  We classify polynomial models for real hypersurfaces in $\mathbb C^N$, which
admit nonlinearizable infinitesimal CR automorphisms. As a consequence, this
provides an optimal 1-jet determination result in the general case. Further we
prove that such automorphisms arise from one common source, by pulling back via
a holomorphic mapping a suitable symmetry of a hyperquadric in some complex
space.
