Soft errors and Hardware Trojans (HTs) constitute major reliability concerns, and in combination they can pose an even greater threat to circuit security. The main aim of this research is to develop and implement a reliability-based HT and to identify the optimal regions for its injection, enabling the creation of challenging benchmarks for evaluating detection techniques. In this context, a reliability-based HT is designed and evaluated using different components to achieve the required time overhead. Next, a method that combines the generation and propagation of Single-Event Transients (SETs), while accounting for both masking effects and the design’s timing constraints, is employed to efficiently identify the most vulnerable and critical gates. The sensitive gates selected for HT insertion exhibit 50–70% vulnerability to soft errors. At the same time, their insertion and the resulting path delay overhead must not violate the design’s timing constraints, and the additional area must remain below 10% of the total area. These three conditions ensure that the inserted HTs remain stealthy and, therefore, challenging to detect. The experimental results demonstrate that selecting this category of gates is highly effective, as it leads to a significant increase in the number of soft errors and, consequently, aggravates circuit vulnerability with minimal impact on the design. On average, the targeted gates exhibit a 130% increase in sensitivity, and the overall Soft Error Rate (SER) increases by 78%, confirming the importance of providing robust benchmarks to combat potential attacks of this kind.