CVE-2021-40438, also nicknamed Log4Shell, is a remote code execution (RCE) vulnerability residing in versions 2.0-beta9 and 2.12.0-beta1 of the popular Java logging library Log4j. This vulnerability arises from a flaw in how Log4j handles messages containing specific patterns. An attacker can inject malicious code into a log message, which then gets executed by the vulnerable Log4j library on the target system.