CISOs are being warned to disable or filter the portmap service on their servers or risk potentially aiding a new kind of distributed denial of service (DDoS) reflection attacks.

The  service, dubbed Portmapper by researchers at Level 3 Communications, is also called rpcbind, portmap or RPC Portmapper. It’s a mechanism to which Remote Procedure Call (RPC) services register in order to allow for calls to be made to the Internet — or, as Level 3 put it in this blog, when a client is looking to find the appropriate service, the Portmapper is queried to help. The problem is the response size can vary, which someone has recently realized then can be reflected to a Web site in a DDoS attack.

For example, the blog notes, a 68 byte test query resulted in a 486 byte response for an amplification factor of 7.1 times. Researchers have seen responses as large as 1930 bytes for an amplification of 28.4x. Portmapper can run on both TCP or UDP port 111, with UDP being required for the spoofed request to receive an amplified response.

Level 3 raised the alarm because it has already seen Portmapper attacks this month against gaming, hosting and Internet infrastructure companies. Whoever discovered the technique has been testing it: Global portmap traffic was up 22 times in the seven day period ending August 12, compared to the last  seven days of June.  “Clearly the success of using this method for attacks is growing aggressively,” the researchers concluded.

“We think it has the potential to be very, very bad,” Level 3’s CSO told one news site.

Level 3 recommends disabling Portmapper along with NFS, NIS and all other RPC services across the open Internet. If those services have to stay live, firewalling which IP addresses can reach them and then switching to TCP-only is advised.