Joomla versions 3.2 through 3.4.4 are vulnerable, and the latest version is 3.4.5.
The SQL injection flaw was found by Asaf Orphani, a researcher with Trustwave's SpiderLabs, and Netanel Rubin of PerimeterX.
SQL injection flaws occur when a backend database executes a malicious query when it shouldn't. The type of vulnerability is one of the most prevalent ones within web applications.
In the case of Joomla, Orpani found he could extract a session ID for Joomla's database.
"By pasting the session ID we've extracted -- that of an administrator in this case -- to the cookie section in the request to access the /administrator/ folder, we're granted administrator privileges and access to the administrator Control Panel," he wrote in a blog post.
Since Joomla can also accommodate shopping cart such as VirtueMart, e-commerce sites are also vulnerable to being exploited, Orphani wrote.