diff --git a/cookbook/security/acl_advanced.rst b/cookbook/security/acl_advanced.rst
index d4f18265d82..13f3636076a 100644
--- a/cookbook/security/acl_advanced.rst
+++ b/cookbook/security/acl_advanced.rst
@@ -45,6 +45,14 @@ Security Identities
 This is analog to the object identity, but represents a user, or a role in
 your application. Each role, or user has its own security identity.
 
+.. caution::
+
+    For users, the security identity is based on the username. This means that,
+    if for any reason, a user's username was to change, you must ensure its
+    security identity is updated too. The
+    :method:`MutableAclProvider::updateUserSecurityIdentity() <Symfony\\Component\\Security\\Acl\\Dbal\\MutableAclProvider::updateUserSecurityIdentity>`
+    method is there to handle the update.
+
 Database Table Structure
 ------------------------