Mattermost Authorization and Validation Flaws

A sensitive information disclosure vulnerability in Mattermost Plugins versions 2.0.3.0 and earlier fails to properly mask sensitive configuration values in support packets, allowing attackers with high privileges to extract original plugin settings from exported configuration data. The vulnerability requires authenticated access with high privileges (CVSS 7.6) and enables attackers to obtain sensitive configuration data that should be masked, potentially exposing API keys, credentials, or other sensitive plugin configurations. No active exploitation or proof-of-concept has been reported, and the vulnerability requires significant access privileges to exploit.

Mattermost Server versions 11.3.0, 11.2.x through 11.2.2, and 10.11.x through 10.11.10 contain a server-side request forgery (SSRF) vulnerability due to improper validation of IPv4-mapped IPv6 addresses, allowing authenticated attackers to bypass reserved IP restrictions and access internal services. An attacker with login credentials can craft requests using IPv6 notation (such as [::ffff:127.0.0.1]) to reach localhost or other restricted internal endpoints that would normally be blocked. No patch is currently available for this vulnerability.

This vulnerability in Mattermost allows guest users to bypass team-specific file upload permissions through a cross-team file metadata reuse attack. Affected versions include Mattermost 11.3.0, 11.2.x up to 11.2.2, and 10.11.x up to 10.11.10. An authenticated guest user can upload a file in a team where they have upload_file permission, then reuse that file's metadata in POST requests to channels in different teams where they lack upload permission, resulting in unauthorized file posting with potential integrity impact.

CVE-2026-21386 is a security vulnerability (CVSS 4.3) that allows an authenticated team member. Remediation should follow standard vulnerability management procedures.

A remote code execution vulnerability (CVSS 4.3) that allows guest users without read permissions. Remediation should follow standard vulnerability management procedures.

Mattermost versions 11.3.x up to and including 11.3.0 contain an information disclosure vulnerability where burn-on-read posts fail to maintain their redacted state when deleted, allowing authenticated channel members to view previously hidden message contents through WebSocket post deletion events. The vulnerability requires low-privilege authenticated access and results in confidentiality loss of sensitive communications that were intentionally designed to be self-destructing. With a CVSS score of 4.3 and network-based attack vector, this represents a meaningful but contained risk primarily affecting organizations relying on Mattermost's burn-on-read feature for secure internal communications.

CVE-2026-22545 is a security vulnerability (CVSS 3.1) that allows an authenticated attacker. Remediation should follow standard vulnerability management procedures.