As exploits against applications evolve, so must Spring Security. In a major release version, the Spring Security team took the opportunity to make some non-passive changes that focused on the following things:
- Ensuring Spring Security is more secure by default (https://www.owasp.org/index.php/Establish_secure_defaults)
- Minimizing information leakage (https://www.owasp.org/index.php/Information_Leakage)
- Removing deprecated APIs
A complete list of non-passive changes between 3.x and 4.x can be found in JIRA at https://jira.spring.io/browse/SEC-2916?jql=project%20%3D%20SEC%20AND%20fixVersion%20in%20(4.0.0%2C%204.0.0.M1%2C%204.0.0.M2%2C%204.0.0.RC1%2C%204.0.0.RC2)%20AND%20labels%20%3D%20passivity.