CVE-2021-1439: Cisco Aironet Access Points FlexConnect Multicast DNS Denial of Service Vulnerability

Severity: high (CVSS 7.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-03-24 · updated 2026-06-17
Weakness: CWE-120

Description

A vulnerability in the multicast DNS (mDNS) gateway feature of Cisco Aironet Series Access Points Software could allow an unauthenticated, adjacent attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability is due to insufficient input validation of incoming mDNS traffic. An attacker could exploit this vulnerability by sending a crafted mDNS packet to an affected device through a wireless network that is configured in FlexConnect local switching mode or through a wired network on a configured mDNS VLAN. A successful exploit could allow the attacker to cause the access point (AP) to reboot, resulting in a DoS condition.

Affected products

Cisco Mobility Express 1800 Access Point Series: all versions
Cisco Catalyst 9800 Firmware: from 17.1 before 17.3.3 — fixed in 17.3.3

How to fix

Upgrade to a fixed version: Cisco Catalyst 9800 Firmware 17.3.3. Check the vendor advisory for mitigations.

References

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