High

CVE-2026-20301 — Cisco IOS Software and IOS XE Software Extensible Messaging Client Protocol Denial of Service Vulnerability

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.

Published Aug 5, 2026 · Updated Oct 15, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: cisco

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

Technical severity (high)+6
Automatable exploitation (SSVC)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

—

No EPSS score.

Affects

Cisco

Cisco IOS XE Software and 1 more

CVSS base

8.6

High · 2 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation

None

none · proof-of-concept · active

Automatable

Yes

can an attacker script the kill-chain steps

Technical impact

Partial

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented mitigation — sec.cloudapps.cisco.com (NIST National Vulnerability Database)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Cisco Cisco IOS XE Software, Cisco Ios).
The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
Apply the documented mitigations until a definitive fix is available.
Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Cisco	Cisco IOS XE Software	version 3.11.14Eversion 16.12.16version 26.2.1eaversion 26.1.1aversion 17.18.3aversion 17.12.7bversion 17.18.3version 17.12.7a and 279 more version 17.18.1yversion 17.15.5aversion 16.12.15version 17.4.1cversion 17.12.6bversion 17.15.4dversion 17.12.1z6version 17.18.1xversion 17.12.1z5version 17.15.5version 17.15.4s1version 16.12.8version 17.8.1version 17.9.1version 3.11.6Eversion 17.6.4version 3.11.7Eversion 17.3.5bversion 17.9.1aversion 17.3.6version 17.10.1version 3.11.8Eversion 16.9.8aversion 17.6.1z1version 17.10.1aversion 17.9.2version 17.6.5version 17.9.1xversion 17.9.1yversion 16.9.8bversion 17.3.7version 17.9.2aversion 17.9.3version 16.12.9version 17.11.1version 17.10.1bversion 17.6.6version 17.9.1x1version 17.11.1aversion 17.9.3aversion 17.12.1version 17.3.8version 17.9.4version 16.12.10version 17.12.1wversion 17.9.1y1version 17.12.1aversion 3.11.9Eversion 17.9.5version 17.13.1version 17.12.1xversion 17.12.2version 17.14.1version 17.9.4aversion 17.3.8aversion 17.6.6aversion 17.6.5aversion 17.6.7version 16.12.10aversion 17.15.1version 17.13.1aversion 17.12.2aversion 16.12.11version 17.12.3version 17.12.1yversion 3.11.10Eversion 17.12.1zversion 17.9.5aversion 17.14.1aversion 17.9.5bversion 17.6.8version 16.12.12version 17.12.4version 17.9.6version 17.17.1version 17.16.1version 17.15.1wversion 17.12.3aversion 3.11.11Eversion 17.9.5cversion 17.15.1aversion 17.12.1z1version 17.15.2version 17.15.1bversion 17.9.5dversion 17.15.1xversion 17.9.6aversion 17.12.1z2version 17.6.8aversion 17.16.1aversion 16.12.13version 17.15.3version 17.9.7version 17.12.5version 17.15.1yversion 17.9.5eversion 17.12.4aversion 17.15.2aversion 17.15.2cversion 17.15.2bversion 17.12.1z3version 17.9.5fversion 17.12.4bversion 3.11.12Eversion 17.12.5aversion 17.18.1version 17.12.6version 17.12.1z4version 17.9.8version 17.9.7aversion 17.15.3aversion 17.15.4version 3.11.13Eversion 17.12.5bversion 17.15.3bversion 16.12.14version 17.18.1zversion 17.9.7bversion 17.12.5cversion 26.1.1version 17.18.1aversion 17.15.4aversion 17.18.2version 17.18.1wversion 17.15.4bversion 17.12.6aversion 17.12.7version 17.15.4cversion 17.9.9version 17.12.5dversion 17.2.1aversion 16.12.1yversion 16.12.3sversion 16.7.1bversion 16.6.2version 16.6.5version 16.12.1wversion 16.9.1dversion 17.3.1version 16.9.4cversion 16.7.1version 17.2.1version 16.6.8version 16.9.1bversion 16.9.2sversion 16.12.4version 16.12.3aversion 17.1.1sversion 16.11.1aversion 16.7.2version 16.11.1bversion 16.9.1sversion 16.9.3hversion 16.9.1cversion 16.6.5aversion 16.6.3version 16.10.1fversion 17.2.1vversion 16.8.1eversion 16.9.3aversion 16.12.1aversion 16.12.1xversion 3.16.10aSversion 16.6.4sversion 16.10.1cversion 16.12.3version 16.11.2version 16.6.5bversion 16.12.2sversion 16.10.1bversion 16.7.3version 16.6.7version 16.9.6version 16.9.2version 16.10.1version 16.12.1tversion 16.9.1version 16.8.1aversion 3.18.8aSPversion 16.9.3sversion 16.6.7aversion 16.12.2version 16.11.1version 16.9.3version 16.11.1sversion 16.12.1version 17.1.1version 17.1.2version 16.10.1dversion 17.1.1tversion 16.9.4version 16.8.3version 16.12.2tversion 16.9.5version 16.8.1sversion 16.10.1eversion 16.8.2version 16.6.4version 16.8.1bversion 16.10.1aversion 16.12.1zversion 16.8.1cversion 16.9.1aversion 16.9.5fversion 16.6.4aversion 16.10.1gversion 16.8.1version 16.6.6version 16.9.2aversion 16.8.1dversion 16.10.3version 16.7.1aversion 16.11.1cversion 16.10.1sversion 17.2.1rversion 17.1.1aversion 16.10.2version 16.12.2aversion 16.12.1cversion 16.12.1sversion 16.7.4version 3.18.9SPversion 17.2.2version 3.11.3Eversion 16.9.8version 16.9.7version 17.1.3version 17.3.1aversion 3.11.4Eversion 17.3.2aversion 16.12.5version 17.3.1wversion 17.3.2version 3.11.3aEversion 17.4.1version 16.12.4aversion 17.4.1aversion 3.16.10bSversion 17.3.1xversion 17.3.3version 16.6.9version 17.2.3version 17.5.1version 16.12.1z1version 16.12.5aversion 17.4.1bversion 17.3.1zversion 16.12.5bversion 17.4.2version 17.3.3aversion 3.11.5Eversion 17.3.5version 17.3.4version 17.5.1aversion 16.12.6version 17.6.1version 16.12.1z2version 16.6.10version 17.3.4aversion 17.6.1aversion 3.8.10cEversion 17.7.1version 17.6.1xversion 17.6.2version 16.12.6aversion 17.3.4cversion 17.4.2aversion 17.3.4bversion 17.7.1aversion 16.12.7version 17.8.1aversion 17.6.3aversion 17.6.3version 17.7.2version 17.7.1bversion 17.9.1wversion 17.3.5aversion 17.6.1zversion 3.8.10dE	Vulnerable	CVE Program (cve.org) e CISA ADP
Cisco	Ios	version 15.5(1)sy9version 15.5(1)sy10version 15.5(1)sy11version 15.5(1)sy12version 15.5(1)sy13version 15.5(1)sy14version 15.5(1)sy15version 15.5(1)sy16 and 5 more version 15.5(1)sy17version 15.6(2)sp4version 15.6(2)sp5version 15.6(2)sp6version 15.6(2)sp7	Vulnerable	NIST National Vulnerability Database

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

None

User interaction

None

Scope

Changed

Confidentiality

None

Integrity

None

Availability

High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
8.6	CVSS 3.1	High	3.9	4.0	psirt@cisco.com via NIST National Vulnerability Database
8.6	CVSS 3.1	High	—	—	cisco via CVE Program (cve.org) e CISA ADP

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H (score shown at the top: psirt@cisco.com)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-606

Other CVEs with this weakness →

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).

web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

2026
Affected products added · cisco/cisco ios xe software, cisco/ios
Oct 15, 2026 · CVE Program (cve.org) e CISA ADP
SSVC assessment updated
Oct 15, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 15, 2026
Fix documented
Oct 15, 2026 · NIST National Vulnerability Database
CVSS changed · 8.6
Oct 15, 2026 · NIST National Vulnerability Database
Source status changed · → Analyzed
Oct 15, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 15, 2026 · NIST National Vulnerability Database
CVE published
Aug 5, 2026 · cisco

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 15, 2026 – Oct 15, 2026

References

1 on the record

sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ios-xmcp-thbAr
Mitigation, Vendor Advisory, NIST National Vulnerability Database

Elsewhere on this site

Cisco
every CVE for this vendor

CWE-606
other pages naming this weakness

Related CVEs (same products)

Medium CVE-2026-20311 Cisco IOS XE Software Web UI Denial of Service Vulnerability Aug 5, 2026
Medium CVE-2026-20308 Cisco IOS XE Software Web-Based Management Interface Vulnerability Aug 5, 2026
High CVE-2026-20273 Cisco IOS XE Software Security Hardening Release Aug 5, 2026
Critical CVE-2026-20272 Cisco IOS XE Software Security Hardening Release Aug 5, 2026
High CVE-2026-20270 Cisco IOS XE Software Security Hardening Release Aug 5, 2026
High CVE-2026-20269 Cisco IOS XE Software Security Hardening Release Aug 5, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No FIRST EPSS score.
No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-20301 is "Cisco IOS Software and IOS XE Software Extensible Messaging Client Protocol Denial of Service Vulnerability", rated High. It affects Cisco IOS XE Software (Cisco); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 8.6 describes technical severity. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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