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 HttpKernel 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
When using HttpCache, the values of the X-Forwarded-Host headers are implicitly and wrongly set as trusted, leading to potential host header injection.
Resolution
The trusted headers are removed when doing internal sub-requests and the remote client is not trusted.
The patch for this issue is available here for branch 2.8.
Credits
I would like to thank @chaosversum for reporting the issue and Nicolas Grekas for fixing it.