CVE-2026-20301

Cisco IOS Software and IOS XE Software Extensible Messaging Client Protocol Denial of Service Vulnerability

State: Published
Severity: 8.6 High
Published: Aug 5, 2026
Updated: Aug 5, 2026
Reserved: Oct 8, 2025

Description

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.

Scores

CVSS	Score	Vector	Source
3.1	8.6 High	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H	cisco

Weaknesses

CWE-606 Unchecked Input for Loop Condition

Affected Products

Product	Versions
Cisco IOS XE SoftwareCisco	17.2.1aAffected 16.12.1yAffected 16.12.3sAffected 16.7.1bAffected 16.6.2Affected 16.6.5Affected 16.12.1wAffected 16.9.1dAffected 17.3.1Affected 16.9.4cAffected 16.7.1Affected 17.2.1Affected 16.6.8Affected 16.9.1bAffected 16.9.2sAffected 16.12.4Affected 16.12.3aAffected 17.1.1sAffected 16.11.1aAffected 16.7.2Affected 16.11.1bAffected 16.9.1sAffected 16.9.3hAffected 16.9.1cAffected 16.6.5aAffected and 262 more versions
IOSCisco	15.2(7)E7Affected 15.2(8)E3Affected 15.7(3)M10Affected 15.8(3)M9Affected 15.9(3)M7Affected 15.2(7)E8Affected 15.2(8)E4Affected 15.9(3)M7aAffected 15.2(2)E10cAffected 15.2(4)E10dAffected 15.9(3)M6aAffected 15.9(3)M6bAffected 15.5(1)SY10Affected 15.5(1)SY11Affected 15.2(6)E1aAffected 15.4(1)SY4Affected 15.5(1)SY1Affected 15.2(2)E8Affected 15.2(6)E1Affected 15.7(3)M2Affected 15.2(4)E6Affected 15.2(1)SY6Affected 15.2(4)EA7Affected 15.2(6)E1sAffected 15.7(3)M3Affected and 115 more versions

Product

Versions

Cisco IOS XE SoftwareCisco

17.2.1aAffected
16.12.1yAffected
16.12.3sAffected
16.7.1bAffected
16.6.2Affected
16.6.5Affected
16.12.1wAffected
16.9.1dAffected
17.3.1Affected
16.9.4cAffected
16.7.1Affected
17.2.1Affected
16.6.8Affected
16.9.1bAffected
16.9.2sAffected
16.12.4Affected
16.12.3aAffected
17.1.1sAffected
16.11.1aAffected
16.7.2Affected
16.11.1bAffected
16.9.1sAffected
16.9.3hAffected
16.9.1cAffected
16.6.5aAffected
and 262 more versions

IOSCisco

15.2(7)E7Affected
15.2(8)E3Affected
15.7(3)M10Affected
15.8(3)M9Affected
15.9(3)M7Affected
15.2(7)E8Affected
15.2(8)E4Affected
15.9(3)M7aAffected
15.2(2)E10cAffected
15.2(4)E10dAffected
15.9(3)M6aAffected
15.9(3)M6bAffected
15.5(1)SY10Affected
15.5(1)SY11Affected
15.2(6)E1aAffected
15.4(1)SY4Affected
15.5(1)SY1Affected
15.2(2)E8Affected
15.2(6)E1Affected
15.7(3)M2Affected
15.2(4)E6Affected
15.2(1)SY6Affected
15.2(4)EA7Affected
15.2(6)E1sAffected
15.7(3)M3Affected
and 115 more versions

References

cisco-sa-ios-xmcp-thbAr34t

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