Using "fbook" as a field name gives Error 403

Hi there,
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Not sure if this has been noticed before, but in both CF5 and CF6, I had a form field named "fbook" intended to store a client's Facebook page URL. I was surprised to find that when this field held a valid URL, the form gave Error 403 when the Submit button was clicked. It did not matter what URL I tried, if it was a valid one it gave the error.
​
I then tried changing the name to "fbook1" but this gave the same result. So I changed it to "Facebook_Page_1" and then it worked.
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Cheers
​
Tim

Probably the exact same thing that happened to me recently - ask your webhost if there's a filter of something in mod_security that's causing the string fbook to be blocked.
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To make sure it's not CF, try putting this in a php file on your server and accessing it. It's just a text field and when you click the button it echos it back.
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DON'T COPY PASTE IT - CF forums keeps adding unprintable characters for some reason. You'll have to type it out manually.
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​

<?php

if(isset($_POST)) {
    echo $_POST["fbook"];
}

?>

<form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
<input name = "fbook"></input><br>
<button type="submit" name = "submit" value = "Submit">TEST</button>
</form>