Alleged British Hacker Fears Guantanamo
Articles 4 and 5 of Convention IV (Relative to the Protection of Civilian Persons in Time of War):
Art. 4. Persons protected by the Convention are those who, at a given moment and in any manner whatsoever, find themselves, in case of a conflict or occupation, in the hands of a Party to the conflict or Occupying Power of which they are not nationals.
Nationals of a State which is not bound by the Convention are not protected by it. Nationals of a neutral State who find themselves in the territory of a belligerent State, and nationals of a co-belligerent State, shall not be regarded as protected persons while the State of which they are nationals has normal diplomatic representation in the State in whose hands they are.
The provisions of Part II are, however, wider in application, as defined in Article 13.
Persons protected by the Geneva Convention for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field of 12 August 1949, or by the Geneva Convention for the Amelioration of the Condition of Wounded, Sick and Shipwrecked Members of Armed Forces at Sea of 12 August 1949, or by the Geneva Convention relative to the Treatment of Prisoners of War of 12 August 1949, shall not be considered as protected persons within the meaning of the present Convention.
Art. 5 Where in the territory of a Party to the conflict, the latter is satisfied that an individual protected person is definitely suspected of or engaged in activities hostile to the security of the State, such individual person shall not be entitled to claim such rights and privileges under the present Convention as would, if exercised in the favour of such individual person, be prejudicial to the security of such State.
Where in occupied territory an individual protected person is detained as a spy or saboteur, or as a person under definite suspicion of activity hostile to the security of the Occupying Power, such person shall, in those cases where absolute military security so requires, be regarded as having forfeited rights of communication under the present Convention.
In each case, such persons shall nevertheless be treated with humanity and, in case of trial, shall not be deprived of the rights of fair and regular trial prescribed by the present Convention. They shall also be granted the full rights and privileges of a protected person under the present Convention at the earliest date consistent with the security of the State or Occupying Power, as the case may be.
See here for a list of signatories to the Geneva conventions. You'll note that Afghanistan signed in 1956, although I'll confess I'm not 100% sure if that was still relevant at the time