Abstract: Joint transmit beamforming and reconfigurable intelligent surface (RIS) phase-shift matrix design is an essential task in uncrewed aerial vehicle (UAV)-assisted integrated sensing and communication (ISAC) systems. However, existing works mainly neglect the complex security environment where spatially diverse and functionally heterogeneous malicious entities coexist in low-altitude activities. Motivated by this, we propose a novel robust RIS-assisted ISAC system where ground passive eavesdroppers, aerial active jammers, and aerial hybrid active-passive malicious users are comprehensively incorporated. To mitigate these security threats, we employ an artificial noise interference (ANI) mechanism to degrade the reception quality of eavesdroppers, while utilizing the RIS enhancement mechanism to enhance the intended signal power against jamming interference. For hybrid malicious users, both mechanisms are simultaneously adopted. An optimization of joint transmit beamforming and RIS phase-shift matrix design is established to maximize the sensing signal-to-clutter-plus-noise ratio (SCNR), subject to communication signal-to-interference-plus-noise ratio (SINR) and security SINR. To tackle this non-convex problem, various mathematical techniques are adopted, including majorization-minimization and semidefinite relaxation. Experiment results demonstrate the effectiveness of the proposed algorithm. In addition, the proposed ANI mechanism and RIS enhancement mechanism increase the sensing SCNR by approximately 49.75% and 22.58% under the security SINR constraints, respectively.