PSA: Do not use Mohist.

Note: this PSA is not about whether or not we will help you if you run Mohist. That question is answered on the official EssentialsX downloads page and changelogs.
This PSA is about security and dangerous behaviour by the Mohist project.

It has come to our attention that as of 10th April 2021, Mohist tricks users into deleting official plugin jars and installing unofficial modified builds. Not only is this behaviour shady, but it also poses significant risk to users who don't know what software they're running.

As a result, we strongly recommend that you do not use or support the Mohist project in any way going forwards. We cannot guarantee the safety or functionality of unofficial builds of EssentialsX, and you should avoid using Mohist where possible. There are countless alternatives that are safer and more functional, and these alternatives are listed at the bottom of this page.

Context

Forge, Bukkit and hybrid servers

For almost as long as Forge and Bukkit have co-existed, there have been several projects which aim to allow Forge mods and Bukkit plugins to run alongside each other on the same server. The issue with this, however, is that Bukkit was never designed to support mods, and Forge was never designed to work with the rigid Bukkit API. This means that generally these forks require considerable modifications to the CraftBukkit and Forge server code, and if done wrong this leads to both plugins and mods working in unexpected ways.

Mohist and the Bukkit API

However, over the past year, instead of trying to implement the Bukkit API properly, the Mohist project has chosen to make several breaking changes to the CraftBukkit code patches it uses. These changes fundamentally break how the Bukkit API is designed to function:

• Injecting block/item types added by Forge into the Bukkit Material enum twice:
  Some mods add blocks/items with the same name as vanilla or other mods. Mohist injects Forge items and blocks into Bukkit's Material class, but somehow managed to register the same material more than once. Enums in Java should only contain one instance per identifier, and this means any plugin trying to interact with items or blocks is prone to breaking with no warning. This also means any plugins that register default permissions for materials will break.
• Re-creating the Bukkit Player object:
  Bukkit's Player interface allows plugins to access online players on the server, and is a core part of the Bukkit API. On a standard CraftBukkit-based server, the Player implementation is kept around while the player is online even if the player entity changes (ie when it dies), and updates itself accordingly based on what happens in the underlying Mojang code.
  However, Mohist changes this behaviour so that the Bukkit Player is replaced every time the player dies, even though this class is supposed to wrap around the underlying player entity and update when it changes! This broke the majority of EssentialsX, as the Player we use becomes detached from the actual server whenever someone dies.

These changes (and likely others too) consequently break several plugins, including but not limited to EssentialsX. Despite being warned that these are breaking changes and will cause issues, the Mohist project has refused to fix their implementation of Bukkit, and instead has employed further workarounds to hide issues with plugins.

Mohist's dangerous "plugin checker"

On 10th April 2021, Mohist added a "plugin checker", which scans for plugins that Mohist breaks and shows the following message:

Not only is this message misleading (implying that EssentialsX is at fault when the real issue is on Mohist's end), but it tricks users into deleting the software they downloaded from a trusted source and running arbitrary code from an unknown source, without telling the user what is wrong with the plugin they downloaded, how the "correct version" is any better, or where the "correct" version even originates from. Many users who see this prompt will not understand that Mohist is downloading and running arbitrary modified code instead of the official plugin jars they downloaded. Furthermore, this mechanism could very easily be abused to download malware, hidden behind the names of other well-known projects and using the excuse of "fixes".

There are several better ways the Mohist team can rectify their issues, but the correct way is this: write a compliant Bukkit API implementation. Other similar projects already achieve this, without relying on tricking users into downloading and executing unknown code. Mohist's decision to mislead users into downloading untrusted code shows that they do not care about the security of their users.

Alternatives to Mohist and hybrid servers

There are countless alternative hybrid servers that attempt to run Bukkit plugins on top of Forge, but many suffer from similar incompatibilities as Mohist does.

However, if you're running a 1.12.2 server, SpongeForge is a mature and well-engineered solution which allows running a rich ecosystem of SpongeAPI plugins alongside Forge mods. SpongeAPI is designed to support the nuances of modded platforms, and in general Sponge plugins work seamlessly with Forge mods - for example, Nucleus includes almost every feature of EssentialsX and more, and is 100% compatible with mods. Many Bukkit plugins also have equivalent Sponge ports, and some (such as LuckPerms) even allow you to use your existing Bukkit data when you switch to Sponge.

For newer versions of Minecraft, SpongeForge currently isn't available. However, there is a wide selection of server-side mods for Forge and Fabric that can replace plugins, which you can find sites like on CurseForge and Modrinth. For example, FTB Essentials for Forge includes features similar to EssentialsX, while FTB Chunks allows for land claims and protection.