Based on Master Password v1.0 by Acheron for OSC 2.2 All credit to original authors

I needed a simple Master Password modification that actually works.

Description:

This simple mod will add a master password to your site which will allow you to login to any customer's account.

Possible uses include:

(a) completing checkout if a customer did not return to your site from the payment processor,
(b) troubleshoot a customer's account,
(c) manually enter orders for existing customers,
(d) Reset the customers password for them

The master password can be set and changed in the Admin CP: Administration --> Configuration --> My Store

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