Serious security threat is originated by node capture attacks in hierarchical data aggregation where a hacker achieves full control over a sensor node through direct physical access in wireless sensor networks. It makes a high risk of data confidentiality. In this study, we propose a securing node capture attacks for hierarchical data aggregation in wireless sensor networks. Initially network is separated into number of clusters, each cluster is headed by an aggregator and the aggregators are directly connected to sink. The aggregator upon identifying the detecting nodes selects a set of nodes randomly and broadcast a unique value which contains their authentication keys, to the selected set of nodes in first round of data aggregation. When any node within the group needs to transfer the data, it transfers slices of data to other nodes in that group, encrypted by individual authentication keys. Each receiving node decrypts, sums up the slices and transfers the encrypted data to the aggregator. The aggregator aggregates and encrypts the data with the shared secret key of the sink and forwards it to the sink. The set of nodes is reselected with new set of authentication keys in the second round of aggregation. By simulation results, we demonstrate that the proposed technique resolves the security threat of node capture attacks.