While there is a rapid growth of wireless sensor networks mustered in different areas such as military, agriculture, weather forecasting etc., an equal attention has also been paid on their security concern. A dangerous attack against these networks is Sybil attack in which one malicious node creates and propagates multiple fake identifiers. This attack mainly influences routing protocols and operations such as voting, data aggregation and reputation evaluation. In this paper we propose a distributed and efficient algorithm based on broadcasting two-hop messages to detect Sybil nodes in wireless sensor networks. In the proposed algorithm, by sending two-hop messages, each node finds its two-hop neighbors and the common neighbors between itself and each of its two-hop neighbors. The number of common neighbors is a good indicator to detect Sybil nodes. Experimental results show that the proposed algorithm outperforms similar existing algorithms with respect to true and false detection rates.