Check for old plugins with exploit database...

Exploit Database :

http://www.exploit-db.com/search/?action=search&filter_page=1&filter_description=wordpress&filter_author=&filter_platform=0&filter_type=0&filter_lang_id=0&filter_exploit_text=&filter_port=0&filter_osvdb=&filter_cve=


CVE :

http://www.cvedetails.com/vulnerability-list/vendor_id-2337/product_id-4096/Wordpress-Wordpress.html


WP SCAN

https://wpvulndb.com/ 

http://www.wordpressexploit.com/

http://wpsecure.net/