CVE-2026-20301: Cisco IOS

High severity, CVSS 8.6. EPSS: 0.6% chance of exploitation in the next 30 days.

A vulnerability in the Extensible Messaging Client Protocol (XMCP), also referred to as the External Client protocol, of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability is due to improper handling of malformed XMCP packets. An attacker could exploit this vulnerability by sending a malformed XMCP packet to an affected device. A successful exploit could allow the attacker to cause the affected device to reload unexpectedly, resulting in a DoS condition. The attacker does not need the XMCP client username to exploit this vulnerability.

Affected products

• Cisco IOS: version 15.2(1)sy6 only; version 15.2(1)sy7 only; version 15.2(1)sy8 only; version 15.2(2)e8 only; version 15.2(2)e9 only; version 15.2(2)e9a only; …
• Cisco IOS XE: version 3.8.10ce only; version 3.8.10de only; version 3.11.3ae only; version 3.11.3e only; version 3.11.4e only; version 3.11.5e only; …

Published 2026-08-05. Last modified 2026-09-17.