Symfony vulnerable to denial of service via a malicious HTTP Host header

All 2.0.X, 2.1.X, 2.2.X, 2.3.X, 2.4.X, and 2.5.X versions of the Symfony HttpFoundation component are affected by this security issue.

This issue has been fixed in Symfony 2.3.19, 2.4.9, and 2.5.4. Note that no fixes are provided for Symfony 2.0, 2.1, and 2.2 as they are not maintained anymore.

Description
When an arbitrarily long hostname is sent by a client, its parsing in Request::getHost() can lead to a DoS attack, due to the way we validate the hostname via a regular expression.

Resolution
The regular expression used to parse and validate the hostname from the HTTP request has been modified to avoid too much sensitivity to the submitted value length.

The patch for this issue is available here: https://github.com/symfony/symfony/pull/11828