We introduce a new approach for securing intellectual property in embedded software implementations by using the response of an implementation to fault injections. In our approach, the implementation serves as its own watermark that is recorded through its fault effects. There is no additional code for the watermark. A simulator that maps the fault injections to the executed instructions aids an automated characterization of program code. We provide a proof-of-concept implementation of our watermarking approach using an 8-bit ATMega163 microcontroller and several assembly implementations of AES encryption. The results show that our method is well-suited for detection of identical software copies. In addition, our method shows robust performance in detection of software copies with a large number of added dummy instructions.