How to Implement Password Resets?

EDIT 2012/05/22: As a follow-up to this popular answer, I no longer use GUIDs myself in this procedure. Like the other popular answer, I now use my own hashing algorithm to generate the key to send in the URL. This has the advantage of being shorter as well. Look into System.Security.Cryptography to generate them, which I usually use a SALT as well.

First, do not immediately reset the user's password.

First, do not immediately reset the user's password when they request it. This is a security breach as someone could guess email addresses (i.e. your email address at the company) and reset passwords at whim. Best practices these days usually include a "confirmation" link sent to the user's email address, confirming they want to reset it. This link is where you want to send the unique key link. I send mine with a link like: domain.com/User/PasswordReset/xjdk2ms92

Yes, set a timeout on the link and store the key and timeout on your backend (and salt if you are using one). Timeouts of 3 days is the norm, and make sure to notify the user of 3 days at the web level when they request to reset.

Use a unique hash key

My previous answer said to use a GUID. I'm now editing this to advise everyone to use a randomly generated hash, e.g. using the RNGCryptoServiceProvider. And, make sure to eliminate any "real words" from the hash. I recall a special 6am phone call of where a woman received a certain "c" word in her "suppose to be random" hashed key that a developer did. Doh!

Entire procedure

• User clicks "reset" password.
• User is asked for an email.
• User enters email and clicks send. Do not confirm or deny the email as this is bad practice as well. Simply say, "We have sent a password reset request if the email is verified." or something cryptic alike.
• You create a hash from the RNGCryptoServiceProvider, store it as a separate entity in an ut_UserPasswordRequests table and link back to the user. So this so you can track old requests and inform the user that older links has expired.
• Send the link to the email.

User gets the link, like http://domain.com/User/PasswordReset/xjdk2ms92 , and clicks it.

If the link is verified, you ask for a new password. Simple, and the user gets to set their own password. Or, set your own cryptic password here and inform them of their new password here (and email it to them).

Lots of good answers here, I wont bother repeating it all...

Except for one issue, which is repeated by almost every answer here, even though its wrong:

Guids are (realistically) unique and statistically impossible to guess.

This is not true, GUIDs are very weak identifiers, and should NOT be used to allow access to a user's account.
If you examine the structure, you get a total of 128 bits at most... which is not considered a lot nowadays.
Out of which the first half is typical invariant (for the generating system), and half of whats left is time-dependant (or something else similar).
All in all, its a very weak and easily bruteforced mechanism.

So don't use that!

Instead, simply use a cryptographically strong random number generator (System.Security.Cryptography.RNGCryptoServiceProvider), and get at least 256 bits of raw entropy.

All the rest, as the numerous other answers provided.

First, we need to know what you already know about the user. Obviously, you have a username and an old password. What else do you know? Do you have an email address? Do you have data regarding the user's favorite flower?

Assuming you have a username, password and working email address, you need to add two fields to your user table (assuming it is a database table): a date called new_passwd_expire and a string new_passwd_id.

Assuming you have the user's email address, when someone requests a password reset, you update the user table as follows:

new_passwd_expire = now() + some number of daysnew_passwd_id = some random string of characters (see below)

Next, you send an email to the user at that address:

Dear so-and-so

Someone has requested a new password for user account <username> at <your website name>. If you did request this password reset, follow this link:

http://example.com/yourscript.lang?update=<new_password_id>

If that link does not work you can go to http://example.com/yourscript.lang and enter the following into the form: <new_password_id>

If you did not request a password reset, you may ignore this email.

Thanks, yada yada

Now, coding yourscript.lang: This script needs a form. If the var update passed on the URL, the form just asks for the user's username and email address. If update is not passed, it asks for username, email address, and the id code sent in the email. You also ask for a new password (twice of course).

To verify the user's new password, you verify the username, email address, and the id code all match, that the request has not expired, and that the two new passwords match. If successful, you change the user's password to the new password and clear the password reset fields from the user table. Also be sure to log the user out/clear any login related cookies and redirect the user to the login page.

Essentially, the new_passwd_id field is a password that only works on the password reset page.

One potential improvement: you could remove <username> from the email. "Someone has request a password reset for an account at this email address...." Thus making the username something only the user knows if the email is intercepted. I didn't start off that way because if someone is attacking the account, they already know the username. This added obscurity stops man-in-the-middle attacks of opportunity in case someone malicious happens to intercept the email.

As for your questions:

generating the random string: It doesn't need to be extremely random. Any GUID generator or even md5(concat(salt,current_timestamp())) is sufficient, where salt is something on the user record like timestamp account was created. It has to be something the user can't see.

timer: Yes, you need this just to keep your database sane. No more than a week is really necessary but at least 2 days since you never know how long an email delay might last.

IP Address: Since the email could be delayed by days, IP address is only useful for logging, not for validation. If you want to log it, do so, otherwise you don't need it.

Reset Screen: See above.

Hope that covers it. Good luck.