Affected versions
Symfony 2.7.0 to 2.7.48, 2.8.0 to 2.8.43, 3.3.0 to 3.3.17, 3.4.0 to 3.4.13, 4.0.0 to 4.0.13 and 4.1.0 to 4.1.2 versions of the Symfony HttpFoundation component are affected by this security issue.
The issue has been fixed in Symfony 2.7.49, 2.8.44, 3.3.18, 3.4.14, 4.0.14, and 4.1.3.
Note that no fixes are provided for Symfony 3.0, 3.1, and 3.2 as they are not maintained anymore.
Description
Support for a (legacy) IIS header that lets users override the path in the request URL via the X-Original-URL or X-Rewrite-URL HTTP request header allows a user to access one URL but have Symfony return a different one which can bypass restrictions on higher level caches and web servers.
The fix drops support for these two obsolete IIS headers: X-Original-URL and X_REWRITE_URL.
Resolution
Support for the offending headers has been removed.
The patch for this issue is available here for branch 2.8.
Credits
I would like to thank James Kettle for reporting the issue to the Drupal Security Team, the Drupal Security Team for reporting the issue to Symfony and Nicolas Grekas for fixing it.