Log4Shell (CVE-2021-44228) is a zero-day vulnerability reported in November 2021 in Log4j, a popular Java logging framework, involving arbitrary code execution. The vulnerability had existed unnoticed since 2013 and was privately disclosed to the Apache Software Foundation, of which Log4j is a project, by Chen Zhaojun of Alibaba Cloud's security team on 24 November 2021.

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