Mattermost versions 9.11.x <= 9.11.0 and 9.5.x <= 9.5.8 fail to validate that the message of the permalink post is a string, which allows an attacker to send a non-string value as the message of a permalink post and crash the frontend.
Exploitability
AV:NAC:HPR:LUI:NScope
S:UImpact
C:NI:NA:L3.1/CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:LResource Management