我的AJAX URL安全吗？ [关闭]

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                    <p class="mb0 mt6">Closed <span title="2014-06-17 09:54:06Z" class="relativetime">5 years ago</span>.</p>
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I writing this question, more from insecurity than from absolute ignorance.

I am writing a WordPress theme for a personal use, and I have made a custom registration form for my site members.

Upon completion of the form, I run several AJAX calls on WordPress to check the availability of several user data.

Among the checks I make, is the username availability.

The process it is very simple, as described below:

1. The user enter's the desired username
2. Then continues to the next form field, and a blur event produced on the username field.
3. In this case, I sending an AJAX call on my server to check the username availability.
4. The server returns a JSON document, with the appropriate information
5. If the username exists, then I display an error message, otherwise nothing happens.

Now, my questions.

The ajax send's an object of data like this:

data : {
    action : "check_username_availability",
    s      : "my_nonce_key",
    u      : "kiven_username"
}

First Question

Can I protect my server from DDoS on my AJAX call ? Is it my script vulnerable by this mean ?

Second Question

Can this AJAX url to be used in brute force attack to give in attacker the username list of my members ?

And thus, if my script is vulnerable in these terms, can I protect my self?

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