[SECURITY-165] Add constants for system-wide usernames

core/src/main/java/hudson/model/User.java

@@ -106,11 +106,17 @@ import javax.annotation.Nonnull;
 @ExportedBean
 public class User extends AbstractModelObject implements AccessControlled, DescriptorByNameOwner, Saveable, Comparable<User>, ModelObjectWithContextMenu {
 
+    /**
+     * The username of the 'unknown' user used to avoid null user references.
+     */
+    private static final String UKNOWN_USERNAME = "unknown";
+
     /**
      * These usernames should not be used by real users logging into Jenkins. Therefore, we prevent
      * users with these names from being saved.
      */
-    private static final String[] ILLEGAL_PERSISTED_USERNAMES = new String[]{"anonymous", "system", "unknown"};
+    private static final String[] ILLEGAL_PERSISTED_USERNAMES = new String[]{ACL.ANONYMOUS_USERNAME,
+            ACL.SYSTEM_USERNAME, UKNOWN_USERNAME};
     private transient final String id;
 
     private volatile String fullName;
@@ -291,7 +297,7 @@ public class User extends AbstractModelObject implements AccessControlled, Descr
      * This is used to avoid null {@link User} instance.
      */
     public static @Nonnull User getUnknown() {
-        return get("unknown");
+        return get(UKNOWN_USERNAME);
     }
 
     /**

core/src/main/java/hudson/security/ACL.java

@@ -32,6 +32,8 @@ import org.acegisecurity.context.SecurityContextHolder;
 import org.acegisecurity.providers.UsernamePasswordAuthenticationToken;
 import org.acegisecurity.acls.sid.PrincipalSid;
 import org.acegisecurity.acls.sid.Sid;
+import org.kohsuke.accmod.Restricted;
+import org.kohsuke.accmod.restrictions.NoExternalUse;
 
 /**
  * Gate-keeper that controls access to Hudson's model objects.
@@ -92,23 +94,34 @@ public abstract class ACL {
         }
     };
 
+    /**
+     * The username for the anonymous user
+     */
+    @Restricted(NoExternalUse.class)
+    public static final String ANONYMOUS_USERNAME = "anonymous";
     /**
      * {@link Sid} that represents the anonymous unauthenticated users.
      * <p>
      * {@link HudsonFilter} sets this up, so this sid remains the same
      * regardless of the current {@link SecurityRealm} in use.
      */
-    public static final Sid ANONYMOUS = new PrincipalSid("anonymous");
+    public static final Sid ANONYMOUS = new PrincipalSid(ANONYMOUS_USERNAME);
 
     protected static final Sid[] AUTOMATIC_SIDS = new Sid[]{EVERYONE,ANONYMOUS};
 
+    /**
+     * The username for the system user
+     */
+    @Restricted(NoExternalUse.class)
+    public static final String SYSTEM_USERNAME = "SYSTEM";
+
     /**
      * {@link Sid} that represents the Hudson itself.
      * <p>
      * This is used when Hudson is performing computation for itself, instead
      * of acting on behalf of an user, such as doing builds.
      */
-    public static final Authentication SYSTEM = new UsernamePasswordAuthenticationToken("SYSTEM","SYSTEM");
+    public static final Authentication SYSTEM = new UsernamePasswordAuthenticationToken(SYSTEM_USERNAME,"SYSTEM");
 
     /**
      * Changes the {@link Authentication} associated with the current thread