Vienna Convention on the Law of Treaties (1969)
art. 53: A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law. For the purposes of the present Convention, a peremptory norm of general international law is a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character.
art. 66 (a) disputes on such a norm's existence can be submitted to ICJ.
An official, internationally agreed list of norms does not (as of Jan 2017) exist but it would - arguably - include both positive eg right to peoples' self determination & negative eg prohibition of genocide
" a concept embodying the community interest and reinforced by its link with public morality [existing] in modern international law as a matter of necessity” ( Peremptory Norms in International Law by Alexander Orakhelashvili at p. 577. See left hand column for this work).
Jus cogens (International law) is a specific subject search term to try in SOLO