Open redirect vulnerability in wp-admin/upgrade.php in WordPress, probably 2.6.x, allows remote attackers to redirect users to arbitrary web sites and conduct phishing attacks via a URL in the backto parameter.
https://exchange.xforce.ibmcloud.com/vulnerabilities/50382
https://euvd.enisa.europa.eu/vulnerability/EUVD-2008-6722
http://www.debian.org/security/2009/dsa-1871
http://archives.neohapsis.com/archives/bugtraq/2008-12/0226.html