In this paper a geometrically nonlinear model for studying the lateral global buckling problem of a generic thin-walled composite beam is presented. The model is based on full second-order deformable beam theory and accounts for axial, flexural, shear, torsional and warping displacements. Moreover, the web/flange junctions are supposed to be deformable. Governing nonlinear equations are derived from the principle of virtual displacements. Initial numerical results, obtained by means of a finite element approximation, deal with open cross-section composite profiles under flexural/torsional loads, making it possible to identify the relevance of connection stiffness over the pre-buckling range of the mechanical response. Comparisons with numerical and experimental results available in the literature are also discussed.