Unvalidated Redirects and Forwards | Server-side URL Redirect

Introduction

Unvalidated Redirects and Forwards also called as URL Redirection or Server-side URL Redirect is a type of vulnerability found in most Web Applications. This vulnerability can be found in Java, Node.js, Python, PHP, Ruby, .NET and many more. 

Unvalidated redirects and forwards occurs when a web application receives untrusted input in an HTTP request that could cause the web application to redirect the request to a URL contained within untrusted input. In this type of vulnerability, the attacker uses to manipulate the untrusted input and successfully launch a phishing scam and can steal user credentials.

In these types of attacks, innocent users can be redirected to a malicious site that looks very similar to the real site they plan to visit, but which is controlled by the attacker.

Vulnerable URL Redirects

1. The following Java code obtains the user input from the parameter named url and redirects to that URL:

response.sendRedirect(request.getParameter("url"));

2. The following PHP code receives a URL from the query string from the parameter named url and then redirects the user to that URL.

$redirect_url = $_GET['url'];
 
header("Location: " . $redirect_url);

3. The following Node.js code accepts an HTTP request parameter which is being used directly in a URL redirect without validating the input, which promotes phishing attacks:

const app = require("express")();

app.get('/some/path', function(req, res) {
  // requested parameter is used without validation into a URL redirect
  res.redirect(req.param("target_path"));
});

Impact of Unvalidated Redirects and Forwards attack

  • Reflected Cross Site Scripting (XSS) can be employed by using this vulnerability.
  • Because the server name in the modified link is identical to the original URL, phishing attempts may have a more trustworthy appearance.
  • CSRF can be chained via this vulnerability.
  • An attacker can steal user credentials.

How to mitigate Unvalidated Redirects and Forwards Vulnerability

  • To guard against untrusted Server-side URL redirection, it is recommended to avoid user input directly into a redirect URL. Use direct links instead of user input.
  •  Maintain a list of authorized redirects on the server, then pick from that list based on the user input provided.
  • Sanitize user input by creating a list of trusted URLs by using lists of hosts or a regex.
  • Force all redirects to first go through a page notifying users that they are going away of your website, with the address clearly displayed, and have them click a link to confirm.

The following example demonstrate how to mitigate the vulnerability in Node.js

const app = require("express")();

const VALID_REDIRECT = "https://topcode.in";

app.get('/some/path', function(req, res) {
  //requested parameter is validated against a known fixed url
  let target = req.param("target_path");
  if (VALID_REDIRECT === target)
    res.redirect(target);
});

References

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