<form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>"> |
When the form is submitted, the form data is sent using the method=”post”.
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What is the $_SERVER[“PHP_SELF”] variable? The $_SERVER[“PHP_SELF”] variable is a superglobal in PHP that contains the filename of the currently executing script. |
Using $_SERVER[“PHP_SELF”] in the form action attribute sends the submitted form data back to the current page, allowing error messages to be displayed alongside the form.
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What is the htmlspecialchars() function? The htmlspecialchars() function in PHP converts special characters into HTML entities. This helps prevent attackers from injecting malicious HTML or JavaScript code (Cross-Site Scripting attacks) into forms by replacing characters like < and > with their corresponding HTML entities such as < and >. |
Using $_SERVER[“PHP_SELF”] in your PHP page without proper validation or escaping can potentially expose your application to Cross-Site Scripting (XSS) attacks. Attackers can manipulate the URL by appending slashes and XSS commands, exploiting vulnerabilities in your site’s security. It’s crucial to validate and sanitize user inputs to mitigate such risks effectively.
| Cross-site scripting (XSS) is a common security vulnerability often found in web applications, allowing attackers to inject malicious client-side scripts into web pages that are then viewed by other users. |
Imagine we have the following form on a page named “test_form.php”:
<form method="post" action="<?php echo $_SERVER["PHP_SELF"];?>"> |
If a user enters the normal URL in the address bar, such as “http://www.example.com/test_form.php”, the provided code will function as follows:
<form method="post" action="test_form.php"> |
So far, so good.
However, consider what happens if a user enters the following URL in the address bar:
| http://www.example.com/test_form.php/%22%3E%3Cscript%3Ealert(‘hacked’)%3C/script%3E |
In this case, the code will be interpreted as:
<form method="post" action="test_form.php/"><script>alert('hacked')</script> |
This code adds a script tag with an alert command. When the page loads, the JavaScript code will execute, displaying an alert box. This example demonstrates a simple and harmless way the PHP_SELF variable can be exploited.
Be aware that any JavaScript code can be added inside the <script> tag! A hacker could redirect the user to a file on another server containing malicious code, potentially altering global variables or submitting the form to a different address to steal user data.