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PHP Secure E-mails

黄舟Original: 2016-12-28 09:34:442380browse

In the PHP e-mail script in the previous section, there is a vulnerability.

PHP E-mail Injection

First, please look at the PHP code in the previous chapter:

<html>
<body>

<?php
if (isset($_REQUEST[&#39;email&#39;]))
//if "email" is filled out, send email
{
//send email
$email = $_REQUEST[&#39;email&#39;] ;
$subject = $_REQUEST[&#39;subject&#39;] ;
$message = $_REQUEST[&#39;message&#39;] ;
mail("someone@example.com", "Subject: $subject",
$message, "From: $email" );
echo "Thank you for using our mail form";
}
else
//if "email" is not filled out, display the form
{
echo "<form method=&#39;post&#39; action=&#39;mailform.php&#39;>
Email: <input name=&#39;email&#39; type=&#39;text&#39;><br>
Subject: <input name=&#39;subject&#39; type=&#39;text&#39;><br>
Message:<br>
<textarea name=&#39;message&#39; rows=&#39;15&#39; cols=&#39;40&#39;>
</textarea><br>
<input type=&#39;submit&#39;>
</form>";
}
?>

</body>
</html>

The problem with the above code is that unauthorized users can Input forms insert data into the email header.

What will happen if the user adds the following text to the email in the input box in the form?

someone@example.com%0ACc:person2@example.com
%0ABcc:person3@example.com,person3@example.com,
anotherperson4@example.com,person5@example.com
%0ABTo:person6@example.com

As usual, the mail() function puts the above text into the email header, so the header now has additional Cc:, Bcc: and To: fields. When the user clicks the submit button, this e-mail will be sent to all the addresses above!

PHP Prevent E-mail Injection

The best way to prevent e-mail injection is to validate the input.

The following code is similar to the one in the previous chapter, but here we have added an input validator to detect the email field in the form:

<html>
<body>
<?php
function spamcheck($field)
{
//filter_var() sanitizes the e-mail
//address using FILTER_SANITIZE_EMAIL
$field=filter_var($field, FILTER_SANITIZE_EMAIL);

//filter_var() validates the e-mail
//address using FILTER_VALIDATE_EMAIL
if(filter_var($field, FILTER_VALIDATE_EMAIL))
{
return TRUE;
}
else
{
return FALSE;
}
}

if (isset($_REQUEST[&#39;email&#39;]))
{//if "email" is filled out, proceed

//check if the email address is invalid
$mailcheck = spamcheck($_REQUEST[&#39;email&#39;]);
if ($mailcheck==FALSE)
{
echo "Invalid input";
}
else
{//send email
$email = $_REQUEST[&#39;email&#39;] ;
$subject = $_REQUEST[&#39;subject&#39;] ;
$message = $_REQUEST[&#39;message&#39;] ;
mail("someone@example.com", "Subject: $subject",
$message, "From: $email" );
echo "Thank you for using our mail form";
}
}
else
{//if "email" is not filled out, display the form
echo "<form method=&#39;post&#39; action=&#39;mailform.php&#39;>
Email: <input name=&#39;email&#39; type=&#39;text&#39;><br>
Subject: <input name=&#39;subject&#39; type=&#39;text&#39;><br>
Message:<br>
<textarea name=&#39;message&#39; rows=&#39;15&#39; cols=&#39;40&#39;>
</textarea><br>
<input type=&#39;submit&#39;>
</form>";
}
?>

</body>
</html>

In the above code, we use PHP Filter to validate the input:

FILTER_SANITIZE_EMAIL Filter removes illegal characters of email from string

FILTER_VALIDATE_EMAIL Filter validates the value of email address

That’s it The content of PHP Secure E-mails. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!

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