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Full vulnerability report · 2026
CVE-2026-20301High confidence

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

Cisco · Cisco IOS XE Software

Official source article: Cisco CISCO-SA-IOS-XMCP-THBAR34T ↗. Check the applicable product and release in the original source.

8.6HighCVSS 3.1
Recommended action
Within 7 days

High technical severity; prioritise exposed affected systems while verifying vendor guidance.

Mitigation available
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:High, unchanged from published severity.unchanged

Evidence used

  • No CISA KEV confirmation is currently recorded.
  • The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
  • EPSS is 0.57% for the current model date.

Compensating controls

  • Apply the linked authoritative mitigation while planning the permanent fix.
  • Restrict the affected network interface to trusted sources where business-safe.
  • Monitor vendor guidance and exploitation sources for a material change.

Verification

  1. Confirm that the asset runs Cisco Cisco IOS XE Software and falls inside the recorded affected range.
  2. Recheck the vendor advisory before scheduling a change because no verified fixed version is currently retained.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 30 daysRemediation target: Within 180 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2026-53467

No EUVD known-exploited evidence

ENISA has published the identifier mapping but no EUVD description has been stored yet.

EUVD state
Present in the current official mapping
Known exploitation
Not present in the current ENISA EUVD known-exploited dataset. This is not proof of no exploitation.
ENISA score
Not supplied in the stored EUVD record
Advisory evidence
No linked advisory details stored yet
Recommended actionWithin 7 days

High technical severity; prioritise exposed affected systems while verifying vendor guidance.

Mitigation available
01

What, why and how

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.

What

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.

Why

The product does not properly check inputs that are used for loop conditions, potentially leading to a denial of service or other consequences because of excessive looping.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may disrupt the affected service.

What

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.

Why

The product does not properly check inputs that are used for loop conditions, potentially leading to a denial of service or other consequences because of excessive looping.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may disrupt the affected service.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
No confirmed evidence

No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path
a network path → Unchecked Input for Loop Condition → disrupt the affected service
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
None
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Changed: exploitation can affect a different security authority
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-606 ↗

CWE-606: Unchecked Input for Loop Condition. The product does not properly check inputs that are used for loop conditions, potentially leading to a denial of service or other consequences because of excessive looping.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.SChangedScope: The attack can affect a component governed by a different security authority.CNoneConfidentiality impact: No direct loss is represented by this metric.INoneIntegrity impact: No direct loss is represented by this metric.AHighAvailability impact: A successful attack can cause a major loss.
Post-exploitation / living off the land
No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
NetworkUnauthenticatedDenial of serviceCWE-606
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

BSI · German · WID-SEC-2026-2664Cisco IOS: Schwachstelle ermöglicht Denial of Service

Ein entfernter, anonymer Angreifer kann eine Schwachstelle in Cisco IOS und Cisco IOS XE ausnutzen, um einen Denial of Service Angriff durchzuführen.

Official advisory ↗
Cyber Security Agency of Singapore · English · CSA-SB-20260812Security Bulletin 12 Aug 2026

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 12 Aug 2026, published on 12 August 2026. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
CERT-FR · French · CERTFR-2026-AVI-0975Multiples vulnérabilités dans les produits Cisco

De multiples vulnérabilités ont été découvertes dans les produits Cisco. Certaines d'entre elles permettent à un attaquant de provoquer une exécution de code arbitraire à distance, un déni de service à distance et une atteinte à la confidentialité des données.

Official advisory ↗
CSIRT Italia · Italian · risolte-vulnerabilita-in-prodotti-cisco-29Risolte vulnerabilità in prodotti Cisco

Cisco ha rilasciato aggiornamenti di sicurezza che risolvono molteplici vulnerabilità, tra cui 5 con gravità "critica" e 11 con gravità “alta”, riguardanti vari prodotti. In particolare si evidenziano la CVE-2026-20316 che risulta attivamente sfruttata in rete e la CVE-2026-20200 , per la quale è disponibile un Proof of Concept (PoC). Di seguito i dettagli: CVE-2026-20316 , di tipo "Static Credential" e con score CVSS v3.1 pari a 5.3, interessa l'interfaccia web di Cisco Secure Firewall Management Center (FMC): la presenza di credenziali statiche associate a un account a basso privilegio consente a un attaccante remoto non autenticato di utilizzare tale account per effettuare il log-in sul sistema e ottenere l'accesso a dati sensibili presenti sul dispositivo; CVE-2026-20200 , di tipo "Argument Injection" e con score CVSS v3.1 pari a 8.8, interessa l'interfaccia di gestione web di Cisco Integrated Management Controller (IMC) ed è dovuta ad una non corretta gestione dell'input fornito dall'utente: un attaccante remoto autenticato con bassi privilegi potrebbe sfruttare tale vulnerabilità, tramite l'invio di input appositamente predisposti all'interfaccia di gestione web, per eseguire comandi arbitrari, con privilegi elevati, sul sistema operativo sottostante.

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-6904Cisco 제품 보안 업데이트 권고

Cisco IOS Software, Cisco IOS XE Software에서 발생하는 Unchecked Input for Loop Condition 취약점(CVE-2026-20301) [4][20]

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Mitigation available
Affected
Fixed
No fixed version is explicitly recorded in the structured CVE data.
Action
Apply the mitigation in the linked authoritative guidance while planning the vendor's permanent fix.
Workaround
A mitigation or workaround reference is available from the source linked below; validate it against the affected product and version.
Official vendor articles1 stored document

Linked articles are downloaded and versioned as source evidence. A CVE mention or approved update relationship does not, by itself, verify a fix for every product branch.

04

Evidence and provenance

Published 5 Aug 2026 · Last source change 5 Aug 2026, 17:45 UTC · CWE-606 · Unchecked Input for Loop Condition

CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2026-53467
Product sourceCNA
Remediation sourceCVE/CNA references
CWE sourceCNA
NVD statusNVD enriched

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Vendor guidanceAuthoritative vendor guidance changed: added mitigation: sec.cloudapps.cisco.com/cisco-sa-ios-xmcp-thbAr34t; added remediation: sec.cloudapps.cisco.com/cisco-sa-ios-xmcp-thbAr34t.
    Before
    no authoritative guidance recorded
    After
    mitigation: sec.cloudapps.cisco.com/cisco-sa-ios-xmcp-thbAr34t · remediation: sec.cloudapps.cisco.com/cisco-sa-ios-xmcp-thbAr34t
    psirt@cisco.com ↗
  2. Affected versionsThe structured affected or fixed version information changed.
    Before
    17.2.1a; 16.12.1y; 16.12.3s; 16.7.1b; 16.6.2; 16.6.5; 16.12.1w; 16.9.1d · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    After
    Cisco IOS XE Software: 17.2.1a, 16.12.1y, 16.12.3s, 16.7.1b, 16.6.2, 16.6.5, 16.12.1w, 16.9.1d, 17.3.1, 16.9.4c, 16.7.1, 17.2.1, 16.6.8, 16.9.1b, 16.9.2s, 16.12.4, 16.12.3a, 17.1.1s, 16.11.1a, 16.7.2, 16.11.1b, 16.9.1s, 16.9.3h, 16.9.1c, 16.6.5a, 16.6.3, 16.10.1f, 17.2.1v, 16.8.1e, 16.9.3a, 16.12.1a, 16.12.1x, 3.16.10aS, 16.6.4s, 16.10.1c, 16.12.3, 16.11.2, 16.6.5b, 16.12.2s, 16.10.1b, 16.7.3, 16.6.7, 16.9.6, 16.9.2, 16.10.1, 16.12.1t, 16.9.1, 16.8.1a, 3.18.8aSP, 16.9.3s, 16.6.7a, 16.12.2, 16.11.1, 16.9.3, 16.11.1s, 16.12.1, 17.1.1, 17.1.2, 16.10.1d, 17.1.1t, 16.9.4, 16.8.3, 16.12.2t, 16.9.5, 16.8.1s, 16.10.1e, 16.8.2, 16.6.4, 16.8.1b, 16.10.1a, 16.12.1z, 16.8.1c, 16.9.1a, 16.9.5f, 16.6.4a, 16.10.1g, 16.8.1, 16.6.6, 16.9.2a, 16.8.1d, 16.10.3, 16.7.1a, 16.11.1c, 16.10.1s, 17.2.1r, 17.1.1a, 16.10.2, 16.12.2a, 16.12.1c, 16.12.1s, 16.7.4, 3.18.9SP, 17.2.2, 3.11.3E, 16.9.8, 16.9.7, 17.1.3, 17.3.1a, 3.11.4E, 17.3.2a, 16.12.5, 17.3.1w, 17.3.2, 3.11.3aE, 17.4.1, 16.12.4a, 17.4.1a, 3.16.10bS, 17.3.1x, 17.3.3, 16.6.9, 17.2.3, 17.5.1, 16.12.1z1, 16.12.5a, 17.4.1b, 17.3.1z, 16.12.5b, 17.4.2, 17.3.3a, 3.11.5E, 17.3.5, 17.3.4, 17.5.1a, 16.12.6, 17.6.1, 16.12.1z2, 16.6.10, 17.3.4a, 17.6.1a, 3.8.10cE, 17.7.1, 17.6.1x, 17.6.2, 16.12.6a, 17.3.4c, 17.4.2a, 17.3.4b, 17.7.1a, 16.12.7, 17.8.1a, 17.6.3a, 17.6.3, 17.7.2, 17.7.1b, 17.9.1w, 17.3.5a, 17.6.1z, 3.8.10dE, 16.12.8, 17.8.1, 17.9.1, 3.11.6E, 17.6.4, 3.11.7E, 17.3.5b, 17.9.1a, 17.3.6, 17.10.1, 3.11.8E, 16.9.8a, 17.6.1z1, 17.10.1a, 17.9.2, 17.6.5, 17.9.1x, 17.9.1y, 16.9.8b, 17.3.7, 17.9.2a, 17.9.3, 16.12.9, 17.11.1, 17.10.1b, 17.6.6, 17.9.1x1, 17.11.1a, 17.9.3a, 17.12.1, 17.3.8, 17.9.4, 16.12.10, 17.12.1w, 17.9.1y1, 17.12.1a, 3.11.9E, 17.9.5, 17.13.1, 17.12.1x, 17.12.2, 17.14.1, 17.9.4a, 17.3.8a, 17.6.6a, 17.6.5a, 17.6.7, 16.12.10a, 17.15.1, 17.13.1a, 17.12.2a, 16.12.11, 17.12.3, 17.12.1y, 3.11.10E, 17.12.1z, 17.9.5a, 17.14.1a, 17.9.5b, 17.6.8, 16.12.12, 17.12.4, 17.9.6, 17.17.1, 17.16.1, 17.15.1w, 17.12.3a, 3.11.11E, 17.9.5c, 17.15.1a, 17.12.1z1, 17.15.2, 17.15.1b, 17.9.5d, 17.15.1x, 17.9.6a, 17.12.1z2, 17.6.8a, 17.16.1a, 16.12.13, 17.15.3, 17.9.7, 17.12.5, 17.15.1y, 17.9.5e, 17.12.4a, 17.15.2a, 17.15.2c, 17.15.2b, 17.12.1z3, 17.9.5f, 17.12.4b, 3.11.12E, 17.12.5a, 17.18.1, 17.12.6, 17.12.1z4, 17.9.8, 17.9.7a, 17.15.3a, 17.15.4, 3.11.13E, 17.12.5b, 17.15.3b, 16.12.14, 17.18.1z, 17.9.7b, 17.12.5c, 26.1.1, 17.18.1a, 17.15.4a, 17.18.2, 17.18.1w, 17.15.4b, 17.12.6a, 17.12.7, 17.15.4c, 17.9.9, 17.12.5d, 16.12.15, 17.15.4s1, 17.15.5, 17.12.1z5, 17.18.1x, 17.12.1z6, 17.15.4d, 17.12.6b, 17.4.1c, 3.11.14E, 17.15.5a, 17.18.1y, 17.12.7a, 17.18.3, 17.12.7b, 17.18.3a, 26.1.1a, 26.2.1ea, 16.12.16; 1 additional product group retained in the authoritative source · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    CNA ↗
  3. Remediation statusRemediation status changed from Awaiting fix to Mitigation available.
    Before
    Awaiting fix
    After
    Mitigation available
    psirt@cisco.com ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Mitigation reference recorded
ENISA EUVD · EUVD-2026-53467↗CVE authority or vendor reference · Cisco CISCO-SA-IOS-XMCP-THBAR34T↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2026-20301 · cve.blacktree.nl