Protecting user sign-up API

Question

I am currently writing an HTML5 web app with a Sails.js (node framework) backend. Right now, most of my APIs are secured against the user authentication system I'm using with PassportJS. Unauthorized users trying to use my APIs will get a 401 error.

However, there's one hole in the system, which is the sign up API itself. I obviously can't secure my sign up API with user authentication (because the user wouldn't have had an account to sign in with yet), therefore anyone could easily spam the API with many fake accounts. On my sign up page, I have a small verification question on the lines of "What is 2+2?" (it is generated randomly) and it is checked on the client and if the answer is correct, the client sends a request to my sign up API route with all the necessary parameters like name, birthday and username. How can I secure this API to ensure that people must go through my sign up page, and cannot simply bypass this security measure and call the API directly?

Just as a note, my APIs are not RESTFul.

Answer 1

It sounds like you're not making full use of your math-problem countermeasure--you're using it as a client-side barrier, but as you observed, a bot could just skip the client side and call your API. To make it more effective, instead of generating the question randomly on the client, you would generate it on the server, save the answer in the session (req.session.mathProblemAnswer = 4) and then send the user's answer with the API call so that it can be checked against the answer in the session.

Using a token as @Qualcuno describes in his answer will be effective against bots spamming your API endpoint directly, but there are bots that are smart enough to load your signup page, scan for hidden fields and submit the form (including the token). It's still a good idea to use CSRF protection though in general, which Sails has built-in support for.

Answer 2

There are multiple possible ways.

First of all, you may consider adding a rate-limit to IP addresses. This is NOT 100% effective, but will certainly slow down some spam attempts. For example, you can limit the number of accounts created by the same IP address to 5 every 5 minutes.

Secondly, if you want to use some sort of captcha, consider reCAPTCHA. Among the different captcha services, this is particularly effective against bots, as they especially use words that fail OCR recognition.

Eventually, to make sure that people actually visit your signup page before calling the API, you can use a "security token". This is the same technique that is used for example to protect against CSRF (Cross-Site Request Forgery) attacks.
When the server generates the signup page, it also passes to the client an hidden field (for example "token") that contains a uniquely-generated value. The client will submit this value along with the form back to the API server when it requests the creation of a user, and the server uses the token to validate the request.

There are basically two approaches to generate these tokens.

First method

The signup page creates a random string/number and stores it in the database to be used as token. When the user submits the form, the server searches for that token into the database: if it's present, then the submission is valid; otherwise it fails. The token is then removed from the database.
Additional security can be obtained by storing into the database, along with the token, an expiration date and the client's user-agent (unlike IP's, user-agents are unlikely to change during the same session).

Pros: each token can be used only once.
Cons: the app needs a database, and it will be queried 3 times just for the token insertion, validation and deletion (requiring time and adding load to the database). You also should to purge regularly expired tokens from the database.

Second method

The signup page creates a token by digitally signing a plain-text string containing all the validation information. For example, suppose that you want to create a token that expires on 1411660627 (UNIX timestamp) and it's associated with the user-agent "Mozilla/5.0 ...". The server also possesses a secret salt (for example "123456abcde") that needs to be unique for the application and kept secret.
The signup page generates the token in a way similar to:

1. Create a plain-text string to be signed, by concatenating all the information. For example, if the expiration is 1411660627 and the MD5-hashed user agent is 0f7aee3e0a65ff9440d2a0183b4b1f49, your base signature would be something similar to: 1411660627-0f7aee3e0a65ff9440d2a0183b4b1f49.
2. Append the secret salt to that string: 1411660627-0f7aee3e0a65ff9440d2a0183b4b1f49_ 123456abcde.
3. Hash that string, using any hashing algorithm (for example MD5). The result is your signature: 0742d84065cb9497c1ba4c1d33190a93.
4. Concatenate your signature to the plain-text string to obtain your security token: 1411660627-0f7aee3e0a65ff9440d2a0183b4b1f49-0742d84065cb9497c1ba4c1d33190a93. This is what the user received and has to submit back.

To verify the token, then, a similar operation is done. When the server receives the token 1411660627-0f7aee3e0a65ff9440d2a0183b4b1f49-0742d84065cb9497c1ba4c1d33190a93, it performs these steps:

1. Extract the expiration from the token and check if it's still valid. If 1411660627 is smaller than the current timestamp, then it's still valid.
2. Compute the hash of the user agent and check if it matches the one on the token: 0f7aee3e0a65ff9440d2a0183b4b1f49.
3. Re-generate the signature as before: expiration-useragent_secretsalt using the data from the security token received from the user. In our example: 1411660627-0f7aee3e0a65ff9440d2a0183b4b1f49_ 123456abcde.
4. Compute the hash of the string as before. If it matches the third parameter from the token (0742d84065cb9497c1ba4c1d33190a93), then the security token is valid.

Pros: this solution does not require a database, but it's equally safe (as long as the salt is kept secret into the server).
Cons: the same security token can be used more than once until it expires.