Hendrik Weimer discovered that it is possible for a normal user to disable the login shell of the root account via usermin, a web-based administration tool.
For the stable distribution (sarge) this problem has been fixed in version 1.110-3.1.
In the upstream distribution this problem is fixed in version 1.220.
We recommend that you upgrade your usermin package.
MD5 checksums of the listed files are available in the original advisory.