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iOSOperating system · Cisco

CVE-2021-34703

MEDIUM · 6.5 CVSS v3.1 Published 2021-09-23
Fix available
A fix is available. Upgrade to after 16.12.3 or later.
See remediation →
71/100
Remediation priority · Elevated
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A buffer initialization flaw in the LLDP message parser of Cisco IOS and IOS XE Software allows an attacker to cause a device crash and reload. Exploitation involves either accessing the LLDP neighbor table while the device is in a specific state, or injecting malicious LLDP frames and then triggering their retrieval via CLI or SNMP.

MitigationApply Cisco-provided software updates for IOS/IOS XE when available. Until then, limit adjacent network access to untrusted segments and restrict CLI/SNMP access to the LLDP neighbor table to trusted, authenticated administrators only.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
iOSOperating system
Affected:<= 16.12.3
Ios XeOperating system
Affected:<= 16.12.3

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify device type and software version
    Execute 'show version' or 'show running-config | include ios' to display the software version. Look for 'Cisco IOS' or 'Cisco IOS XE' and note the release number.
    Affected if The version number is 16.12.3 or lower for either Cisco iOS or Cisco IOS XE.
  2. Verify LLDP is enabled on the device
    Execute 'show lldp' or 'show lldp neighbors' to check if LLDP is globally enabled and running.
    Affected if LLDP is enabled and the device is transmitting or receiving LLDP frames.
  3. Confirm LLDP neighbor table is accessible
    Execute 'show lldp neighbors' or use SNMP to query the LLDP neighbor table (such as 'snmpwalk' on OID 1.0.8802.1.1.2).
    Affected if The LLDP neighbor table can be retrieved via CLI or SNMP without error, indicating the feature is active.
  4. Check LLDP configuration in running config
    Execute 'show running-config | include lldp' to view the LLDP configuration settings.
    Affected if LLDP is configured with 'lldp run' or similar enabling statements present.

A device is affected if it runs Cisco iOS or IOS XE version 16.12.3 or lower and has LLDP enabled, as exploitation requires LLDP to be active and its neighbor table accessible.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Upgrade available Upgrade to a release after 16.12.3
Interim mitigation

Apply Cisco-provided software updates for IOS/IOS XE when available. Until then, limit adjacent network access to untrusted segments and restrict CLI/SNMP access to the LLDP neighbor table to trusted, authenticated administrators only.

Fix this in iOS Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing5.0 h
  • Review / QA2.0 h
16.0 hours of engineering $2,790
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-34703 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data