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Fencer Vulns
high 7.4

CVE-2021-34714 json

Multiple Cisco Operating Systems Unidirectional Link Detection Denial of Service Vulnerability

published 2021-09-23 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-21364

cvss
7.4
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 33%
public exploits
0
none known

Summary

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.

Patch / fix

  • cisco fxos 2.2.2.148, 2.3.1.216, 2.4.1.273, 2.6.1.224, 2.7.1.143, 2.8.1.143, 2.9.1.135

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
2 y ago

Exploit prediction

0.39% chance of exploitation in the next 30 days, higher than 33% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 7.4 CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
NVD 3.1 7.4 CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
NVD 2.0 5.7 AV:A/AC:M/Au:N/C:N/I:N/A:C

Affected products

  • Cisco Cisco IOS

Classification

Weakness
  • CWE-20 Improper Input Validation
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.