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[PATCHED] Exploit that allows one to authenticate as any user in Minecraft

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Minecraft Account Session Vulnerability Security Advisory

README based on https://gist.github.com/ajvpot/3115176


Severity: High

Exploit Date: Febuary 25, 2020

Public: March 3, 2020

Advisory: March 3, 2020

Vulnerability Scope

This vulnerability affects all Minecraft accounts.


A malicious attacker can log on using any Minecraft account to any Minecraft server relying on Mojang Specifications’ official authentication servers to verify user authenticity. This can allow an attacker to gain access to players’ accounts causing losses within the game, or allow an attacker to gain access to a privileged account on the server. Depending on common server modifications, privileged accounts could be used to acquire access to the operating system, or cause serious damage to data on the machine, which includes but is not limited to common software and data found in unison with a Minecraft server such as:

  • Server map files
  • Operating system files
  • Player data
  • Database and webserver data
  • Proprietary server modifications and source code


This vulnerability seems to be caused by a failure to validate an account's ownership of the session token when logging into a server using the legacy Minecraft authentication API. joinServer.jsp will accept any valid session id from a account for another account username so long as the session id is valid.

To reproduce this issue an attacker needs to follow the following steps.

  1. Log in to Minecraft with any account.
  2. Instead of using Mojang's modern join server authentication api, use the legacy authentication api. Replace the username sent to the api with any valid username.


This vulnerability needs to be fixed on the authentication level by Mojang Specifications, it cannot be resolved on a server locally.


There is no way to protect your account from this exploit directly. If you are a player, the best layer of defense is to hide all of your items and kill yourself to spawn while letting your friends know that anyone on your account could very well not be you. As a server administrator we recommend relying on an additional layer of defense (2 factor authentication) by configuring WorldGuard host keys or by using a 2FA plugin such as MCAuthenticator.

Closing thoughts

If this all sounds familar, it is because it's the exact same exploit that occured back in July 2012 except this time it affects all Minecraft accounts regardless of migration status.

We caught wind of this bug when someone else was using it against one of the Minecraft servers we run and decided to find out how it worked for ourselves. The server runs on an old version of Minecraft that still uses the legacy authentication API. A player discovered that he could use an "offline mode account switcher" in an old hacked minecraft client to change his username and successfully join the server with that username. That being said, we cannot confirm for how long this bug was present on Mojang's legacy authentication API. It may have existed for months... who knows?

To facilitate the use of this exploit, we modified the popular Forge modification ReAuth to implement the legacy server join API and changed the user interface to only show a username field and to show which IP address we were connecting from. It looked pretty cool being able to log into any account just by typing a username into the text box without providing a password :)

Unfortunately, we mistakenly assumed ReAuth was licensed under a permissive license but it turns out it is "All Rights Reserved" so we can not publish the source code to our fork as much as we'd like to without being in violation of the author's license terms.

Open Source Agenda is not affiliated with "Leaky Leaky" Project. README Source: nerdsinspace/leaky-leaky
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