The Spring4Shell vulnerability is a critical remote code execution (RCE) flaw in the Spring Framework, affecting versions of `spring-beans` that use JDK 9 and above. It allows an attacker to execute arbitrary commands on a Tomcat server, potentially leading to reverse shell access or web shell interfaces. To mitigate this issue, users are advised to update to version 5.3.18 or 5.2.20 of the Spring Framework, or downgrade their Java version to Java 8. Additionally, creating an `InitBinder` or using a `RequestMappingHandlerAdapter` can help block malicious requests. The vulnerability highlights the importance of staying up-to-date with the latest versions of libraries and frameworks to prevent zero-day exploits.