This security release disables post fetching for relayables. Due to an insecure implementation, fetching of root posts for relayables could allow an attacker to distribute malicious/spoofed/modified posts for any person.
Disabling the fetching will make the current federation a bit less reliable, but for a hotfix, this is the best solution. We will re-enable the fetching in 0.6.0.0 when we moved out the federation into its own library and are able to implement further validation during fetches.