CVE-2021-34767

VLAN traffic disruption via reflected IPv6 packets

Published Sep 23, 2021 · Updated Nov 7, 2024

Denial of service in Cisco IOS XE Software on Catalyst 9800 Wireless Controllers allows attackers to disrupt traffic in a configured VLAN. The controller incorrectly reflects crafted link-local IPv6 traffic through its wired interfaces instead of only encapsulating and relaying it to access points, creating a Layer 2 loop. Default configuration is exposed when the traffic enters a wireless-network VLAN through a wired interface; successful exploitation causes traffic drops in that VLAN.

CVSS severity7.4
High
EPSS probability0.78%
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 XE Software on Catalyst 9800 Wireless Controllers allows attackers to disrupt traffic in a configured VLAN. The controller incorrectly reflects crafted link-local IPv6 traffic through its wired interfaces instead of only encapsulating and relaying it to access points, creating a Layer 2 loop. Default configuration is exposed when the traffic enters a wireless-network VLAN through a wired interface; successful exploitation causes traffic drops in that VLAN.

The record

CVE
CVE-2021-34767
Published
Sep 23, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco IOS XE Software
Classifications
CWE-670, T1498
Attack vector
adjacent
Privileges
unauthenticated

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 privilegesunauthenticated

What is affected?

ClamAV · Cisco IOS XE Softwareversion=n/a

Published CVSS scores

7.4CiscoCVSS:3.1/AV:A/AC:L/PR:N/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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-670Always-Incorrect Control Flow Implementation
T1498Network 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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