CVE-2026-10556: mattermost server from 10.11.0 (inclusive), before 10.11.23 (exclusive), from 11.7.0 (inclusive), before 11.7.8…

Affected technology

mattermost server · from 10.11.0 (inclusive), before 10.11.23 (exclusive)
mattermost

mattermost server · from 11.7.0 (inclusive), before 11.7.8 (exclusive)
mattermost

mattermost server · from 11.8.0 (inclusive), before 11.8.5 (exclusive)
mattermost

mattermost server · from 11.9.0 (inclusive), before 11.9.1 (exclusive)
mattermost

Mattermost · 11.9.0 through 11.9.0, 11.8.0 through 11.8.4, 11.7.0 through 11.7.7, 10.11.0 through 10.11.22
Mattermost

Description’s affected range: versions 11.9.x <= 11.9.0

Component: Not specified by the source

Attack conditions (Vendor/CNA, CVSS 3.1): Network (remote) · No privileges required · No user interaction required

What an attacker can do

An unauthenticated attacker can crash the Microsoft Calendar plugin process and deny calendar integration service to all users on the instance via a crafted {{POST}} request to the public webhook endpoint. Vendor/CNA’s CVSS 3.1 assessment (base score 5.3/10) rates confidentiality and integrity impact as none; availability impact as low.