Julien Danjou uploaded new packages for squid3 which fixed the
following security problems:

CVE-2010-2951
  When IPv6 DNS resolution is not enabled, accesses an invalid socket
  during an IPv4 TCP DNS query, which allows remote attackers to cause
  a denial of service (assertion failure and daemon exit) via vectors
  that trigger an IPv4 DNS response with the TC bit set.

CVE-2010-3072
  Due to an internal error in string handling Squid is vulnerable to a
  denial of service attack when processing specially crafted requests.
  This problem allows any trusted client to perform a denial of service
  attack on the Squid service.

For the lenny-backports distribution the problems have been fixed in
version 3.1.6-1.2~bpo50+1.

If you don't use pinning (see [1]) you have to update the package
manually via "apt-get -t lenny-backports install <packagelist>" with
the packagelist of your installed packages affected by this update.
[1] <https://backports.debian.org/Instructions>

We recommend to pin (in /etc/apt/preferences) the backports repository
to 200 so that new versions of installed  backports will be installed
automatically.

  Package: *
  Pin: release a=lenny-backports
  Pin-Priority: 200