Mastodon is a free, open-source social network server based on ActivityPub. Starting in version 3.5.0 and prior to versions 3.5.9, 4.0.5, and 4.1.3, attackers using carefully crafted media files can cause Mastodon's media processing code to create arbitrary files at any location. This allows attackers to create and overwrite any file Mastodon has access to, allowing Denial of Service and arbitrary Remote Code Execution. Versions 3.5.9, 4.0.5, and 4.1.3 contain a patch for this issue.