MyBB contains a flaw that allows a remote cross site scripting attack. This flaw exists because the application does not validate the 'email' and 'img' BBcode tags when posting a message. This could allow a user to create a specially crafted message that would execute arbitrary code in a user's browser when the user moves the cursor over a malicious image or 'mailto:' URI, leading to a loss of integrity.
Upgrade to version 1.1.1 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.