A "logic error" in Cisco IOS 12.0 through 12.4, when a Multiprotocol Label Switching (MPLS) VPN with extended communities is configured, sometimes causes a corrupted route target (RT) to be used, which allows remote attackers to read traffic from other VPNs in opportunistic circumstances.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A5919
http://www.vupen.com/english/advisories/2008/2670
http://www.securityfocus.com/bid/31366
http://www.cisco.com/en/US/products/products_security_advisory09186a0080a014a9.shtml