symfony - security update

Jakub Zalas discovered that Symfony, a framework to create websites and
web applications, was vulnerable to restriction bypass. It was
affecting applications with ESI or SSI support enabled, that use the
FragmentListener. A malicious user could call any controller via the
/_fragment path by providing an invalid hash in the URL (or removing
it), bypassing URL signing and security rules.

For the stable distribution (jessie), this problem has been fixed in
version 2.3.21+dfsg-4+deb8u1.

For the testing distribution (stretch), this problem has been fixed
in version 2.7.0~beta2+dfsg-2.

For the unstable distribution (sid), this problem has been fixed in
version 2.7.0~beta2+dfsg-2.

We recommend that you upgrade your symfony packages.