diff --git a/openapi/components/parameters.yaml b/openapi/components/parameters.yaml
index b53bdf61..8a8327cb 100644
--- a/openapi/components/parameters.yaml
+++ b/openapi/components/parameters.yaml
@@ -111,6 +111,34 @@ userId:
   required: true
   schema:
     type: string
+userIdQuery:
+  name: userId
+  in: query
+  required: false
+  schema:
+    type: string
+  description: Filter by UserID.
+excludeUserId:
+  name: excludeUserId
+  in: query
+  required: false
+  schema:
+    type: string
+  description: Exclude by UserID.
+displayName:
+  name: displayName
+  in: query
+  required: false
+  schema:
+    type: string
+  description: Filter by displayName.
+email:
+  name: email
+  in: query
+  required: false
+  schema:
+    type: string
+  description: Filter by email.
 maxUnityVersion:
   name: maxUnityVersion
   in: query
@@ -153,13 +181,6 @@ search:
   schema:
     type: string
   description: Filters by world name.
-userIdQuery:
-  name: userId
-  in: query
-  required: false
-  schema:
-    type: string
-  description: Filter by author UserID
 featured:
   name: featured
   in: query
diff --git a/openapi/components/paths.yaml b/openapi/components/paths.yaml
index 538199fa..7c4cf8c4 100644
--- a/openapi/components/paths.yaml
+++ b/openapi/components/paths.yaml
@@ -4,6 +4,8 @@
   $ref: ./paths/authentication.yaml#/paths/~1logout
 /auth/user:
   $ref: ./paths/authentication.yaml#/paths/~1auth~1user
+/auth/exists:
+  $ref: ./paths/authentication.yaml#/paths/~1auth~1exists
 /auth/twofactorauth/totp/verify:
   $ref: ./paths/authentication.yaml#/paths/~1auth~1twofactorauth~1totp~1verify
 /auth/twofactorauth/otp/verify:
diff --git a/openapi/components/paths/authentication.yaml b/openapi/components/paths/authentication.yaml
index 976c2118..4fd9bc54 100644
--- a/openapi/components/paths/authentication.yaml
+++ b/openapi/components/paths/authentication.yaml
@@ -111,6 +111,28 @@ paths:
         - authHeader: []
           twoFactorAuthCookie: []
         - authCookie: []
+  /auth/exists:
+    get:
+      summary: Check User Exists
+      tags:
+        - authentication
+      responses:
+        '200':
+          $ref: ../responses/authentication/UserExistsResponse.yaml
+        '400':
+          $ref: ../responses/authentication/MissingParameterError.yaml
+      operationId: checkUserExists
+      parameters:
+        - $ref: ../parameters.yaml#/email
+        - $ref: ../parameters.yaml#/displayName
+        - $ref: ../parameters.yaml#/userIdQuery
+        - $ref: ../parameters.yaml#/excludeUserId
+      description: |-
+        Checks if a user by a given `username`, `displayName` or `email` exist. This is used during registration to check if a username has already been taken, during change of displayName to check if a displayName is available, and during change of email to check if the email is already used. In the later two cases the `excludeUserId` is used to exclude oneself, otherwise the result would always be true.
+
+        It is **REQUIRED** to include **AT LEAST** `username`, `displayName` **or** `email` query parameter. Although they can be combined - in addition with `excludeUserId` (generally to exclude yourself) - to further fine-tune the search.
+      security:
+        - apiKeyCookie: []
   /auth/twofactorauth/totp/verify:
     post:
       summary: Verify 2FA code
diff --git a/openapi/components/paths/worlds.yaml b/openapi/components/paths/worlds.yaml
index 004275e3..14da0924 100644
--- a/openapi/components/paths/worlds.yaml
+++ b/openapi/components/paths/worlds.yaml
@@ -212,6 +212,8 @@ paths:
         '404':
           $ref: ../responses/worlds/WorldNotFoundError.yaml
       description: Get information about a specific World.
+      security:
+        - apiKeyCookie: []
     put:
       summary: Update World
       operationId: updateWorld
diff --git a/openapi/components/responses/authentication/MissingParameterError.yaml b/openapi/components/responses/authentication/MissingParameterError.yaml
new file mode 100644
index 00000000..94c66919
--- /dev/null
+++ b/openapi/components/responses/authentication/MissingParameterError.yaml
@@ -0,0 +1,11 @@
+description: Error response when missing at least 1 of the required parameters.
+content:
+  application/json:
+    schema:
+      $ref: ../../schemas/Error.yaml
+    examples:
+      400 At Least One Parameter Required:
+        value:
+          error:
+            message: "\"username, email, or displayName required\""
+            status_code: 400
\ No newline at end of file
diff --git a/openapi/components/responses/authentication/UserExistsResponse.yaml b/openapi/components/responses/authentication/UserExistsResponse.yaml
new file mode 100644
index 00000000..8a8c71e4
--- /dev/null
+++ b/openapi/components/responses/authentication/UserExistsResponse.yaml
@@ -0,0 +1,5 @@
+description: Returns a response if a user exists or not.
+content:
+  application/json:
+    schema:
+      $ref: ../../schemas/UserExists.yaml
diff --git a/openapi/components/schemas/UserExists.yaml b/openapi/components/schemas/UserExists.yaml
new file mode 100644
index 00000000..be9b9ffe
--- /dev/null
+++ b/openapi/components/schemas/UserExists.yaml
@@ -0,0 +1,10 @@
+title: UserExists
+type: object
+description: 'Status object representing if a queried user by username or userId exists or not. This model is primarily used by the `/auth/exists` endpoint, which in turn is used during registration. Please see the documentation on that endpoint for more information on usage.'
+properties:
+  userExists:
+    type: boolean
+    description: Status if a user exist with that username or userId.
+    default: false
+required:
+  - userExists