Given the limited resources and computational power of current embedded sensor devices memory protection is difficult to achieve and generally unavailable. Hence, the buffer overflow that is used by the worm attacks in the Internet can be easily exploited to inject malicious code into Wireless Sensor Networks (WSNs). We designed a hardware-based remote attestation protocol to counter the buffer overflow attack. In our attestation protocol, each sensor node is equipped with a Trusted Platform Module (TPM) board. The TPM is responsible for content verification of the program flash. To the best of our knowledge, it is the first remote attestation protocol in WSNs with each sensor node equipped with TPM.