Samarium hexaboride (SmB 6 ), a well-known Kondo insulator in which the insulating bulk arises from strong electron correlations, has recently attracted great attention owing to increasing evidence for its topological nature, thereby harboring protected surface states. However, corroborative spectroscopic evidence is still lacking, unlike in the weakly correlated counterparts, including Bi 2 Se 3 . Here, we report results from planar tunneling that unveil the detailed spectroscopic properties of SmB 6 . The tunneling conductance obtained on the (001) and (011) single crystal surfaces reveals linear density of states as expected for two and one Dirac cone(s), respectively. Quite remarkably, it is found that these topological states are not protected completely within the bulk hybridization gap. A phenomenological model of the tunneling process invoking interaction of the surface states with bulk excitations (spin excitons), as predicted by a recent theory, provides a consistent explanation for all of the observed features. Our spectroscopic study supports and explains the proposed picture of the incompletely protected surface states in this topological Kondo insulator SmB 6 .topological Kondo insulator | samarium hexaboride | planar tunneling spectroscopy | inelastic tunneling | spin excitons T opological insulators are an emerging class of quantum matter in which the nontrivial topology of the bulk band structure naturally gives rise to topologically protected, i.e., robust, surface states (1, 2). Several dozens of such materials have been discovered but most of them, including Bi 2 Se 3 , are weakly correlated band insulators. A recent theoretical proposal (3) that certain Kondo insulators (4), which are insulating in the bulk due to strong electron correlations, could also be topological has stimulated vigorous research in the field. In particular, samarium hexaboride (SmB 6 ) has drawn great attention owing to its telltale resistivity behavior saturating below 4 K (5). Various experiments have been implemented to investigate this possibility (6-24), establishing the robustness of the surface states and the Kondo hybridization leading to a formation of the bulk gap, but their topological origin and nature has not been unambiguously confirmed. Factors contributing to this situation include their inherently complex nature due to strong correlations, nontrivial surface chemistry, and insufficient energy resolution. A recent report of quantum oscillations in magnetic torque supports the topological origin of the surface states (22), but conflicting results have also been reported (23).Planar tunneling spectroscopy, inherently surface sensitive with high energy resolution and momentum selectivity, is ideally suited for the study of surface states, particularly so in SmB 6 where the bulk hybridization gap is much smaller than the band gap in Bi 2 Se 3 . Lead (Pb) is chosen as the counter electrode in this study because the quality of its measured superconducting density of states (DOS) is an important junction diagn...