Page "Invariant mass" Paragraph 3

Note that for reasons above, such a rest frame does not exist for single photons, or rays of light moving in one direction.

When two or more photons move in different directions, however, a center of mass frame ( or " rest frame " if the system is bound ) exists.

Thus, the mass of a system of several photons moving in different directions is positive, which means that an invariant mass exists for this system even though it does not exist for each photon.