Only the latest stable release gets security updates.
If the vulnerability could be dangerous to existing Tiled users, please report it to me (@bjorn) privately. I can be reached by email to bjorn@lindeijer.nl, via Telegram at https://t.me/thorbjorn or on Discord (best to first join the Tiled Discord server at https://discord.gg/39wbTv7).
In case it seems urgent to fix the vulnerability I will make a new patch release for the current stable version.