We just released security patches for versions 3.5, 4.0, 4.1 and 4.2. If you are using nightly, you can upgrade to 4.2.0-rc2 safely. We strongly suggest you upgrade to one of those versions in the coming days.
Please note that Mastodon 3.5 will reach end-of-life at the end of the year, and 4.0 at the end of October. We strongly suggest you upgrade to 4.1 or 4.2 by that point! Mastodon 4.2 will be released this Thursday.