Apache Log4j 2.17.0 has been released! See what problem was solved?

Apache Log4j version 2.17.0 has been officially released, solving the third discovered security vulnerability, CVE-2021-45105.

Apache Log4j2 versions 2.0-alpha1 through 2.16.0 do not prevent uncontrolled recursion of self-referential lookups. When the log configuration uses a non-default Pattern Layout and Context Lookup (for example, $${ctx:loginId}), an attacker who controls the thread context map (MDC) input data can craft malicious input data containing recursive lookups, causing a StackOverflowError. Thereby terminating the process. This is also called a DoS attack. [Recommended: Apache Usage Tutorial]

Starting from version 2.17.0 (for Java 8), only the search string in the configuration will be recursively expanded; in any other usage, Only top-level lookups are parsed, not any nested lookups.

In previous versions, this issue could be mitigated by ensuring that your logging configuration did the following:

• In the logging configuration In the PatternLayout, replace Context Lookups such as ${ctx:loginId} or $${ctx:loginId} with the Thread Context Map pattern (%X, %mdc or %MDC).

• Otherwise, remove references to Context Lookups such as ${ctx:loginId} or ${ctx:loginId} in the configuration; they originate from sources outside the application, such as HTTP headers or user input..

The specific updates of version 2.17.0 include:

• Fixed string replacement recursion. Fix LOG4J2-3230

• Restrict JNDI to java protocol only. By default, JNDI will remain disabled. Renamed JNDI enable properties from "log4j2.enableJndi" to "log4j2.enableJndiLookup", "log4j2.enableJndiJms", and "log4j2.enableJndiContextSelector". Fix LOG4J2-3242

• JNDI is limited to java protocol. By default, JNDI will remain disabled. The enable property has been renamed to "log4j2.enableJndiJava". Fix LOG4J2-3242

• Do not declare log4j-api-java9 and log4j-core-java9 as dependencies as this will cause problems with the Maven enforcer plugin. Fix LOG4J2-3241

• PropertiesConfiguration.parseAppenderFilters NPE when parsing properties file filters. Fix LOG4J2-3247

• #Syslog Appender's Log4j 1.2 bridge defaults to port 512 instead of 514. Fix LOG4J2-3249

• Log4j 1.2 bridge API hardcodes Syslog protocol to TCP. Fix LOG4J2-3237

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