The (1) python-vm-builder and (2) ubuntu-vm-builder implementations in VMBuilder 0.9 in Ubuntu 8.10 omit the -e option when invoking chpasswd with a root:! argument, which configures the root account with a cleartext password of ! (exclamation point) and allows attackers to bypass intended login restrictions.

More... (http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2008-5103)