CVE-2026-6339

Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636

CVSS v3 4.3 MEDIUM

4.3^/10

CVSS v3 : MEDIUM

Vector :

Exploitability : 2.8 / Impact : 1.4

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required LOW

User Interaction NONE

Confidentiality Impact NONE

Integrity Impact NONE

Availability Impact LOW

Scope UNCHANGED

References

Link	Resource
https://mattermost.com/security-updates	Vendor Advisory

Configurations

Configuration 1 (hide)

OR	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server::::::::

History

14 Oct 2026, 20:10

Type	Values Removed	Values Added
Summary		(es) Las versiones de Mattermost 11.5.x menor o igual que 11.5.1, 11.4.x menor o igual que 11.4.3 no validan el encabezado X-Requested-With en el endpoint de revelación de mensajes de lectura única, lo que permite a un miembro autenticado del canal forzar la revelación de un mensaje de lectura única sin el consentimiento del destinatario a través de una etiqueta de imagen Markdown manipulada. ID de Aviso de Mattermost: MMSA-2026-00636

Information

Published : 2026-05-18 09:16

Updated : 2026-10-14 20:10

NVD link : CVE-2026-6339

Mitre link : CVE-2026-6339

CVE.ORG link : CVE-2026-6339

JSON object : View

Products Affected

mattermost

mattermost_server

CWE

CWE-346

Origin Validation Error

{"id": "CVE-2026-6339", "cveTags": [], "metrics": {"ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"id": "CVE-2026-6339", "role": "CISA Coordinator", "options": [{"exploitation": "none"}, {"automatable": "no"}, {"technicalImpact": "partial"}], "version": "2.0.3", "timestamp": "2026-05-18T12:41:50.625706Z"}}], "cvssMetricV31": [{"type": "Secondary", "source": "responsibledisclosure@mattermost.com", "cvssData": {"scope": "UNCHANGED", "version": "3.1", "baseScore": 4.3, "attackVector": "NETWORK", "baseSeverity": "MEDIUM", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L", "integrityImpact": "NONE", "userInteraction": "NONE", "attackComplexity": "LOW", "availabilityImpact": "LOW", "privilegesRequired": "LOW", "confidentialityImpact": "NONE"}, "impactScore": 1.4, "exploitabilityScore": 2.8}, {"type": "Primary", "source": "nvd@nist.gov", "cvssData": {"scope": "UNCHANGED", "version": "3.1", "baseScore": 4.3, "attackVector": "NETWORK", "baseSeverity": "MEDIUM", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N", "integrityImpact": "NONE", "userInteraction": "NONE", "attackComplexity": "LOW", "availabilityImpact": "NONE", "privilegesRequired": "LOW", "confidentialityImpact": "LOW"}, "impactScore": 1.4, "exploitabilityScore": 2.8}]}, "affected": [{"source": "responsibledisclosure@mattermost.com", "affectedData": [{"vendor": "Mattermost", "product": "Mattermost", "versions": [{"status": "affected", "version": "11.5.0", "versionType": "semver", "lessThanOrEqual": "11.5.1"}, {"status": "affected", "version": "11.4.0", "versionType": "semver", "lessThanOrEqual": "11.4.3"}, {"status": "unaffected", "version": "11.6.0"}, {"status": "unaffected", "version": "11.5.2"}, {"status": "unaffected", "version": "11.4.4"}], "defaultStatus": "unaffected"}]}], "published": "2026-05-18T09:16:23.573", "references": [{"url": "https://mattermost.com/security-updates", "tags": ["Vendor Advisory"], "source": "responsibledisclosure@mattermost.com"}], "vulnStatus": "Analyzed", "weaknesses": [{"type": "Secondary", "source": "responsibledisclosure@mattermost.com", "description": [{"lang": "en", "value": "CWE-346"}]}], "descriptions": [{"lang": "en", "value": "Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636"}, {"lang": "es", "value": "Las versiones de Mattermost 11.5.x menor o igual que 11.5.1, 11.4.x menor o igual que 11.4.3 no validan el encabezado X-Requested-With en el endpoint de revelaci\u00f3n de mensajes de lectura \u00fanica, lo que permite a un miembro autenticado del canal forzar la revelaci\u00f3n de un mensaje de lectura \u00fanica sin el consentimiento del destinatario a trav\u00e9s de una etiqueta de imagen Markdown manipulada. ID de Aviso de Mattermost: MMSA-2026-00636"}], "lastModified": "2026-10-14T20:10:00.190", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "CF171039-837A-4D23-87EB-F328AD04976C", "versionEndExcluding": "11.4.4", "versionStartIncluding": "11.4.0"}, {"criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "726AD6AD-6C01-45BB-9115-B8209717A6D4", "versionEndExcluding": "11.5.2", "versionStartIncluding": "11.5.0"}], "operator": "OR"}]}], "sourceIdentifier": "responsibledisclosure@mattermost.com"}