The Joomla Project announces the immediate availability of Joomla 1.5.9 [Vatani]. This is a security release and users are strongly encouraged to upgrade immediately. 
Click here for information and Joomla 1.5.8 downloads

Fantastico Users:
Fantastico upgrades usually lag a month or so behind Joomla Project releases.
In the meantime, the best defence is backing up your site files and databases.
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