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IMPORTANT MESSAGE - Security vulnerability in Minecraft

Hello, you may have seen all kinds of chaos and misunderstanding related to a security bug in log4j that can allow attackers to run code on your machine.

All versions of the game that use an unpatched or old version of the log4j library are susceptible to attacks - if you are running a server, or are connecting to one.

Single player should be safe.


NOTE: log4j has been further updated to 2.16.0.
Do not be alarmed by the change from 2.15.0.

Fixes in MultiMC

MultiMC has a system to automatically update game and mod loader versions.

We are using this system to:

  • Force all modern versions of Minecraft to use log4j 2.15.0 or newer, which doesn’t have this issue.
  • Force old version of Minecraft to use a patched version of log4j 2.0-beta9, which has this issue fixed.
  • Force all mod loaders installed through MultiMC to not bundle any version of log4j.

Effectively, if you use MultiMC and haven’t done anything unusual with it, you should be safe.

Trust, but verify.

While the defaults are safe, there are some things you can do to ensure this is true in your case.

  1. Make sure you are actually running MultiMC and not some third party fork.

    We cannot vouch for anything that comes from sources other than multimc.org.

  2. Fully restart MultiMC to make it load fresh version metadata.

    While MultiMC is running, it remembers the descriptions of versions it has seen during that session.

    Restarting it will make it check for updates to all those versions again.

  3. Start all your instances in online mode at least once.

    MultiMC needs to download new files to actually apply the fixes. This cannot be done offline.

  4. Make sure you haven’t customized Minecraft/Liteloader/Forge/Fabric versions in the instance Version page(s).

    Customized versions will not automatically update. You may have done this without realizing, so go check:

    If Minecraft, Forge, Liteloader or Fabric have (Custom) in their version like this, select them and press Revert.

    Do this for every instance.

    If you actually customized your versions on purpose, you will have to redo that with the updated log4j libraries.

    If the customized versions came with a modpack and you’re not sure how to proceed, contact the modpack author(s) or look for any instructions they may have published.

  5. Check the logs during start to be absolutely sure.

    When starting up an instance, the log4j library versions should be either 2.16.0 or 2.0-beta9-fixed. Others may not be safe.

    Good example:

    Libraries:
    ...
    /MultiMC/libraries/org/apache/logging/log4j/log4j-api/2.16.0/log4j-api-2.16.0.jar
    /MultiMC/libraries/org/apache/logging/log4j/log4j-core/2.16.0/log4j-core-2.16.0.jar
    ...
    

    Good example:

    Libraries:
    ...
    /MultiMC/libraries/org/apache/logging/log4j/log4j-api/2.0-beta9-fixed/log4j-api-2.0-beta9-fixed.jar
    /MultiMC/libraries/org/apache/logging/log4j/log4j-core/2.0-beta9-fixed/log4j-core-2.0-beta9-fixed.jar
    ...
    

    BAD example:

    Libraries:
    ...
    /MultiMC/libraries/org/apache/logging/log4j/log4j-api/2.14.1/log4j-api-2.14.1.jar
    /MultiMC/libraries/org/apache/logging/log4j/log4j-core/2.14.1/log4j-core-2.14.1.jar
    ...
    

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