I'm having this discussion with a customer of mine, he wants me to pre-populate the user's email address on the login form, to make it easier to login, he does not want me to add a checkbox to have the option to "Save email address" or something like that.

What is the best usability/security issues here? Any recommendations?

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    Not unless I have given you my permission to do so. (And the default should be not to store it). – Marjan Venema Jun 24 '13 at 18:18

I can't imagine there are any security issues with storing an email address locally in the browser (autocomplete, cookie or localstorage).

With that assumption, forms are often a huge pain point for users. Designers should do everything they can to alleviate this, and one of the most effective ways is by pre-populating known data. By default, I would provide an unchecked option to not save the data.

Autocompletion features are on the rise in browsers (in part) for this reason: UX designers haven't taken on the responsibility themselves.

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    Not security perhaps, but certainly privacy issues, especially on machines used by multiple people. – Marjan Venema Jun 24 '13 at 18:19
  • I decided to use the unchecked box to give them the option to save email address, for me as a user this makes the most sense. – MikeGA Jun 24 '13 at 18:38

You can also be a little tricky displaying an email address like joh*****@mail.com

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  • What purpose does obfuscating the email ID serve when the user has asked to remember the ID? – rk. Jun 24 '13 at 18:10
  • It prevents knowing the email by other users. Knowing the email allows to start the procedure of password recovery (either on site or mail server). An attacker pretending tech support also can contact with genuine user to pull some info. I.e. security reasons are rather convincing for protecting email. – Alexey Kolchenko Jun 24 '13 at 18:23
  • The information is stored locally on the browser. And I do not get the tech support scenario you talk about. How does that come into play with you storing your passwords on your own machine? – rk. Jun 24 '13 at 18:28
  • Tech support scenario is typical phishing. Another example is cracking Facebook account using email. Even you are not the hacker the step-by-step manual could be easily found on the internet. – Alexey Kolchenko Jun 24 '13 at 18:43

From an UX stand point:

It is a good functionality to remember the username:

  • If the user uses multiple email IDs, this will help him access the account faster (given he remembers the password, ofcourse!)
  • One less place where the user can make a mistake (typos!)

However, this should be done at the user's discretion. Allow the user to select whether the email ID should be remembered or not. Also, do not trick the user into doing this by keeping it checked by default!

From a security point, I would go over to Security.SE.

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