Hardware security vulnerabilities to hardware Trojans in widely used filter structures are identified. The widely used two-integrator loop filter architecture known as the Kerwin-Huelsman-Newcomb (KHN) Biquad is used to demonstrate the vulnerability. It is shown that the relationship between the passive component values and the nonlinear amplifier parameters, the slew rate and the output saturation voltages, determine the presence or absence of a stationary nonlinear undesired oscillatory mode of operation. Experimental results obtained from a discrete component filter demonstrate the vulnerability to the Trojan mode of operation in this filter structure.