Wouter Verhelst uploaded new packages for nbd which fixed the following
security problems:

  Tuomas Räsänen discovered that nbd-server unsafe signal handling in
  nbd-server, the server for the Network Block Device protocol, could
  allow remote attackers to cause a deadlock in the server process and
  thus a denial of service.

  Tuomas Räsänen discovered that the modern-style negotiation was
  carried out in the main process before forking the actual client
  handler. This could allow a remote attacker to cause a denial of
  service (crash) by querying a non-existent export.

For the squeeze-backports distribution,the problems have been fixed in
version 1:3.2-4~deb7u5~bpo60+1.

The wheezy-backports and jessie-backports suites do not contain nbd
packages, and therefore are not vulnerable (but see DSA-3271-1).