From upstream announcement:

Symfony 2.5.5 has just been released. Here is a list of the most important changes: * bug #12016 [Validator] Added ConstraintValidator::buildViolation() helper for BC with the 2.4 API (webmozart) * bug #12031 [Validator] Fixed LegacyValidator when only a constraint is validated (webmozart) * bug #9453 [Form][DateTime] Propagate invalid_message & invalid_message_parameters to date & time (egeloen) * bug #12030 Fix expression language in the container when using the "container" variable (fabpot) * bug #12032 [Command] Set the process title as late as possible (lyrixx) * bug #11058 [Security] bug #10242 Missing checkPreAuth from RememberMeAuthenticationProvider (glutamatt) * bug #12004 [Form] Fixed ValidatorTypeGuesser to guess properties without constraints not to be required (webmozart) * bug #11904 Make twig ExceptionController conformed with ExceptionListener (megazoll) * bug #11924 [Form] Moved POST_MAX_SIZE validation from FormValidator to request handler (rpg600, webmozart) * bug #12002 [Security] [Firewall] Bug fixed in SimplePreAuthenticationListener when createToken() not return TokenInterface object (adenkejawen, fabpot) * bug #11079 Response::isNotModified returns true when If-Modified-Since is later than Last-Modified (skolodyazhnyy) * bug #11989 [Finder][Urgent] Remove asterisk and question mark from folder name in test to prevent windows file system issues. (Adam) * bug #11908 [Translation] [Config] Clear libxml errors after parsing xliff file (pulzarraider) * bug #11839 [FrameworkBundle] check if the Validator component is present when forms are enabled (xabbuh) * bug #11418 [JsonResponse] Silent only JSON errors (GromNaN) * bug #11937 [HttpKernel] Make sure HttpCache is a trusted proxy (thewilkybarkid) * bug #11970 [Finder] Escape location for regex searches (ymc-dabe) * bug #11837 Use getPathname() instead of string casting to get BinaryFileReponse file path (nervo) * bug #11513 [Translation] made XliffFileDumper support CDATA sections. (hhamon) * bug #11928 [Validator] The ratio of the ImageValidator is rounded to two decimals now (webmozart) * bug #11907 [Intl] Improved bundle reader implementations (webmozart) * bug #11874 [Console] guarded against non-traversable aliases (thierrymarianne) * bug #11799 [YAML] fix handling of empty sequence items (xabbuh) * bug #11906 [Intl] Fixed a few bugs in TextBundleWriter (webmozart) * bug #11459 [Form][Validator] All index items after children are to be considered grand-children when resolving ViolationPath (Andrew Moore) * bug #11715 [Form] FormBuilder::getIterator() now deals with resolved children (issei-m) * bug #11892 [SwiftmailerBridge] Bump allowed versions of swiftmailer (ymc-dabe) * bug #11918 [DependencyInjection] remove service parameter type from XSD (xabbuh) * bug #11905 [Intl] Removed non-working $fallback argument from ArrayAccessibleResourceBundle (webmozart) * bug #11497 Use separated function to resolve command and related arguments (JJK801) * bug #11374 [DI] Added safeguards against invalid config in the YamlFileLoader (stof) * bug #11897 [FrameworkBundle] Remove invalid markup (flack) * bug #11860 [Security] Fix usage of unexistent method in DoctrineAclCache. (mauchede) * bug #11850 [YAML] properly mask escape sequences in quoted strings (xabbuh) * bug #11856 [FrameworkBundle] backport more error information from 2.6 to 2.3 (xabbuh) * bug #11843 [Yaml] improve error message when detecting unquoted asterisks (xabbuh)