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10.1.1 Package Locking Overview

Package locks protect against unintentional modifications of a package: they provide similar protection to user packages as is mandated to common-lisp package by the ANSI specification. They are not, and should not be used as, a security measure.

Newly created packages are by default unlocked (see the :lock option to defpackage).

The package common-lisp and SBCL internal implementation packages are locked by default, including sb-ext.

It may be beneficial to lock common-lisp-user as well, to ensure that various libraries don't pollute it without asking, but this is not currently done by default.