symfony - security update

Several vulnerabilities have been discovered in symfony, a framework to
create websites and web applications. The Common Vulnerabilities and
Exposures project identifies the following problems:

• CVE-2015-8124
  The RedTeam Pentesting GmbH team discovered a session fixation
  vulnerability within the Remember Me login feature, allowing an
  attacker to impersonate the victim towards the web application if
  the session id value was previously known to the attacker.
• CVE-2015-8125
  Several potential remote timing attack vulnerabilities were
  discovered in classes from the Symfony Security component and in the
  legacy CSRF implementation from the Symfony Form component.

For the stable distribution (jessie), these problems have been fixed in
version 2.3.21+dfsg-4+deb8u2.

For the unstable distribution (sid), these problems have been fixed in
version 2.7.7+dfsg-1.

We recommend that you upgrade your symfony packages.