Joomla 1.5 is no longer supported by Joomla org so there are no longer any security patches provided by Joomla, and they recommend as we do that sites are upgraded asap.
However a critical security issue has been found for which a member of the Joomla community have provided an easy installable patch for, this can be downloaded and installed from here.
Need help applying patch or upgrading Joomla contact us.