compromised, unmanaged, or untrusted. Devices with UDLD disabled are not described as exposed by the source bundle.

Exploitation context

The sources do not report known active exploitation, and the CVE is not marked KEV. Exploitation conditions are strict: the attacker must control a directly connected device and reach the affected UDLD feature.

Researcher notes

Key constraints are adjacent attack vector, no authentication, availability-only impact, and CWE-20 input validation failure in UDLD packet handling. Affected-version detail is incomplete in the provided bundle, so rely on Cisco advisory matching for final scope.

Mitigation direction

  • Confirm whether UDLD is enabled; leave it disabled where not operationally required.
  • Review Cisco advisory for product-specific fixed releases and upgrade guidance.
  • Prioritize edge, partner, lab, and unmanaged-neighbor links for exposure reduction.
  • Ensure directly connected devices on UDLD-enabled links are trusted and controlled.
  • Monitor affected devices for unexpected reloads or UDLD process restarts.

Validation and detection

  • Inventory Cisco FXOS, IOS, IOS XE, IOS XR, and NX-OS devices.
  • Check device configuration for UDLD enabled on interfaces.
  • Map UDLD-enabled links to directly connected neighbor ownership and trust status.
  • Review maintenance records for Cisco advisory remediation or fixed software.
  • Inspect logs for unexplained device reloads or IOS XR UDLD process reloads.
Prepared
Confidence
medium
Sources
3

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-20: Exact CWE lookup

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cve · low confidence lookup

CVE-2021-34714 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.4 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

Official CVE source material

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.

1CVSS vectors
0Timeline events
0ADP providers
2Source links

CVSS vector scores

1 official score

We 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.

ScoreVersionSeverityVectorExploitImpactSource
7.4CVSS 3.1HighCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H2.84Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

7.4High
CVSS 3.1 vector shape for CVE-2021-34714Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
CiscoCisco IOSn/aListed
Weakness

CWE details

CWE-20

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.