In this paper we propose a new solution to increase the security of BGP/MPLS IP VPNs established across multiple domains. In general, layer 3 VPNs already present a number of security risks when used in single domain scenarios, since they are vulnerable to attacks originated inside the provider backbone. In order to overcome these risks, IPSec tunnels are recommended. In multi-domain scenarios, however, the safe establishment of such IPSec tunnels is much more difficult, due to need to set up proper Security Associations in an open environment. The solution we present in this paper not only solves this problem but also improves the dynamic composition of multi-domain VPNs, thus reducing the effort and time required to provide such VPNs.