php hidden redirect

PHP hidden redirection

In web application development, redirection (redirect) is a common jump method. Redirection means that when you enter a URL in the browser, the server returns a 302 jump response, and the browser automatically jumps to a new URL. In actual development, we may need to use redirection to implement page jumps, parameter transfer and other functions.

However, there are also some security issues with redirection. Attackers can use redirection vulnerabilities to implement phishing attacks, XSS attacks, etc. Therefore, developers need to take some measures to prevent redirection vulnerabilities from occurring.

This article will introduce a hidden redirection method implemented in PHP and how to use this method to prevent the occurrence of redirection vulnerabilities.

Header function in PHP

The header function in PHP can set HTTP header information, including Cookie, Content-Type, Cache-Control, etc. At the same time, the header function can also implement the redirection function. For example, the following code implements a simple redirect function:

<?php
header('Location: http://www.example.com/');
?>

When the browser requests this script, the server will return a 302 jump response and set the Location header to http://www.example .com/. The browser will automatically jump to this URL.

However, this method also has some security issues. An attacker can redirect to a malicious website by constructing a URL. For example, the following code exists in the website:

<?php
$redirect_url = $_GET['url'];
header("Location: $redirect_url");
?>

An attacker can construct the following URL:

http://www.example.com/redirect.php?url=http://www.bad-site.com/

When the user clicks on this URL, the server will redirect the user to http://www. bad-site.com/, thereby achieving phishing attacks or XSS attacks.

Implementation of PHP hidden redirection

In order to solve the above security problems, we can implement redirection on the server side. Specifically, we can put the redirected URL in a session variable and jump to a transit page. In the transfer page, we then take out the redirect URL in the session variable and jump. In this way, attackers cannot directly construct URLs to implement redirection, thus ensuring security.

The following is the specific implementation of this solution.

The first step is to save the redirect URL in the session variable. We can set a redirect button in the original page and pass the URL that needs to be redirected to the server:

session_start();
$_SESSION['redirect_url'] = 'http://www.example.com/';
?>

<form method="post" action="redirect.php">
  <input type="submit" value="redirect">
</form>

The second step is to jump to a transfer page. In the transfer page, we can take out the redirect URL in the session variable and jump:

session_start();
$redirect_url = $_SESSION['redirect_url'];

if (!empty($redirect_url)) {
  header('Location: ' . $redirect_url);
} else {
  echo 'Error: redirect_url not found';
}
?>

It should be noted that we need to call the session_start function at the beginning of each page in order to create the session variable. At the same time, in order to ensure security, we need to filter and verify the redirect URL. For example, we can use the filter_var function to filter the URL to determine whether it is a legal URL; we can also use the parse_url function to parse the URL and determine whether the host is legal, etc.

How to prevent redirection vulnerabilities

In addition to taking the above hidden redirection method, we also need to pay attention to the following points when writing programs to prevent the occurrence of redirection vulnerabilities:

1. Strictly filter and verify the entered URL to ensure that only legal URLs are allowed to jump. You can use filter_var, parse_url, urlencode and other functions to verify and process URLs.
2. Do not use the URL or parameters entered by the user as the value of the Location header to prevent attackers from constructing malicious URLs for attacks.
3. Try to use relative paths for redirection. In this way, attackers can be prevented from using absolute paths to jump to other websites.
4. Verify and authorize jumps involving sensitive operations to ensure that only users who have logged in and have permission can make jumps.

Summary

Redirect vulnerability is one of the common security issues in web applications. Attackers can use redirection vulnerabilities to implement phishing attacks, XSS attacks, etc. In order to avoid the occurrence of these security problems, we can take some measures, such as using hidden redirection and other methods; at the same time, we also need to pay attention to security issues such as filtering, verification, and authorization when writing programs.

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