Mastodon versions 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6 have a vulnerability affecting the experimental FASP feature. This flaw allows unconfirmed FASP registrations to make subscriptions and request content backfill without administrator approval. The issue only impacts servers that have enabled the experimental 'fasp' feature via the EXPERIMENTAL_FEATURES environment variable. Exploitation can lead to minor information leaks of publicly available URIs and, if repeated, can cause denial-of-service conditions by overloading the sidekiq worker handling the 'fasp' queue. The vulnerability is fixed in Mastodon versions 4.4.14 and 4.5.7. Servers not using the experimental feature are not affected.