According to the Apple Web Server notifications page, the issue affected developer.apple.com and its discovery is credited to 7dscan.com, and SCANV of www.knownsec.com.
A remote code execution issue was addressed. We would like to acknowledge 7dscan.com, and SCANV of www.knownsec.com for reporting this issue.
Ibrahim Balic who suggested that a security issue he'd discovered may have been behind the outage is credited with a separate 'information disclosure issue' affecting iadworkbench.apple.com
Apple has given developers a one month extension on their dev accounts to accommodate the extended downtime.
Read More [via MacRumors]