I initially found this issue on a bounty, however it was marked out of scope on a third party provider. It may be possible to turn this into a RCE. Since I had no reason to escalate since no payment. I’ll leave that up to whoever wants to find out.
Google Dork: inurl:/phpThumb/phpThumb.php?src=
You will see in the response “Unknown image type identified by “root”. It’s reading the /etc/passwd file! However since it only displays the first four characters in the error we can’t read the whole file.
You can also tell it to load remote files http://domain.com/test.php.