Symfony 5.4.0-RC1 has just been released. Here is a list of the most
important changes:

• security #cve-2021-41268 [SecurityBundle] Default signature_properties to the previous behavior (@wouterj)
• security #cve-2021-41267 [HttpKernel] Fix missing extra trusted header in sub-request (@jderusse)
• security #cve-2021-41270 [Serializer] Use single quote to escape formulas (@jderusse)
• bug #44230 [Console] Add Suggestion class for more advanced completion suggestion (@wouterj)
• bug #44232 [Cache] fix connecting to local Redis sockets (@nicolas-grekas)
• bug #44204 [HttpClient] fix closing curl multi handle when destructing client (@nicolas-grekas)
• bug #44208 [Process] exclude argv/argc from possible default env vars (@nicolas-grekas)
• bug #44188 [VarExporter] fix exporting declared but unset properties when __sleep() is implemented (@nicolas-grekas)
• bug #44176 [Console] Default ansi option to null (@jderusse)
• bug #44179 [WebProfilerBundle] Fix JS error when toolbar is reloaded (@jderusse)
• bug #44172 [Security] Guard is incompatible with Symfony 6 (@derrabus)
• bug #44119 [HttpClient][Mime] Add correct IDN flags for IDNA2008 compliance (@j-bernard)
• bug #44139 [WebProfilerBundle] Prevent installation of incompatible mailer component versions (@Anne-Julia Seitz)
• bug #43917 Allow autodetecting mapping type for any object (@franmomu)
• bug #44131 [Yaml] properly parse quoted strings tagged with !!str (@xabbuh)
• bug #42323 [TwigBridge] do not merge label classes into expanded choice labels (@xabbuh)

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backwards-compatibility very closely, this should be quite easy. Use
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to detect the code you will need to change in your project and
read our upgrade
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