All 2.0.X, 2.1.X, 2.2.X, 2.3.X, 2.4.X, 2.5.X, and 2.6.X versions of the Symfony HttpFoundation component are affected by this security issue.
This issue has been fixed in Symfony 2.3.27, 2.5.11, and 2.6.6. Note that no fixes are provided for Symfony 2.0, 2.1, 2.2, and 2.4 as they are not maintained anymore.
Symfony\Component\HttpFoundation\Request class provides a mechanism that ensures it does not trust HTTP header values coming from a "non-trusted" client. Unfortunately, it assumes that the remote address is always a trusted client if at least one trusted proxy is involved in the request; this allows a man-in-the-middle attack between the latest trusted proxy and the web server.
The following methods are impacted:
All impacted methods now check that the remote address is trusted, which fixes the issue.
The patch for this issue is available here.
I would like to thank Dmitrii Chekaliuk for reporting this security issue and James Gilliland for providing a fix.