THE VIENNA CONVENTION ON THE LAW OF TREATIES

PART V
INVALIDITY, TERMINATION AND SUSPENSION OF THE OPERATION OF TREATIES
SECTION 2. INVALIDITY OF TREATIES
Article 53
Treaties conflicting with a peremptory norm of general international law
(jus cogens)
  

A treaty is void if, at the time of its conclusion, it conflicts with
a peremtory 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.