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How to secure Ajax link requests?

Posted by: admin November 30, 2017 Leave a comment


Imagine the next scenario: a user wants to register to a webpage and fills a form. While he is filling the form, jQuery keeps checking through a regular expression if fields are valid, etc…

Taking the email as the primary key which the user will use after registering to login, the email field needs to be checked with Ajax to let the user know if that email is registered or not. I want to check it with Ajax to avoid sending the full form and emptying it, refreshing page, etc…

So, when the user has ended filling the email field, the Ajax request is sent to the server, something like the next link:

example.com/[email protected]

When check.php receives the email, it asks the database if it exists or not and returns a message like: User already exists if user exists or null if user does not exist.

The question is: if someone digs through my .js and finds out links similar to that, they could use that link to send a large number of requests to find out if those random emails exist. This could lead to heavy use of the database or in the worst cases even crashing and private information leaks.

Someone could do a huge for loop to check emails like:

//Getting the response of the next links
example.com/[email protected] // Returns null
example.com/[email protected] // Returns null
example.com/[email protected] // Returns null
example.com/[email protected] // Returns User already exists


Since i last accepted the answer, i kept investigating this and found the solution to avoid this behaviour. The following code is for JAVA but the logic can be applied to any other server-side language.

Before doing ANY ajax request to the server, I request a token to the server. This token looks like this fmf5p81m6e56n4va3nkfu2ns8n it is made by a simple method, it can, however, be more complex, but this is good to go.

public String getToken() throws UnsupportedEncodingException {
    return new BigInteger(130, new SecureRandom()).toString(32);

When requesting the token, the server does not only return the token, but also a small script that in case someone uses browser to inspect element (and browser navbar) and such the script will run and the token will be cleared. Servlet returns something like this:

_html += "<head>"
    + "<script> "
    + "window.onload=function(){\n"
    + "       document.body.innerHTML = \"\";\n"
    + "    }"
    + "window.location.href='http://mywebsite.com' "
    + "</script>"
    + "</head>"
    + "<body>"
    + "[" + token+ "]"
    + "</body>"
    + "</html>";

First empties the body then navigates back to wherever we want. javascript/jquery will however, catch the entire content as string, then I simply extract the string between [ and ]. This token is only available for the next request, so every AJAX request will have its unique token. On the 2nd reques the token just used is deleted.

After I get the token I append it as parameter to whatever link i request, something like this:

ajaxRequestObjet = $.ajax({
    url: "http://localhost:8084/mywebsite.com/servlet", //<-- local tomcat server
    method: "POST",
    data: "type=AJAX&page=some-article&token=fmf5p81m6e56n4va3nkfu2ns8n"

This method works fine against someone who inspects the website manually and try to use the links, but what about java/php/IIS servers that do this automaticly?

For this ask for header! Something like this:

boolean isAjax = "XMLHttpRequest".equals(request.getHeader("X-Requested-With")); 

It will be true only and only if XMLHttpRequest exists….

There is one last thing to keep in mind. Make sure 'Access-Control-Allow-Origin' header is NOT present in your app to make sure that any javascript NOT in your server wont get the server resources. If this header does not exist, chrome will return this:

XMLHttpRequest cannot load http://localhost:8084/mywebsite.com/servlet. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost' is therefore not allowed access.

Java server was in tomcat and I had another apache for this tests, this is the small html present in apache which gave the error above:

    <script src="http://code.jquery.com/jquery-latest.min.js" type="text/javascript"></script>

    ajaxRequestObjet = $.ajax({
        url: "http://localhost:8084/mywebsite.com/servlet",
        method: "POST",
        data: "type=AJAX&page=Token"

    ajaxRequestObjet.done(function (msg) { 



While you can not control this 100%… there are a few options..

Try using the same methods that people use with Captcha scripts..

Basically when the user loads the form / page.. You generate a random string/id in their PHP session and store it.. When they send the ajax requests, have your ajax check also append the string/id and require it before allowing a check to perform else return a header of 500 or something..

Using this approach with sessions, you could set a allowed limit of checks (say 5) and once the user has tried more than 5 checks, They are required to reload the page or perform a human check (eg Captcha).. Then it resets their count.. Even allow a total of say 30 within 1 hour / per IP or something.

Also use smart events to trigger when the ajax check is done, eg field/tab change or on a button press.. Or when a valid email is detected.. but say .com.au would trigger twice.

Basically this way, even if someone sniffed your JS files and tried to automate the email checker.. It would require them finding a way to append the string/id that you generate and also limit their amount of requests performed.

Beyond this, there is not to much more you can do easily.. But there are still a few other idea’s.

Most of them would work around using a PHP session / cookie.. Say for example if they check and find 3 email addresses.. Then again you set that as a limit and force them to require a manual submission or something.

See how the above suggestion goes for you, any questions do feel free to ask. But may take me a day or two to reply as weekend.. Also research how Captcha scripts work as plenty of source code for them.. As they work on the same idea.

Time Delays will simply look bad / make your site appear slow / bug the user with waiting for a response.

You need to limit the amount of look up’s per session / ip address.. Otherwise there is always a way to get past these checks.. Basically once they hit a limit.. Force the user/ip/session to wait a few minutes/hours and verify them with a Captcha script so it can not be scripted…

Javascript Security / Hiding The Source

While you can not do this truly, you can do certain things generate the JS using a PHP page with a JS header.. so <script src='myjscode.php'></script> and this allows PHP to check for a valid session.. So stops external requests to an extent.. But this is mostly useful for allowing JS to be only available behind a membership/login..

Multiple Checks / If Possible In This Case

Depending on your approach, is this for a user to check if they already have an account? If so.. you could combine the email check with something like their name/country/age/dob … So they would need to select two or three correct matching values before being able to get a check/response from the ajax call?

Maybe not in your case, but just thought would add this as well.


The JavaScript code on your website is executed on the computer of the user, so there is no way you could stop him from digging through your code. Even if you use a code obfuscator (for example, https://www.javascriptobfuscator.com/), the hacker could debug your application and record all requests send to the server.

Everything security-relevant has to happen on the server. You could limit the amount of requests from a specific IP address.


You could protect against brute force attacks with something similar to CSRF tokens:

Assign a server-generated ID to every client session. Each request to check.php should include this ID.

check.php should reject requests that do not include an ID, or include an ID that the server did not generate (to prevent attacks with spoofed IDs). It should also rate limit on ID – if a given ID has made a request in (say) the last second, or a given ID makes more than n requests in a 10 second interval, it should return an error response. This protects against requests from a single session arriving from several IP addresses.

You should also rate limit by IP address to prevent brute-forcing by opening a large number of web application sessions.

There isn’t much you can do to prevent an attacker looking up a single, or small number, of specific email addresses – it’s an inherent risk with this type of validation.


This type of attack can be treated the same as any other brute force attack, where the only effective solution is to use a Captcha. But of course, Captchas are a detriment to UX, so you have to consider if the additional security is worth it, especially for an attack that is very unlikely to happen anyway. That said, you may want to use a Captcha on your registration form anyway, to prevent bots from creating accounts.

This sort of attack has a huge cost for little reward for the attacker. There are billions of possible email addresses to test for. It could only be worth going to great lengths such as this, if the site in question was particularly sensitive, such as some kind of adult site, where the attacker hopes to blackmail users that he finds.


Not as good as a Captcha solution but the brute force attack might be detected and prevented by CloudFlare’s DDoS system. Also, CF can force Tor users to solve a Captcha before accessing your site, which would prevent an attacker from using Tor as a vehicle for the attack.

IP Rate Limiting

Rate limiting on an IP basis has problems because if an attacker decided to undertake a task as huge as this, he will likely be using a Botnet or some other system of multiple machines to launch the attack.

Consider a large organisation such as a University, where all users share the public IP. One of the users launches an attack on your site, and you block his IP, and in the processes blocking everyone else. This countermeasure could actually be used to launch a DoS attack.

Session ID/CRSF Token

Definitely not a solution because the attacker needs to simply make a request to the page first, to obtain the token. It’s an additional request to make but only an inconvenience for the attacker.


First of all: I’d URL-encode the mail-address. 'example.com/check.php?email=' . urlencode([email protected])

Ad your question: when check.php is called, you can

  • check, if the user’s session and his IP have sent a request during the last seconds
  • if not, write the user’s session, the user’s IP plus the current timestamp to a helper-table plus to a cookie and hit your DB
  • if yes, block the request

But I’m afraid this won’t help you from fraud because everyone can check your JavaScript and if someone want’s to exploit this, he will find ways.


check.php should depending on the setup either only be accessible internally, or verify from where the connection is made. Take a look at this previous question- I hope it might be what you’re looking for. how to verify the requesting server in php?


You could use a CSRF token and exit early from your script if you detect that no or an invalid CSRF token. Almost (if not all) PHP frameworks come with support for this.

Also check this question from the security community: https://security.stackexchange.com/questions/23371/csrf-protection-with-custom-headers-and-without-validating-token