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

Severity: high (CVSS 7.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-09-23 · updated 2026-06-17
Weakness: CWE-20

Description

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.

Affected products

Cisco Firepower Extensible Operating System (FXOS): through 8.4\(3.115\); through 7.0\(3\)i7\(9\); through 7.3\(8\)n1\(1\); through 3.2\(3o\)a; through 4.1\(1a\)a
Cisco Fxos: from 2.2 before 2.2.2.148; from 2.3 before 2.3.1.216; from 2.4 before 2.4.1.273; from 2.6 before 2.6.1.224; from 2.7 before 2.7.1.143; from 2.8 before 2.8.1.143 — fixed in 2.2.2.148, 2.3.1.216, 2.4.1.273, 2.6.1.224, 2.7.1.143, 2.8.1.143
Cisco IOS: through 8.4\(3.115\); through 7.0\(3\)i7\(9\); through 7.3\(8\)n1\(1\); through 3.2\(3o\)a; through 4.1\(1a\)a
Cisco IOS XE: through 8.4\(3.115\); through 7.0\(3\)i7\(9\); through 7.3\(8\)n1\(1\); through 3.2\(3o\)a; through 4.1\(1a\)a
Cisco IOS XR Software: through 8.4\(3.115\); through 7.0\(3\)i7\(9\); through 7.3\(8\)n1\(1\); through 3.2\(3o\)a; through 4.1\(1a\)a
Cisco NX-OS: through 8.4\(3.115\); through 7.0\(3\)i7\(9\); through 7.3\(8\)n1\(1\); through 3.2\(3o\)a

How to fix

Upgrade to a fixed version: Cisco Fxos 2.2.2.148, 2.3.1.216, 2.4.1.273, 2.6.1.224. Check the vendor advisory for mitigations.

References

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