Medium WID-SEC-2026-1243 6 CVEs

Mattermost: Mehrere Schwachstellen ermöglichen nicht spezifizierten Angriff

Published Apr 22, 2026 · Updated May 25, 2026 · Revision 2 · de-DE

Vendor: Mattermost Product: Mattermost Platform: Linux Platform: Windows

Summary

Ein Angreifer kann mehrere Schwachstellen in Mattermost ausnutzen, um einen nicht näher spezifizierten Angriff durchzuführen.

Product

Mattermost ist ein webbasierter Instant-Messaging-Dienst.

Vulnerabilities 6

High 7.5

CVE-2026-5740

Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and c…

Mattermost Mattermost github.com/mattermost/mattermost-server github.com/mattermost/mattermost-server/v6 Go

May 22, 2026
Medium 6.5

CVE-2026-5755

Denial of service via crafted TIFF file upload

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.2, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate the TIFF IFD offset in the image header before allocating memory, which allows authenticated users with file upload or posting per…

Mattermost Mattermost github.com/mattermost/mattermost-server Go

May 22, 2026
Medium 5.9

CVE-2026-3473

Improper file ownership validation in the Boards API allows unauthorised file access

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate file ownership and access control, which allows an authenticated user to access and download files belonging to other users or teams via crafted Boa…

Mattermost Mattermost github.com/mattermost/mattermost-server Go

May 22, 2026
Medium 4.9

CVE-2026-5308

Missing request body size limits on Zoom plugin HTTP endpoints

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to enforce request body size limits on plugin HTTP endpoints which allows an attacker to cause a denial of service via crafted oversized HTTP requests.. Matterm…

Mattermost Mattermost github.com/mattermost/mattermost-server github.com/mattermost/mattermost-plugin-github Go

May 22, 2026
Medium 4.3

CVE-2026-3636

Sanitize team member data returned by API

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to sanitize team member data when returned via API to users without elevated permissions which allows a user without permissions to get data about team members …

Mattermost Mattermost github.com/mattermost/mattermost-server Go

May 22, 2026
Medium 4.3

CVE-2026-4646

Insufficient input validation in GitHub plugin API causes denial of service

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate user-supplied input in API request handlers which allows an authenticated attacker to crash the plugin process via a crafted HTTP request to the PR …

Mattermost Mattermost github.com/mattermost/mattermost-server github.com/mattermost/mattermost-plugin-github Go

May 22, 2026

Revision history

2026-05-25 #2
CVE aufgenommen
2026-04-22 #1
Initiale Fassung

References

Mattermost Security Updates vom 2026-04-22
https://mattermost.com/security-updates/#server