电子邮件验证链接是个坏主意

Question

In my registration process, the user registers, they get emailed a verification link, and if they click it, only then would their account be verified. But isn't this verification method too easy for the bots?

I think an email could be created by a bot, but for sure if the verification is just clicking a link, it could also be automated by a bot. I'm not sure since I haven't done this and don't care to test it just to know, but my question is isn't this verification method flawed?

I'm thinking about sending the verification code to the user as a text which they would have to copy/paste manually into a form AND the form is captcha protected. Is this a better idea? any flaws with it?

Answer 1

Most of the suggestions are about verifying emails and using CAPTCHAs which of course you should do, but keep in mind that none of these methods is completely bulletproof.

Email verification

A bot can easily "click" on links in any email. Copying and pasting something would be slightly more annoying for the bot author but not much. Generally email verification is just that - email verification.

You verify if the email is likely to be controlled by whoever tries to register, but of course since email is usually sent in cleartext over untrusted TCP and relies on insecure DNS, then until we're all using DNSSEC and encrypt all traffic it will be easy to sniff emails and spoof servers and clients. The important thing to realize is that using email verification you get only a certain degree of confidence that whoever or whatever you are talking to is really a user of that email address.

Turing test

Answering a question that only human should know the answer to would be still more annoying but considering that you probably wouldn't have an infinite number of questions, the bot author might redirect unknown question to a real human and use cached answers if any question repeats more than once. Answering a question like "what is 12+8" like I've seen in some websites lately as a Turing test is completely counterproductive since this question is actually easier for bots than for humans. Probably the most popular Turing test for that are CAPTCHAs but here you also have to realize that they can be fooled.

First of all people are showing methods of circumventing CAPTCHAs, for example see the Decoding reCAPTCHA talk from DEFCON 18. Many CAPTCHAs are much easier for robots to decipher since they are generated by algorithms that are trivial to reverse. The reCAPTCHA distortions are also pretty simple but the words that they use are real scanned words that was hard for OCRs so in principle it should be much harder for bots, but it is not always the case.

And there is also a possibility to display captchas that you want to guess on other websites and have people answer it for you. Also there is a black market of people actually solving captchas so if your bot author doesn't mind paying something like two cents for a dozen then no matter how hard it is for humans, actual humans will solve it anyway.

Bottom line

The bottom line is that using any of the bot-stopping techniques will always be a compromise of how much would a bot owner (a spammer or anyone else who wants to register a lot of users in your system) be willing to spend time, effort and money to do it, and how much inconvenience for your users are you going to tolerate, because ultimately you will never be able to do any automated test to tell humans and bots apart without actually annoying humans and alienating people with disabilities (has anyone ever tried to guess the audio version of reCAPTCHA?), and still your bots may actually be human-powered, so not really bots but cyborgs, so to speak.

It's an arms race for which your honest users are paying a price. Please keep all of that in mind.

Answer 2

The questions is what are you trying to verify? When you send a link to an email address, what you can know is that whoever registered that account has access to the email address. It doesn't tell you anything about them other than that.

So yeah, bots can create an account, and use it for registration. If you want to stop bots, then yeah, a captcha is what you need to add. Note that there's little point in adding the code to copy/paste - that's both easy for a bot to do, and also doesn't gain you anything over the captcha.

Answer 3

As always, security and convenience are generally competing with each other.

A link in an email simply validates that it is an active email address. Yes, it's easy for the bots to handle this. But is your service so valuable that bots will be attacking it?

A CAPTCHA is always the way to go to ensure your users are human. The additional coding and frustrations involved with it are a trade-off.

In the end, keep things as simple as possible, but not simpler.

Answer 4

As pointed out already, you simply have some CAPTCHA validation.

My suggestion is though do human validation before your app creates the user account and sends the verification email. Added value of your site can't easily be forced to just spam verification emails and create bogus waiting to be verified accounts.

Nothing wrong with a link if you do that.

Answer 5

Yes, bots can enter emails and check the responses. I've also heard of endeavors toward bots getting better at image recognition and answering captchas, although I can't say for sure how good they are. If you are really really concerned, I would go with:

1. Email verification
2. Captcha
3. Simple random questions (How many ears/fingers do most humans have?)
4. Cell phone number that sends a code via SMS

The last one might prove to be the best in eliminating bots, but it will also limit who signs up for your website. Also, the more validations you have, the more you'll annoy users and the more you'll increase the barriers to getting them to sign up, which could also be a pretty big drawback. Personally, I think captchas are a good balance of bot protection vs. user inconvenience.

Answer 6

Are you verifying an email only or doing a full registration?

I always verify the email account first. then once verified complete the registration process.

so add a captcha at the verify email step.

In other words, ask the user to enter their email address, enter the captcha and submit the form.

That way only real people get the verification email sent.

It doesn't prevent human bots of course.

DC

It also means you don't need to store failed/bad registration data.

One problem is a user validating with one email address and then changing it during the registration process, I handle that this way..

When a user submits their email address the data is not stored at all. Instead I use $validation_code = md5(trim($email)+$secret) to generate the verification code. That way they can't change the email address on the actual registration form. The email and verification code is carried as a hidden field to the end to validate the email address. if the email address is altered from the verified one, registration will fail as the md5 no longer matches.

DC

Answer 7

I ran into similar problems with verification emails and testing. If you want to end-to-end test email verification try EmailE2E.com — it's free.

You can send and receive emails from randomly generated inboxes via an API.

It's perfect for testing Firebase, Amazon Cognito, or other OAuth providers that use email verification codes during sign up. Plus it has clients in Java and JS.