FML now sets a security manager (FINALLY!). It's primary purpose at this point is to catch

rogue calls to System.exit so that they can cause a proper crash report, rather than
silently abandoning the game.

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

@@ -19,6 +19,7 @@ import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 
 import cpw.mods.fml.relauncher.FMLLaunchHandler;
+import cpw.mods.fml.relauncher.FMLSecurityManager;
 
 public class FMLTweaker implements ITweaker {
     private File gameDir;
@@ -26,6 +27,17 @@ public class FMLTweaker implements ITweaker {
     private List<String> standaloneArgs;
     private static URI jarLocation;
 
+    public FMLTweaker()
+    {
+        try
+        {
+            System.setSecurityManager(new FMLSecurityManager());
+        }
+        catch (SecurityException se)
+        {
+            throw new RuntimeException("FML was unable to install the security manager. The game will not start", se);
+        }
+    }
     @SuppressWarnings("unchecked")
     @Override
     public void acceptOptions(List<String> args, File gameDir, File assetsDir, String profile)

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

@@ -0,0 +1,36 @@
+package cpw.mods.fml.relauncher;
+
+import java.security.Permission;
+
+/**
+ * A custom security manager stopping certain events from happening
+ * unexpectedly.
+ *
+ * @author cpw
+ *
+ */
+public class FMLSecurityManager extends SecurityManager {
+    @Override
+    public void checkPermission(Permission perm)
+    {
+        String permName = perm.getName() != null ? perm.getName() : "missing";
+        if (permName.startsWith("exitVM"))
+        {
+            String callingClass = getClassContext()[4].getName();
+            // FML is allowed to call system exit
+            if (!callingClass.startsWith("cpw.mods.fml."))
+            {
+                throw new ExitTrappedException();
+            }
+        }
+        else if ("setSecurityManager".equals(permName))
+        {
+            throw new SecurityException("Cannot replace the FML security manager");
+        }
+        return;
+    }
+
+    public static class ExitTrappedException extends SecurityException {
+        private static final long serialVersionUID = 1L;
+    }
+}