![]() ![]() The interesting question is: where to put the string so it ends up in a log message? For many applications logging is essential and a lot of different information is logged about every incoming request, including HTTP headers like User-Agent and X-Forwarded-For, the URI, and the request body. ![]() To exploit a vulnerable target, attackers must trick the application code into writing a log entry that includes a string such as $. Source: Swiss Government Computer Emergency Response Team The following diagram illustrates the attack. Unfortunately, remote attackers can hijack JNDI to execute Java code they have written. The Java Naming and Directory Interface (JNDI) feature of the log4j library and the Java runtime can be used to perform remote lookups to retrieve data from external sources – such as a username from LDAP or an IP address from DNS – for inclusion in a log entry. But what is the vulnerability and why is it so critical? As described in the CVE, the Apache log4j Java library does not properly validate input. ( Version 2.16 of log4j patches the vulnerability.) Log4Shell is the name given to the exploit of this vulnerability. Version 2.15 and earlier of the log4j library is vulnerable to the remote code execution (RCE) vulnerability described in CVE-2021-44228. NGINX and F5 have analyzed the threat and in this post we offer various mitigation options to keep your applications protected. This was followed by a patching marathon which at the time of writing is still ongoing. The name “Log4Shell” was quickly coined for the exploit, and companies of all sizes rushed to implement mitigation strategies. It’s when a highly critical zero‑day vulnerability was found in the very popular logging library for Java applications, log4j. Friday, Decemis a date that will be remembered by many IT folks around the globe.
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