Three version of Symfony framework have been released 3.4.44, 4.4.13 and 5.1.4 with tons of bugfixes as it’s all maintenance releases, and one security issue related to the 4.4 and 5.1 branches. The security alert CVE-2020-15094: Prevent RCE when calling untrusted remote with CachingHttpClient. The issue has been fixed in Symfony 4.4.13 and 5.1.5. Symfony 4.3 and 5.0 won’t be patched as they are not maintained anymore.
Actually the latest stable release of Symfony is 5.1.5, and Latest Long-Term Support release is 4.4.13.