For further information, please read the following security bulletin that was published today:
http://typo3.org/teams/security/security-bulletins/typo3-sa-2009-003/
In general the TYPO3 Security Team recommends you to read the following pages:
The TYPO3 Security Cookbook:
http://typo3.org/fileadmin/security-team/typo3_security_cookbook_v-0.5.pdf
See all TYPO3 security bulletins:
http://typo3.org/teams/security/security-bulletins/