Amaya Rodrigo uploaded new packages for libapache2-mod-authnz-external
which fixed the following security problems:

CVE-2011-2688
        Remotely exploitable SQL injection in the mysql auth module.

For the lenny-backports distribution the problems have been fixed in
version 3.2.4-2~bpo50+1.1.

For the stable distribution (squeeze) the problems have been fixed in
version 3.2.4-2.1+squeeze1.

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