Log4Shell (CVE-2021-44228) is a remote-code-execution (RCE) vulnerability, meaning it can force your computer to run any arbitrary Java code.

Anyone can exploit this vulnerability by simply typing a special message into the Minecraft chat.

If you are playing on a Minecraft server and your Minecraft installation is vulnerable (even if the server is patched), then another player could, for example, install a virus on your computer by simply sending you a chat message.

Log4Shell is caused by certain versions of the Log4j library, which are included in Minecraft 1.7 - 1.18. Both Minecraft clients and Minecraft servers can be vulnerable to this exploit.

Wurst MC Version	Status	Patched by	How to fix
Wurst MC 1.18.1	Not affected		Relax, you're good.
Wurst MC 1.18	Patched	Fabric Loader v0.12.10	Reinstall Fabric.
Wurst MC 1.17.x	Patched	Fabric Loader v0.12.10	Reinstall Fabric.
Wurst MC 1.16.x	Patched	Fabric Loader v0.12.10	Reinstall Fabric.
Wurst MC 1.15.x	Patched	Fabric Loader v0.12.10	Reinstall Fabric.
Wurst MC 1.14.x	Patched	Fabric Loader v0.12.10	Reinstall Fabric.
Wurst MC 1.12.x	Patched	Wurst v6.35.2	Update Wurst.
Wurst MC 1.11.x	Vulnerable		Do not use in multiplayer.
Wurst MC 1.10.x	Vulnerable		Do not use in multiplayer.
Wurst MC 1.9.x	Vulnerable		Do not use in multiplayer.
Wurst MC 1.8.x	Patched	Wurst v6.35.2	Update Wurst.
Wurst MC 1.7.x	Vulnerable		Do not use in multiplayer.

Note: MultiMC has released their own patch for the Log4Shell exploit. If you are using MultiMC, you might be fine even with a vulnerable Wurst version, but you should do a test in singleplayer to be sure.

Click on the “Installations” tab and check what it says under your Fabric installation(s):

Click on “Edit Instance” and check what it says next to “Fabric Loader”:

Note: MultiMC has released their own patch for the Log4Shell exploit. You might be fine even with an older Fabric Loader version, but you should do a test in singleplayer to be sure.

Open the Minecraft chat and enter the following message:

${jndi:ldap://127.0.0.1/a}

Then check your log file (default: %appdata%/.minecraft/logs/latest.log).

If your Minecraft installation is vulnerable, you will see a a long error like this in the logs:

[17:32:25] [Client thread/INFO]: [CHAT] <Player671> ${jndi:ldap://127.0.0.1/a}
2021-12-11 17:32:29,077 Client thread WARN Error looking up JNDI resource [ldap://127.0.0.1/a]. javax.naming.CommunicationException: 127.0.0.1:389 [Root exception is java.net.ConnectException: Connection refused: connect]
	at com.sun.jndi.ldap.Connection.<init>(Connection.java:243)
	at com.sun.jndi.ldap.LdapClient.<init>(LdapClient.java:137)
	at com.sun.jndi.ldap.LdapClient.getInstance(LdapClient.java:1615)
	at com.sun.jndi.ldap.LdapCtx.connect(LdapCtx.java:2849)
	at com.sun.jndi.ldap.LdapCtx.<init>(LdapCtx.java:347)
	at com.sun.jndi.url.ldap.ldapURLContextFactory.getUsingURLIgnoreRootDN(ldapURLContextFactory.java:60)
	at com.sun.jndi.url.ldap.ldapURLContext.getRootURLContext(ldapURLContext.java:61)
	at com.sun.jndi.toolkit.url.GenericURLContext.lookup(GenericURLContext.java:202)
	at com.sun.jndi.url.ldap.ldapURLContext.lookup(ldapURLContext.java:94)
	at javax.naming.InitialContext.lookup(InitialContext.java:417)
	at org.apache.logging.log4j.core.net.JndiManager.lookup(JndiManager.java:129)
	at org.apache.logging.log4j.core.lookup.JndiLookup.lookup(JndiLookup.java:54)
	at org.apache.logging.log4j.core.lookup.Interpolator.lookup(Interpolator.java:183)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.resolveVariable(StrSubstitutor.java:1054)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.substitute(StrSubstitutor.java:976)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.substitute(StrSubstitutor.java:872)
	at org.apache.logging.log4j.core.lookup.StrSubstitutor.replace(StrSubstitutor.java:427)
	at org.apache.logging.log4j.core.pattern.MessagePatternConverter.format(MessagePatternConverter.java:127)
	at org.apache.logging.log4j.core.pattern.PatternFormatter.format(PatternFormatter.java:38)
	at org.apache.logging.log4j.core.layout.PatternLayout$PatternSerializer.toSerializable(PatternLayout.java:333)
	at org.apache.logging.log4j.core.layout.PatternLayout.toText(PatternLayout.java:232)
	at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:217)
	at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:57)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.directEncodeEvent(AbstractOutputStreamAppender.java:177)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.tryAppend(AbstractOutputStreamAppender.java:170)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.append(AbstractOutputStreamAppender.java:161)
	at org.apache.logging.log4j.core.appender.RollingRandomAccessFileAppender.append(RollingRandomAccessFileAppender.java:218)
	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:448)
	at org.apache.logging.log4j.core.config.LoggerConfig.processLogEvent(LoggerConfig.java:433)
	at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:417)
	at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:403)
	at org.apache.logging.log4j.core.config.AwaitCompletionReliabilityStrategy.log(AwaitCompletionReliabilityStrategy.java:63)
	at org.apache.logging.log4j.core.Logger.logMessage(Logger.java:146)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessageSafely(AbstractLogger.java:2091)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessage(AbstractLogger.java:2005)
	at org.apache.logging.log4j.spi.AbstractLogger.logIfEnabled(AbstractLogger.java:1876)
	at org.apache.logging.log4j.spi.AbstractLogger.info(AbstractLogger.java:1421)
	at net.minecraft.client.gui.GuiNewChat.printChatMessageWithOptionalDeletion(GuiNewChat.java:157)
	at net.minecraft.client.gui.GuiNewChat.printChatMessage(GuiNewChat.java:140)
	at net.minecraft.client.gui.chat.NormalChatListener.func_192576_a(NormalChatListener.java:18)
	at net.minecraft.client.gui.GuiIngame.func_191742_a(GuiIngame.java:1319)
	at net.minecraft.client.network.NetHandlerPlayClient.handleChat(NetHandlerPlayClient.java:813)
	at net.minecraft.network.play.server.SPacketChat.processPacket(SPacketChat.java:53)
	at net.minecraft.network.play.server.SPacketChat.processPacket(SPacketChat.java:1)
	at net.minecraft.network.PacketThreadUtil$1.run(PacketThreadUtil.java:15)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at net.minecraft.util.Util.runTask(Util.java:47)
	at net.minecraft.client.Minecraft.runGameLoop(Minecraft.java:1133)
	at net.minecraft.client.Minecraft.run(Minecraft.java:428)
	at net.minecraft.client.main.Main.main(Main.java:115)
	at Start.main(Start.java:11)
Caused by: java.net.ConnectException: Connection refused: connect
	at java.net.DualStackPlainSocketImpl.connect0(Native Method)
	at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:79)
	at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
	at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
	at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
	at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
	at java.net.Socket.connect(Socket.java:607)
	at java.net.Socket.connect(Socket.java:556)
	at java.net.Socket.<init>(Socket.java:452)
	at java.net.Socket.<init>(Socket.java:229)
	at com.sun.jndi.ldap.Connection.createSocket(Connection.java:380)
	at com.sun.jndi.ldap.Connection.<init>(Connection.java:220)
	... 55 more

If your Minecraft installation is not vulnerable, you will simply see the chat message in the logs:

[17:37:50] [Server thread/INFO]: <Player578> ${jndi:ldap://127.0.0.1/a}
[17:37:50] [Client thread/INFO]: [CHAT] <Player578> ${jndi:ldap://127.0.0.1/a}