What can you do to prevent automatic confirmation in that case?

I run an authorization service that allows to log-in using magic links and we managed to solve this. First approach was for the link opening GET request to do not log the user in, but to include an HTML page with JavaScript that issued a POST request with a code from the link to log the user in. This worked well for a long time, because email scanners were fetching links from emails with GET requests but did not execute JavaScript on the fetched pages. Unfortunately, some time ago Microsoft tools indeed started to render the fetched pages and execute JavaScript on them which broke the links. What works now is to check if the link is open in the same browser that requested the link (you can use a cookie to do it) and only automatically login the user in these cases. If a link is open in a different browser, show an additional button ('Login as <email address>') that the user needs to click to finish the login action. MS tools render the login page but do not click buttons on it.

The issue that MS tools introduced is broader, because it affects also email confirmation flows during signups. This is less visible, because usually the scanners will confirm emails that the user would like to confirm anyway. But without additional protection steps, the users can be signed up for services that they didn't request and MS tools will automatically confirm such signups.

> check if the link is open in the same browser that requested the link (you can use a cookie to do it) and only automatically login the user in these cases. If a link is open in a different browser, show an additional button ('Login as <email address>') that the user needs to click to finish the login action.

Thanks for checking if it's the same browser. Some companies don't care about that (cough booking cough) so harmful actors just spam users with login attempts in hope a user will click by accident. And puff, random guy gets full access to your account. I got those every day, if I ever needed to login this way I would not be able to figure out which request is mine.

Wouldn't that just log you in on the browser doing the clicking, instead of the attackers browser?

You mean in the booking example? They logged in the browser that... requested access. So basically anyone that knew your login/email.

I think it should check if browser requesting is the same as the one confirming, or just drop that whole dumb mechanism entirely.

Ok, what if an email has "click this link if it was you who tried to log-in", or "if it wasn't you"?

Will Microsoft automatically authenticate malicious actors, or block yourself from services built with assumptions that the email client won't auto-click everything?

Login links from my service were automatically clicked and rendered and I know that other services discovered similar problems. Based on this I think that it is very likely the case with all the links in emails, but I don't know if there is any additional heuristic involved that would treat some links differently.

See also this issue which suggests that all links are opened: https://techcommunity.microsoft.com/discussions/microsoftdef...

Note that this doesn't affect all Outlook users, this Microsoft Defender for Office 365 is a separate product that only some companies use.

> But without additional protection steps, the users can be signed up for services that they didn't request and MS tools will automatically confirm such signups.

Indeed it's a bad thing but how bad?

The admins of some web service get a database of emails, send them those registration links, make their mail software create the accounts and? They end up with a service with accounts that they could create without sending those emails, before they send some emails to solicit users to perform some action on their (long forgotten?) account. There is no additional threat unless I'm missing something.

The admins have only an extra thin layer of protection because of the confirmation step but I think that any court can see through it.

The exploitation and potential damage would be service specific. Say a Dropbox like service for computer file syncing: An attacker creates an account for '[email protected]' and gets the signup email automatically confirmed. The attacker uploads some malware files to the account. After some time Alice attempts to create a valid account and resets password for '[email protected]'. Then Alice installs a desktop file syncing client provided by the service and malware files from the attacker get downloaded to her machine.

Another example would be if a company hosted a web app for employees that allowed signups only from @company.com addresses. In such case an attacker could be able to signup with such an address.

You are right. I didn't think about 3rd parties creating accounts on a service they don't control.

It defeats the email verification entirely. If that weren't necessary for something, why would the site require it?

That check for the same browser is a great idea. Thanks!

This can be annoying when you intentionally want to log in to a browser that is not the one making the request, such as an in-app browser.

The link leads to a page where you need to press a button (HTTP POST).

As far as I know there’s nothing.

The alternative is to send an OTP in the mail and tell the user to enter that.

In that way there is no link to auto confirm.

However, if you do that ensure that you have a way to jump straight to the page to enter the OTP because (looking at you Samsung) the account registration process can expire or the app is closed (not active long enough) and your user is stuck

That seems like a really thoughtless idea.

I've had this problem as a user, accidentally previewing a link in iOS by tapping for too long.

This is even worse for copying the link. On iOS the contextual menu comes with the preview, which will destroy the magic link.

> These days there's also to consider that some Mail Threat Protection Tools (at least Microsoft Defender in Exchange Online does this) click links in Mails to check them.

What an insane policy, why am I surprised Microsoft came up with it…

It's not actually insane if the application hosting the link follows the principle that GET requests should not mutate state.

This problem is ~20 years old from when CMS platforms had GET links in the UI to delete records and "browsing accelerator" browser extensions came along that pre-fetched links on pages, and therefore deleted resources in the background.

At the time the easiest workaround was to use Javascript to handle the link click and dynamically build a form to make a POST request instead (and update your endpoint to only act on POST requests), before the fetch API came along.

It is insane because it brings literally nothing security-wise (an attacker can easily detect that the link is being opened from something else than an end-user's browser, and not deliver the payload) while actualy compromising the security of their users (by allowing an attacker to know which addresses exist and which do not, which is very useful if you want to attack companies).

The idea is that the pre-fetching is done by an environment that looks similar to the end-user's browser.

It does not only show to attackers that your address exists, it also shows that it is hosted on Microsoft 365 and is ATP is licensed.

Come to think of it, magic links by definition violate the principle that GET requests should not change state. Defender & preview tools are actually following the established norms here - norms that were established decades ago precisely because we hit the more broad problem with C, U & D parts of CRUD, and collectively agreed that doing destructive operations on GET requests is stupid.

You can GET a <form> which POSTs when you click the "log in" button.

Yes, but the GET itself isn't changing any state. The state changes only after clicking on the button. This is OP's point.

TeMPOraL said, "magic links by definition violate the principle that GET requests should not change state". That is a reasonable thing to think, but it is not true, because you can GET a <form> which POSTs when you click the "log in" button, unless you think a link to such a <form> page should be excluded from the definition of "magic link".

In your example, it seems to me that the POST request is the action that changes the state.

Agreed.

> unless you think a link to such a <form> page should be excluded from the definition of "magic link".

Yes. Linking to a form requiring user to press a button to submit an actual POST request is one proper way of doing it, and won't confuse prefetchers, previewers and security scanners - but it lacks the specific "magic" in question, which is that clicking on a link alone is enough to log you in.

Can't really have both - the "magic" is really just violating the "GET doesn't mutate" rule, rebranding the mistake we already corrected 20+ years ago.

(EDIT: Also the whole framing of "magic links" vs. passkeys reads to me like telling people that committing sins is the wrong way of getting to hell, because you can just ask the devil directly instead.)

Aha, then we agree on the facts, just disagree about nomenclature.

Your theological analogy is hilarious!

This is the way.