The continuous quest for performance pushed processors to incorporate elements such as multiple cores, caches, acceleration units, or speculative execution that make systems very complex. On the other hand, these features often expose unexpected vulnerabilities that pose new challenges. For example, the timing differences introduced by caches or speculative execution can be exploited to leak information or detect activity patterns. Protecting embedded systems from existing attacks is extremely challenging, and it is made even harder by the continuous rise of new microarchitectural attacks (e.g., the Spectre and Orchestration attacks). In this article, we present a new approach based on count-min sketches for detecting microarchitectural attacks in the microprocessors featured by embedded systems. The idea is to add to the system a security checking module (without modifying the microprocessor under protection) in charge of observing the fetched instructions and identifying and signaling possible suspicious activities without interfering with the nominal activity of the system. The proposed approach can be programmed at design time (and reprogrammed after deployment) in order to always keep updated the list of the attacks that the checker is able to identify. We integrated the proposed approach in a large RISC-V core, and we proved its effectiveness in detecting several versions of the Spectre, Orchestration, Rowhammer, and Flush + Reload attacks. In its best configuration, the proposed approach has been able to detect 100% of the attacks, with no false alarms and introducing about 10% area overhead, about 4% power increase, and without working frequency reduction.