Cisco IOS and IOS XE Software Link Layer Discovery Protocol Denial of Service Vulnerability

published 2021-09-23 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-21353

cvss: 6.8; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 1.2%; EPSS 1.2% chance of exploitation in 30 days, percentile 68%
public exploits: 0; none known

A vulnerability in the Link Layer Discovery Protocol (LLDP) message parser of Cisco IOS Software and Cisco IOS XE Software could allow an attacker to trigger a reload of an affected device, resulting in a denial of service (DoS) condition. This vulnerability is due to improper initialization of a buffer. An attacker could exploit this vulnerability via any of the following methods: An authenticated, remote attacker could access the LLDP neighbor table via either the CLI or SNMP while the device is in a specific state. An unauthenticated, adjacent attacker could corrupt the LLDP neighbor table by injecting specific LLDP frames into the network and then waiting for an administrator of the device or a network management system (NMS) managing the device to retrieve the LLDP neighbor table of the device via either the CLI or SNMP. An authenticated, adjacent attacker with SNMP read-only credentials or low privileges on the device CLI could corrupt the LLDP neighbor table by injecting specific LLDP frames into the network and then accessing the LLDP neighbor table via either the CLI or SNMP. A successful exploit could allow the attacker to cause the affected device to crash, resulting in a reload of the device.

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: partial
Assessed: 2 y ago (2024-11-07)

Exploit prediction

1.19% chance of exploitation in the next 30 days, higher than 68% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	6.8	CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H
NVD	3.1	6.5	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
NVD	2.0	6.8	AV:N/AC:L/Au:S/C:N/I:N/A:C

Affected products

Cisco Cisco IOS

Classification

Weakness

CWE-456 Missing Initialization of a Variable
CWE-665 Improper Initialization

Technology feeds

Cisco IOS rss critical rss

References

tools.cisco.com /security/center/content/CiscoSecurityAdvisory/cisco-sa-lldp-dos-sBnuHSjT
vendor-advisory

Sources

CVE List CVE-2021-34703
NVD CVE-2021-34703

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.

Summary