Only the last release of Lufi is supported and will receive security fixes, but you can also report vulnerability found in develop branch.
To report a vulnerability, please create a confidential issue on https://framagit.org/fiat-tux/hat-softwares/lufi/-/work_items.
Please note that this GitLab instance moderate inscriptions. If your account is not accepted in 24 hours, please go to https://contact.framasoft.org/ and write a message for Luc.
There is no bounty program in Lufi at this time.