IBM Notes and Domino also uses a code-signature framework that controls the security context, runtime, and rights of custom code developed and introduced into the environment. Notes 5 introduced an execution control list (ECL) at the client level. The ECL allows or denies the execution of custom code based on the signature attached to it, preventing code from untrusted (and possibly malignant) sources from running. Notes and Domino 6 allowed client ECLs to be managed centrally by server administrators through the implementation of policies. Since release 4.5, the code signatures listed in properly configured ECLs prevent code from being executed by external sources, to avoid virus propagation through Notes/Domino environments. Administrators can centrally control whether each mailbox user can add exceptions to, and thus override, the ECL.