CVE-2026-19867 Uncapped mention keys in user notification properties allow CPU exhaustion during post processing

Mattermost versions 11.10.x = 11.10.0, 11.9.x = 11.9.1, 11.8.x = 11.8.5, 11.7.x = 11.7.10 fail to cap the number and size of mention keys stored in a user's notification properties, and to bound the quadratic multibyte-keyword matching performed for each post, which allows an authenticated user t...