The realities of contemporary armed conflicts with a complex interweaving net of actors are rarely reminiscent of classic combat scenarios envisaged by the drafters of the Geneva Conventions. The scarcity of conventional regulation of non-international armed conflicts (NIACs), coupled with the non-state character of the majority of detaining powers, lead to lack of clarity regarding the legal regime of detention of persons captured by non-state armed groups (NSAGs). In the absence of an explicit authorisation for internment under the international humanitarian law applicable to NIACs, recent developments in case law have induced a scholarly debate on what is the legal basis for administrative detention carried out by these actors. The article analyses key arguments presented by both sides of the debate, concluding that neither side can demonstrate either the existence or the absence of the authorisation in question, while the discussion itself has limited practical value in regulating the conduct of NSAGs. At the same time, the practice of states, although still ambivalent, points to the gradual transformation of mere legality, or the so-called ‘inherent power’ to intern, into a customary provision providing a legal basis for administrative detention by NSAGs.