What Is an Open Redirection Vulnerability and How to Prevent it?

An Open Redirection is when a web application or server uses a user-submitted link to redirect the user to a given website or page. Even though it seems like a harmless action, to let a user decide on which page he wants to be redirected to, if exploited such a technique can have a serious impact, especially when combined with other vulnerabilities and tricks.

How Can an Open Redirect Web Vulnerability be Exploited?

Abusing the Trust Users Have in the Vulnerable Website

Since the domain name in a URL is typically the only indicator for a user to recognize a legitimate website from a non-legitimate one, an attacker can abuse this trust to exploit an open redirect vulnerability on the vulnerable website, and redirect the user to a malicious page to execute further attacks, as explained in the following sections.

Exploiting an Open Redirect Vulnerability for a Phishing Attack

When the user clicks on a link of a legitimate website he often won't be suspicious if suddenly a login prompt shows up. To launch a successful phishing attack the attacker sends the victim a link, for example via email, which exploits the vulnerability on the vulnerable website example.com:

https://example.com/redirect.php?go=http://attacker.com/phish/

By exploiting the open redirect vulnerability on the legitimate website, the attacker is redirecting the victim to, http://attacker.com/phish which is a phishing page that is similar to the legit website. Once the visitor is on the attacker's malicious website, he enters his credentials on the login form which points to a script that is controlled by the attacker. The script is typically used to save the username and the password that is being typed in by the victim, which attackers typically use at a later stage to impersonate the victim on the legitimate website.

The probability of a successful phishing attack is quite high since the domain example.com is shown when the user clicks on the link.

Exploiting an Open Redirect Vulnerability to Redirect Victims to Malicious Websites

It is also possible to redirect an otherwise careful internet user to a site hosting attacker-controlled content, like a browser exploit or a page executing a CSRF attack. As above, the chances that the victim clicks the link are higher if the site the link points to is trusted by the victim. An example is an open redirect in a trustworthy page like a banking site, that directs the victim to a page with a CSRF exploit against a vulnerable WordPress plugin.

Exploiting an Open Redirection Vulnerability to Execute Code

Redirecting to JavaScript: URIs

An open redirection vulnerability in a web application can also be used to execute an XSS payload by redirecting to JavaScript: URIs. Those can be used to directly execute JavaScript code in the context of the vulnerable website. An example would be this:

https://example.com/index.php?go=javascript:alert(document.domain)

The above would show an alert window with the content from example.com. However, in most modern browsers this only works when the redirection is JavaScript based. That means that a location header with javascript: will not execute the code and might show an error message instead.

Another URI scheme that's useful for an attacker is data:. While this does not work in WebKit-based Browsers like Google Chrome or Opera anymore, in Mozilla FireFox the attacker can still redirect to it. What this does is write data directly to the browser window, which could ease the process of creating phishing pages, even without using a web server to host them.

What Is the Impact of an Open Redirection Vulnerability?

As mentioned above, the impacts can be many, and vary from theft of information and credentials, to the redirection to malicious websites containing attacker-controlled content, which in some cases even cause XSS attacks. So even though an open redirection might sound harmless at first, the impacts of it can be severe should it be exploitable.

How Can You Prevent Open Redirection Vulnerabilities?

The easiest and most effective way to prevent vulnerable open redirects would be to not let the user control where your page redirects him to. If you have to redirect the user based on URLs, you should always use an ID which is internally resolved to the respective URL. If you want the user to be able to issue redirects you should use a redirection page that requires the user to click on the link instead of just redirecting them. You should also check that the URL begins with http:// or https:// and also invalidate all other URLs to prevent the use of malicious URIs such as javascript:.

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