Mattermost disclosed a bug submitted by c0rydoras: https://hackerone.com/reports/2541027 – Bounty: $150
[Source]
Warning: Trying to access array offset on value of type null in /home/bugbount/public_html/wp-content/plugins/citadela-pro/plugin/Custom_Header/Feature.php on line 309
Let's see what this Bug Bounty thing is all about