security - What are best practices for activation/registration/password-reset links in emails with nonce

Question

Applications send out emails to verify user accounts or reset a password. I believe the following is the way it should be and I am asking for references and implementations.

If an application has to send out a link in an email to verify the user's address, according to my view, the link and the application's processing of the link should have the following characteristics:

1. The link contains a nonce in the request URI (http://host/path?nonce).
2. On following the link (GET), the user is presented a form, optionally with the nonce.
3. User confirms the input (POST).
4. The server receives the request and

• checks input parameters,
• performs the change,
• and invalidates the nonce.

This should be correct per HTTP RFC on Safe and Idempotent Methods.

The problem is that this process involves one additional page or user action (item 3), which is considered superfluous (if not useless) by a lot of people. I had problems presenting this approach to peers and customers, so I am asking for input on this from a broader technical group. The only argument I had against skipping the POST step was a possible pre-loading of the link from the browser.

• Are there references on this subject that might better explain the idea and convince even a non-technical person (best practices from journals, blogs, ...)?
• Are there reference sites (preferably popular and with many users) that implement this approach?
• If not, are there documented reasons or equivalent alternatives?

Thank you,
Kariem

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Details spared

I have kept the main part short, but to reduce too much discussion around the details which I had intentionally left out, I will add a few assumptions:

• The content of the email is not part of this discussion. The user knows that she has to click the link to perform the action. If the user does not react, nothing will happen, which is also known.
• We do not have to indicate why we are mailing the user, nor the communication policy. We assume that the user expects to receive the email.
• The nonce has an expiration timestamp and is directly associated with the recipients email address to reduce duplicates.

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Notes

With OpenID and the like, normal web applications are relieved from implementing standard user account management (password, email ...), but still some customers want 'their own users'

Strangely enough I haven't found a satisfying question nor answer here yet. What I have found so far:

• Answer by Don in HTTP POST with URL query parameters — good idea or not?
• Question from Thomas -- When do you use POST and when do you use GET?

Answer 1

This question is very similar to Implementing secure, unique “single-use” activation URLs in ASP.NET (C#).

My answer there is close to your scheme, with a few issues pointed out - such as short period of validity, handling double signups, etc.
Your use of a cryptographic nonce is also important, that many tend to skip over - e.g. "lets just use a GUID"...

One new point that you do raise, and this is important here, is wrt the idempotency of GET.
Whilst I agree with your general intent, its clear that idempotency is in direct contradiction to one-time links, which is a necessity in some situations such as this.

I would have liked to posit that this doesn't really violate the idempotentness of the GET, but unfortunately it does... On the other hand, the RFC says GET SHOULD be idempotent, its not a MUST. So I would say forgo it in this case, and stick to the one-time auto-invalidated links.

If you really want to aim for strict RFC compliance, and not get into non-idempotent(?) GETs, you can have the GET page auto-submit the POST - kind of a loophole around that bit of the RFC, but legit, and you dont require the user to double-optin, and you're not bugging him...

You dont really have to worry about preloading (are you talkng about CSRF, or browser-optimizers?)... CSRF is useless because of the nonce, and optimizers usually wont process javascript (used to auto-submit) on the preloaded page.