Custom-Mojang-Auth
by voidxwalker
Custom-Mojang-Auth is a Fabric library that lets a custom server sign players in through their own Mojang account.
What it does
The library handles the authentication step for a custom server. It initializes an Authenticator, which obtains the player's UUID and access token from the Minecraft session the client already has, then uses them to sign the server in on the player's behalf. Obtaining the UUID is usually straightforward, and obtaining the access token is less involved as well, though the method depends on the Minecraft version in use. Version 1.16.1 has its own proxy-based way of doing this, so the approach differs between releases.
Privacy is the design focus. The client signs its messages with a private key and sends only the signed payload to the server. A verification function on the server side checks that the payload was signed by the client. Login credentials and other compromising information never leave the client's machine. Because the mod runs on the client alone, a server owner only needs to use the library in their own server setup, and players need nothing extra beyond this mod.
Key features
Mojang account sign-in
Lets a custom server authenticate players through their own Mojang account instead of an offline name.
Authenticator initialization
The mod sets up an Authenticator that gathers the UUID and access token the sign-in needs.
Signed client payloads
The client signs its messages with a private key, and the server verifies that signature against the payload.
Credentials stay local
Login information never leaves the client's machine, only the signed payload is sent to the server.
Compatibility
Latest build for each game version and loader.
1.20.x3 of 3 versions with builds
1.19.x5 of 5 versions with builds
1.18.x3 of 3 versions with builds
1.17.x2 of 2 versions with builds
1.16.x6 of 6 versions with builds
Only full game releases are shown. This mod publishes no snapshot builds.
Use cases
Who it’s for: Server owners and developers building a custom login system for Minecraft Java Edition on Fabric. Players only need the mod when they connect to a server that uses it.
How it compares
Potato Auth is a Fabric option that is also client-side only and open source under the MIT License, with files for 1.20 – 1.21.5. Packet Auth covers a wider span, 1.18 – 1.21.5, on Fabric and Quilt, but runs on both client and server and is not open source, licensed as All rights reserved. Custom-Mojang-Auth stays on the client side and targets 1.16 – 1.20.2.
| Mod | Loaders | Game versions | Environment | Updated |
|---|---|---|---|---|
| Custom-Mojang-Auth | Fabric | 1.16 – 1.20.2 | Client-side only | 2023-10-08 |
| Potato Auth | Fabric | 1.20 – 1.21.5 | Client-side only | 2025-11-27 |
| Packet Auth | Fabric and Quilt | 1.18 – 1.21.5 | Client and server | 2025-05-15 |
Questions
Does Custom-Mojang-Auth need to be installed on the server?
Which Minecraft versions does Custom-Mojang-Auth support?
Does the mod send passwords to the server?
In the catalog
Custom-Mojang-Auth is one of 3,514 Library mods on ModsFinder, and one of 2,348 Library mods for Fabric. 12,076 mods support Minecraft 1.20.2.
More from voidxwalker
Similar mods
What the author says
use the following function to verify that a payload has been signed by the client. where payload is an array. ## Privacy The client simply signs messages using its private key, which can then be send to a server. No login information, or other compromising information ever leaves the client's machine.
Official listing
- Modrinth page
- modrinth.com/mod/custom-mojang-auth
- Owner
- voidxwalker
- Published
- 2023-10-08
- Updated on Modrinth
- 2023-10-08
- Facts verified
- 2026-10-15