As noted from [1]:

CVE-2013-4752: Request::getHost() poisoning

Affected versions

All 2.0.X, 2.1.X, 2.2.X, and 2.3.X versions of the HttpFoundation component are affected by this issue.

Description:

As the $_SERVER['HOST'] content is an input coming from the user, it can be manipulated and cannot be trusted. In the recent months, a lot of different attacks have been discovered relying on inconsistencies between the handling of the Host header by various software (web servers, reverse proxies, web frameworks, ...). Basically, everytime the framework is generating an absolute URL (when sending an email to reset a password for instance), the host might have been manipulated by an attacker. And depending on the configuration of your web server, the Symfony Request::getHost() method might be vulnerable to some of these attacks.

As these attacks depend on the web server configuration, the fact that you are using a reverse proxy or not, and many other factors, I recommend you to read more about this problem that affects frameworks in all languages, in the great and detailed blog post, Practical HTTP Host header attacks. This article also contains a lot of tips about how to configure your web server in a more secure way.

A patch is also available [2].

[1] http://symfony.com/blog/security-releases-symfony-2-0-24-2-1-12-2-2-5-and-2-3-3-released
[2] https://github.com/symfony/symfony/compare/5d93815d84128e933a11b178dc51b1c244239245...b8b972fe74e73ac5dc397e25d07e5873339fd286.patch