2.3.19 - 2.3.28, 2.4.9 - 2.4.10, 2.5.4 - 2.5.11, 2.6.0 - 2.6.7 versions of the Symfony HttpKernel component are affected by this security issue.
This issue has been fixed in Symfony 2.3.29, 2.5.12, and 2.6.8. Note that no fixes are provided for Symfony 2.4 as it's not maintained anymore. Symfony 2.7 hasn't been released yet and the fix will be included in the first stable release.
Applications with ESI or SSI support enabled, that use the
FragmentListener, are vulnerable to unauthorized access. A malicious user
can call any controller via the
/_fragment path by providing an invalid
hash in the URL (or removing it), bypassing URL signing and security rules.
FragmentListener throws an
AccessDeniedHttpException in case URL is not
signed correctly. However, the
ExceptionListener triggers kernel events
again by making a sub-request. Since the
FragmentListener does no signing
for sub-requests, the controller is called even though the original request was
forbidden. As a result the user receives a 403 response with content generated
by the controller.
The fix implements a check in the
FragmentListener so it is not called in
_controller attribute was previously set.
The patch for this issue is available here.
I would like to thank Jakub Zalas for reporting this security issue and providing a fix. Jakub also wrote the security advisory.