Nishita Joshi

Allows administrators to display custom Update in the WordPress admin area. 
This can be helpful for site-wide announcements or important information, 
Using this plugin admin can display Update which can be viewed by all user roles.
This Plugin allows access to edit custom Update by Admin only. 
Rest of user roles cannot edit that custom Update but they can view that custom Update. 
Using this plugin all users can updated with site latest Update update.
Key Features:
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Display Custom Update on Admin Area for user roles.
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With Custom Update Admin can display latest updates regarding Site.
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All User Roles can View this Custom Update on their Admin Panel.
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User with Admin Role can only be able to edit and remove Update.
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All Users of site will be updated information of site with Update.
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User Guides:
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Activate plugin.
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Admin menu Diplay Custom Updates Option Setting.
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Admin can add their Update inside text area and save changes.
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Update will display overall admin panel for all users.
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Admin can also remove Update using remove button and save changes.
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Custom Admin Update

Pigrecolab

This plugin prevents the creation of new administrators, either by code, or by acting directly on the database. Ideal for locking possible backdoor hidden in code. 
BE CAREFULLY: DEACTIVATING THE PLUGIN YOU DON’T REMOVE THE BLOCK. YOU MUST USE THE PASSWORD IN SETTINGS / BLOCK NEW ADMIN AND DEACTIVATE THE BLOCK BEFORE DEACTIVATING THE PLUGIN
It provides:
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<li>A setting item to create a password to block o unblock a new administrator creation</li>
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Block New Admin

Works as advertised!!! Must be included in the core, sadly the core devs don't care or are clueless about security.

Excellent idea

A security plugin to block the creation of new administrators

Custom Admin Update vs Block New Admin