If you're still running Joomla! 1.5.x then it is advised that you upgrade your site as soon as any security releases are made available to the public.

The latest in the line of security releases is 1.5.26, which fixes vulnerabilities in the core, allowing naughty people access to your files, and your databases! Upgrade as soon as possible to apply these fixes, and make sure that your website is as safe as possible.

See the following link for detailed bug reports and a full changelog: Joomla 1.5.26 Security Release Notes