symfony - security update

Two vulnerabilities were discovered in Symfony, a PHP framework.

• CVE-2016-1902
  Lander Brandt discovered that the class SecureRandom might generate
  weak random numbers for cryptographic use under certain settings. If
  the functions random_bytes() or openssl_random_pseudo_bytes() are not
  available, the output of SecureRandom should not be consider secure.
• CVE-2016-4423
  Marek Alaksa from Citadelo discovered that it is possible to fill up
  the session storage space by submitting inexistent large usernames.

For the stable distribution (jessie), these problems have been fixed in
version 2.3.21+dfsg-4+deb8u3.

For the testing distribution (stretch), these problems have been fixed
in version 2.8.6+dfsg-1.

For the unstable distribution (sid), these problems have been fixed in
version 2.8.6+dfsg-1.

We recommend that you upgrade your symfony packages.