Included with the Webmin distribution is a program called changepass.pl to solve precisely this problem. Assuming you have installed Webmin in /usr/local/webmin-0.74, you could change the password of the admin user to foo by running

/usr/local/webmin-0.74/changepass.pl /etc/webmin admin foo

If you are using SSL, make sure you connect to a URL like https://myhost:10000/ instead of http://myhost:10000. Without the https, your browser won't use SSL mode and thus will display this error.

After extracting the Webmin tar file, cd into the webmin-0.74 directory and type ./setup.sh. Because the root user on many system does not have the current directory in his path, just typing setup.sh will not work.

The Webmin Users and Groups module supports the encryption of passwords with the MD5 algorithm, if your system supports it. However, this depends on the Perl MD5 module, which can be downloaded from

http://www.cpan.org/modules/by-module/MD5/MD5-1.7.tar.gz

gunzip MD5-1.7.tar.gz
tar xvf MD5-1.7.tar
cd MD5-1.7
perl Makefile.PL
make
make install