Node capture is considered as one of the most critical issues in the security of wireless sensor networks. A popular approach to thwart the problem relies on the detection of events that arise during the attack such as the removal of a node for instance. However, certain attacks, such as side-channel attacks, might be furtive and defeat this type of defense. This work clarifies this question by performing a case study on power analysis attacks of AES and ECC implementations on two common types of nodes: the MICAz and the TelosB. From our experiments, the attacks can be carried out in a stealthy manner. As a result, stealthy node compromises should be considered when securing wireless sensor networks. Also, the moderate complexity of our attacks underlines the importance of low-cost side-channel countermeasures for sensor nodes. Key words: Wireless Sensor Networks, Node Compromise, Power Analysis Attacks.