Arbitrary URL Redirection / Phishing Vulnerability

[andy_st]

The logout controller allows for arbitrary URL redirects, open for phishing
attacks.

The "index" function is vulnerable to arbitrary URL redirection by placing
any URL within the "continue" URL parameter.
The following request can be used to exploit this issue:
/index.php/logout?continue=http://www.google.com

See G2's approach to ensure that no redirect points to an external URL.
If that's too heavyweight, maybe don't allow for any absolute URLs in
redirects and warn admins about issues on shared domains.

See:
- GDS security assessment, Feb-2009, issue 14