CVE-2021-34703

Device reload via uninitialized LLDP parser buffer

Published Sep 23, 2021 · Updated Nov 7, 2024

Denial of service in Cisco IOS allows attackers to crash and reload affected devices through LLDP neighbor-table processing. The LLDP message parser leaves a buffer improperly initialized, so processing a corrupted neighbor-table state during CLI or SNMP retrieval crashes the device. LLDP must be enabled; exploitation requires authenticated table access or injected adjacent LLDP frames followed by table retrieval.

CVSS severity6.8
Medium
EPSS probability1.19%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Cisco IOS allows attackers to crash and reload affected devices through LLDP neighbor-table processing. The LLDP message parser leaves a buffer improperly initialized, so processing a corrupted neighbor-table state during CLI or SNMP retrieval crashes the device. LLDP must be enabled; exploitation requires authenticated table access or injected adjacent LLDP frames followed by table retrieval.

The record

CVE
CVE-2021-34703
Published
Sep 23, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco IOS
Classifications
CWE-456, T1499
Attack vector
adjacent
Privileges
authenticated

Timeline

How it unfolded

  1. Sep 23, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 7, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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  1. Affected versionversion=n/a

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesauthenticated

What is affected?

ClamAV · Cisco IOSversion=n/a

Published CVSS scores

6.8CiscoCVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-456Missing Initialization of a Variable
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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