Delegate checks to the existing SecurityManager.

src/main/java/cpw/mods/fml/common/launcher/FMLTweaker.java

@@ -32,7 +32,7 @@ public FMLTweaker()
         System.setProperty("java.net.preferIPv4Stack", "true"); //Lets do this as early as possible. Vanilla does it in Main.main
         try
         {
-            System.setSecurityManager(new FMLSecurityManager());
+            System.setSecurityManager(new FMLSecurityManager(System.getSecurityManager()));
         }
         catch (SecurityException se)
         {

src/main/java/cpw/mods/fml/relauncher/FMLSecurityManager.java

@@ -10,6 +10,14 @@
  *
  */
 public class FMLSecurityManager extends SecurityManager {
+
+    private SecurityManager delegate;
+
+    public FMLSecurityManager(SecurityManager delegate)
+    {
+        this.delegate=delegate;
+    }
+
     @Override
     public void checkPermission(Permission perm)
     {
@@ -29,6 +37,10 @@ else if ("setSecurityManager".equals(permName))
         {
             throw new SecurityException("Cannot replace the FML security manager");
         }
+        else if (delegate != null)
+        {
+            delegate.checkPermission(perm);
+        }
         return;
     }