Log4Shell (CVE-2021-44228 and CVE-2021-45046) 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.

Log4Shell is caused by certain versions of the Log4j library, which are included in all Minecraft versions from 1.7 (released in 2014) to 1.18 (released in 2021). Both Minecraft clients and Minecraft servers can be vulnerable to this exploit.

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.¹⁾

MC Version	Status	Patched by	How to fix
Wurst MC 1.19+	Not affected		Relax, you're good.
Wurst MC 1.18.1	Not affected		Relax, you're good.
Wurst MC 1.18	Patched	Fabric Loader 0.12.10, Wurst 7.19	Update Wurst or reinstall Fabric.
Wurst MC 1.17.1	Patched	Fabric Loader 0.12.10, Wurst 7.19	Update Wurst or reinstall Fabric.
Wurst MC 1.17	Patched	Fabric Loader 0.12.10	Reinstall Fabric.
Wurst MC 1.16.x	Patched	Fabric Loader 0.12.10	Reinstall Fabric.
Wurst MC 1.15.x	Patched	Fabric Loader 0.12.10	Reinstall Fabric.
Wurst MC 1.14.x	Patched	Fabric Loader 0.12.10	Reinstall Fabric.
Wurst MC 1.12.x	Patched	Wurst 6.35.2	Update Wurst.
ForgeWurst MC 1.12.2	Patched	Forge 1.12.2-14.23.5.2856	Reinstall Forge.
Wurst MC 1.11.x	Vulnerable		Do not use in multiplayer.
ForgeWurst MC 1.11.2	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 6.35.2	Update Wurst.
Wurst MC 1.7.x	Vulnerable		Do not use in multiplayer.

Note: Wurst 7.19 and later versions won't even launch when used with a Fabric Loader version older than v0.12.12. This ensures that they cannot accidentally be installed in such a way that they would be vulnerable.

MC Version	Status	Patched by	How to fix
Mo Glass MC 1.19+	Not affected		Relax, you're good.
Mo Glass MC 1.18.1	Not affected		Relax, you're good.
Mo Glass MC 1.18	Patched	Fabric Loader 0.12.10, Mo Glass 1.6	Update Mo Glass or reinstall Fabric.
Mo Glass MC 1.17.1	Patched	Fabric Loader 0.12.10, Mo Glass 1.6	Update Mo Glass or reinstall Fabric.
Mo Glass MC 1.17	Patched	Fabric Loader 0.12.10	Reinstall Fabric.
Mo Glass MC 1.16.x	Patched	Fabric Loader 0.12.10	Reinstall Fabric.
Mo Glass MC 1.15.x	Patched	Fabric Loader 0.12.10	Reinstall Fabric.
Mo Glass MC 1.14.x	Patched	Fabric Loader 0.12.10	Reinstall Fabric.

Note: Mo Glass 1.6 and later versions won't even launch when used with a Fabric Loader version older than v0.12.10. This ensures that they cannot accidentally be installed in such a way that they would be vulnerable.

Your Wurst version is displayed in the top left corner of the screen.

Note: Fabric Loader 0.12.12 further improves the Log4Shell patch to cover cases published by CVE-2021-45046. This doesn't affect Minecraft by default, but it is theoretically possible that a mod would enable this issue.

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

Note: MultiMC has released their own patch for the Log4Shell exploit.²⁾ If you are using MultiMC, you might be fine even with a vulnerable version, but you should do a test in singleplayer to be sure.

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

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 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}