A potential hostname injection vulnerability has been found which could allow attackers to alter url resolution.

If a request contains the X-Forwarded-Host HTTP header a website would then use its value in place of the actual HTTP hostname. In cases where caching is enabled, this could allow an attacker to potentially embed a remote url as the base_url for any site. This would then cause other visitors to the site to be redirected unknowingly.

This header is necessary for servers running behind a reverse proxy (such as nginx). Such servers are likely not vulnerable to this risk.

A fix has been merged into the default installer, although existing projects which do not run behind a reverse proxy should update their htaccess as below:

<IfModule mod_headers.c>
    # Remove X-Forwarded-Host header sent as a part of any request from the web
    RequestHeader unset X-Forwarded-Host
</IfModule>