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Ios XeOperating system · Cisco

CVE-2021-34767

HIGH · 7.4 CVSS v3.1 Published 2021-09-23
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
79/100
Remediation priority · High
No privileges 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 IPv6 traffic processing of Cisco IOS XE Wireless Controller Software for Cisco Catalyst 9000 Family Wireless Controllers could allow an unauthenticated, adjacent attacker to cause a Layer 2 (L2) loop in a configured VLAN, resulting in a denial of service (DoS) condition for that VLAN. The vulnerability is due to a logic error when processing specific link-local IPv6 traffic. An attacker could exploit this vulnerability by sending a crafted IPv6 packet that would flow inbound through the wired interface of an affected device. A successful exploit could allow the attacker to cause traffic drops in the affected VLAN, thus triggering the DoS condition.

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 logic error in IPv6 link-local traffic processing in Cisco IOS XE Wireless Controller Software for Catalyst 9000 Family Wireless Controllers allows an adjacent, unauthenticated attacker to cause an L2 loop in a VLAN by sending a crafted IPv6 packet through the wired interface, resulting in traffic drops and DoS for the affected VLAN.

MitigationApply the Cisco software update (patch) for CVE-2021-34767 to affected Cisco Catalyst 9000 Family Wireless Controllers. Until patched, monitor for unexpected L2 loops and IPv6 traffic anomalies in VLANs served by affected devices.

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
Ios XeOperating system
Affected:all versions= 3.15.1xbs= 3.15.2xbs= 16.7.1= 16.7.1a= 16.7.1b= 16.7.2= 16.7.3= 16.7.4= 16.8.1= 16.8.1a= 16.8.1b

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
Adjacent
Complexity
Low
Privileges
None
User interaction
None
Scope
Changed
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/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 the device model and software
    Run 'show version' or 'show controller' to confirm the device is a Cisco Catalyst 9000 Family Wireless Controller running Cisco IOS XE software
    Affected if Device is a Cisco Catalyst 9000 Family Wireless Controller running IOS XE
  2. Check the IOS XE version
    Run 'show version' and examine the IOS XE release number (for example, 16.7.1, 16.8.1, 3.15.1xbs). Compare against the affected version list: 3.15.1xbs, 3.15.2xbs, 16.7.1, 16.7.1a, 16.7.1b, 16.7.2, 16.7.3, 16.7.4, 16.8.1, 16.8.1a, 16.8.1b, or any version if the 'all versions' note applies
    Affected if The installed IOS XE version matches or falls within the affected version ranges
  3. Verify IPv6 is enabled
    Run 'show ipv6 interface brief' to confirm IPv6 is configured and active on any interfaces
    Affected if IPv6 is enabled on the device (the vulnerability affects IPv6 link-local traffic processing)
  4. Monitor for L2 loop symptoms
    Run 'show interface' and 'show spanning-tree' to check for unusually high traffic counts, CRC errors, or spanning-tree topology changes. Use 'show vlan' and 'show interface switchport' to identify VLANs experiencing traffic drops or loop conditions
    Affected if L2 loop indicators are present in any VLAN served by the affected device

The device is affected if it is a Cisco Catalyst 9000 Family Wireless Controller running a vulnerable IOS XE version with IPv6 enabled, and unexpected L2 loops or VLAN traffic anomalies are observed.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the Cisco software update (patch) for CVE-2021-34767 to affected Cisco Catalyst 9000 Family Wireless Controllers. Until patched, monitor for unexpected L2 loops and IPv6 traffic anomalies in VLANs served by affected devices.

Fix this in Ios Xe Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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Scan for this in your stack

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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-2021-34767 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