Anisotropy-Gated Densification Against Poison-splat Attacks

3D Gaussian Splatting (3DGS) achieves efficient novel-view synthesis through explicit Gaussian representations and Adaptive Density Control (ADC). However, recent Poison-splat attacks exploit the gradient-driven densification mechanism to trigger excessive clone and split operations, leading to rapid Gaussian proliferation and substantial computational overhead. We observe that poisoned scenes exhibit stronger spectral anisotropy than clean scenes, suggesting that frequency-domain characteristics provide useful cues for regulating densification. Based on this observation, we propose Anisotropy-Gated Densification (AGD), a defense method that integrates spectral anisotropy into ADC. AGD adaptively adjusts the densification threshold according to scene-level spectral statistics and guides gradient accumulation using local spectral orientation, suppressing unnecessary Gaussian generation while preserving legitimate scene structures. Extensive experiments on Mip-NeRF360, NeRF Synthetic, and Tanks and Temples demonstrate that AGD effectively mitigates Poison-splat attacks by reducing Gaussian proliferation, GPU memory consumption, and training overhead, while maintaining rendering quality comparable to the original 3DGS on clean scenes.