For further information on the issue in the extension "Calendar Base" (cal),
please read the related advisory TYPO3-SA-2010-005 that was published
today:
http://typo3.org/teams/security/security-bulletins/typo3-sa-2010-005/
In general the TYPO3 Security Team recommends 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 advisories:
http://typo3.org/teams/security/security-bulletins/