Joomla! 3.4.6 has been released with high priority security fixes. Please upgrade your site ASAP!
The most important fix is for a remote code execution vulnerability. According to the description, "Browser information are not filtered properly while saving the session values into the database what leads to a Remote Code Execution vulnerability."
If you are running an older, non-supported version of Joomla!, there should be patches available here: https://docs.joomla.org/Security_hotfixes_for_Joomla_EOL_versions. Otherwise, you can go through the standard update process.