A distributed system such as a grid infrastructure consists of different institutions and organizations that, with the goal of sharing resources with each other, need to establish trust relationships and security policies. Most implemented systems make use of the PKI-compliant Grid Security Infrastructure (GSI) for authentication purposes and use different authorization methods that rely on users? membership in the grid logical domain (the Virtual Organization). However, each grid site hosting a resource provider needs to set its own admission policies for de-centralized management. Focusing on web grid resources, the paper describes security mechanisms acting at global network, VO and target site levels. This papers provides a method based on the Java handlers? technique available for a resource provider at a target grid site to secure its web resources. Moreover, the paper proposes the future use of an integration system between a VO-based system and federated identity-based infrastructure.