In this section, we'll explain reflected cross-site scripting, describe the impact of reflected XSS attacks, and spell out how to find reflected XSS vulnerabilities.
Reflected cross-site scripting (or XSS) arises when an application receives data in an HTTP request and includes that data within the immediate response in an unsafe way.
Suppose a website has a search function which receives the user-supplied search term in a URL parameter:
The application echoes the supplied search term in the response to this URL:
<p>You searched for: gift</p>
Assuming the application doesn't perform any other processing of the data, an attacker can construct an attack like this:
This URL results in the following response:
<p>You searched for: <script>/* Bad stuff here... */</script></p>
If another user of the application requests the attacker's URL, then the script supplied by the attacker will execute in the victim user's browser, in the context of their session with the application.
If an attacker can control a script that is executed in the victim's browser, then they can typically fully compromise that user. Amongst other things, the attacker can:
There are various means by which an attacker might induce a victim user to make a request that they control, to deliver a reflected XSS attack. These include placing links on a website controlled by the attacker, or on another website that allows content to be generated, or by sending a link in an email, tweet or other message. The attack could be targeted directly against a known user, or could an indiscriminate attack against any users of the application:
The need for an external delivery mechanism for the attack means that the impact of reflected XSS is generally less severe than stored XSS, where a self-contained attack can be delivered within the vulnerable application itself.
There are many different varieties of reflected cross-site scripting. The location of the reflected data within the application's response determines what type of payload is required to exploit it and might also affect the impact of the vulnerability.
In addition, if the application performs any validation or other processing on the submitted data before it is reflected, this will generally affect what kind of XSS payload is needed.
The vast majority of reflected cross-site scripting vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner.
Testing for reflected XSS vulnerabilities manually involves the following steps:
What is the difference between reflected XSS and stored XSS? Reflected XSS arises when an application takes some input from an HTTP request and embeds that input into the immediate response in an unsafe way. With stored XSS, the application instead stores the input and embeds it into a later response in an unsafe way.
What is the difference between reflected XSS and self-XSS? Self-XSS involves similar application behavior to regular reflected XSS, however it cannot be triggered in normal ways via a crafted URL or a cross-domain request. Instead, the vulnerability is only triggered if the victim themselves submits the XSS payload from their browser. Delivering a self-XSS attack normally involves socially engineering the victim to paste some attacker-supplied input into their browser. As such, it is normally considered to be a lame, low-impact issue.