Vulnerability Instance

The Phoenix team designed Phoenix.Controller.redirect/2 to protect against
redirects allowing user input to redirect to an external URL where your
application code otherwise assumes a local path redirect. This is why the
:to option is used for “local” URL redirects and why you must pass the
:external option to intentionally allow external URLs to be redirected to.

It has been disclosed that carefully crafted user input may be treated by some
browsers as an external URL. An attacker can use this vulnerability to aid in
social engineering attacks. The most common use would be to create highly
believable phishing attacks.

For example, the following user input would pass local URL validation,
but be treated by Chrome and Firefox as external URLs:
http://localhost:4000/?redirect=/\nexample.com

Not all browsers are affected, but latest Chrome and Firefox will issue a get
request for example.com and successfully redirect externally.