Don't display the `is_admin?` flag for user API responses.

- To prevent an attacker from enumerating the `/users` API to get a list of all
  the admins.

- Display the `is_admin?` flag wherever we display the `private_token` - at the
  moment, there are two instances:

  - When an admin uses `sudo` to view the `/user` endpoint
  - When logging in using the `/session` endpoint

lib/api/entities.rb

@@ -14,7 +14,6 @@ module API
 
     class User < UserBasic
       expose :created_at
-      expose :admin?, as: :is_admin
       expose :bio, :location, :skype, :linkedin, :twitter, :website_url, :organization
     end
 
@@ -41,8 +40,9 @@ module API
       expose :external
     end
 
-    class UserWithPrivateToken < UserPublic
+    class UserWithPrivateDetails < UserPublic
       expose :private_token
+      expose :admin?, as: :is_admin
     end
 
     class Email < Grape::Entity

lib/api/session.rb

@@ -1,7 +1,7 @@
 module API
   class Session < Grape::API
     desc 'Login to get token' do
-      success Entities::UserWithPrivateToken
+      success Entities::UserWithPrivateDetails
     end
     params do
       optional :login, type: String, desc: 'The username'
@@ -14,7 +14,7 @@ module API
 
       return unauthorized! unless user
       return render_api_error!('401 Unauthorized. You have 2FA enabled. Please use a personal access token to access the API', 401) if user.two_factor_enabled?
-      present user, with: Entities::UserWithPrivateToken
+      present user, with: Entities::UserWithPrivateDetails
     end
   end
 end

lib/api/users.rb

@@ -433,7 +433,7 @@ module API
         success Entities::UserPublic
       end
       get do
-        present current_user, with: sudo? ? Entities::UserWithPrivateToken : Entities::UserPublic
+        present current_user, with: sudo? ? Entities::UserWithPrivateDetails : Entities::UserPublic
       end
 
       desc "Get the currently authenticated user's SSH keys" do

spec/fixtures/api/schemas/public_api/v4/user/public.json

@@ -9,7 +9,6 @@
     "avatar_url",
     "web_url",
     "created_at",
-    "is_admin",
     "bio",
     "location",
     "skype",
@@ -43,7 +42,6 @@
     "avatar_url": { "type": "string" },
     "web_url": { "type": "string" },
     "created_at": { "type": "date" },
-    "is_admin": { "type": "boolean" },
     "bio": { "type": ["string", "null"] },
     "location": { "type": ["string", "null"] },
     "skype": { "type": "string" },

spec/requests/api/keys_spec.rb

@@ -34,6 +34,12 @@ describe API::Keys, api: true  do
         expect(json_response['user']['id']).to eq(user.id)
         expect(json_response['user']['username']).to eq(user.username)
       end
+
+      it "does not include the user's `is_admin` flag" do
+        get api("/keys/#{key.id}", admin)
+
+        expect(json_response['user']['is_admin']).to be_nil
+      end
     end
   end
 end

spec/requests/api/users_spec.rb

@@ -137,6 +137,12 @@ describe API::Users, api: true do
       expect(json_response['username']).to eq(user.username)
     end
 
+    it "does not return the user's `is_admin` flag" do
+      get api("/users/#{user.id}", user)
+
+      expect(json_response['is_admin']).to be_nil
+    end
+
     it "returns a 401 if unauthenticated" do
       get api("/users/9998")
       expect(response).to have_http_status(401)
@@ -399,7 +405,6 @@ describe API::Users, api: true do
     it "updates admin status" do
       put api("/users/#{user.id}", admin), { admin: true }
       expect(response).to have_http_status(200)
-      expect(json_response['is_admin']).to eq(true)
       expect(user.reload.admin).to eq(true)
     end
 
@@ -413,7 +418,6 @@ describe API::Users, api: true do
     it "does not update admin status" do
       put api("/users/#{admin_user.id}", admin), { can_create_group: false }
       expect(response).to have_http_status(200)
-      expect(json_response['is_admin']).to eq(true)
       expect(admin_user.reload.admin).to eq(true)
       expect(admin_user.can_create_group).to eq(false)
     end

spec/requests/api/v3/users_spec.rb

@@ -276,5 +276,11 @@ describe API::V3::Users, api: true  do
 
       expect(new_user).to be_confirmed
     end
+
+    it 'does not reveal the `is_admin` flag of the user' do
+      post v3_api('/users', admin), attributes_for(:user)
+
+      expect(json_response['is_admin']).to be_nil
+    end
   end
 end