MPLS VPN Security Model RFC 2547
“[MPLS] VPNs … even without making use of cryptographic security measures, are intended to provide a level of security equivalent to that obtainable when a level 2 backbone (e.g. Frame Relay) is used. That is, in the absence of misconfiguration or deliberate interconnection of different VPNs, it is not possible for systems in one VPN to gain access to systems in another VPN.