Reflected cross-site scripting
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.
Impact of reflected XSS vulnerabilities
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:
Perform any action within the application that the user can perform.
View any information that the user is able to view.
Modify any information that the user is able to modify.
Initiate interactions with other application users, including malicious attacks, that will appear to originate from the initial victim user.
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.
Reflected XSS in different contexts
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.
Finding reflected XSS vulnerabilities
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:
Test separately every entry point for data within the application's HTTP requests. This includes parameters or other data within the URL query string and message body, and the URL file path. It also includes HTTP headers, although XSS-like behavior that can only be triggered via certain HTTP headers may not be exploitable in practice.
For each entry point, submit a unique random value and determine whether the value is reflected in the response. The value should be designed to survive most input validation, so needs to be fairly short and contain only alphanumeric characters. But it needs to be long enough to make accidental matches within the response highly unlikely. A random alphanumeric value of around 8 characters is normally ideal. You can use Burp Intruder's number payloads [https://portswigger.net/burp/documentation/desktop/tools/intruder/payloads/types#numbers] with randomly generated hex values to generate suitable random values. And you can use Burp Intruder's grep payloads option to automatically flag responses that contain the submitted value.
If the candidate XSS payload was modified by the application, or blocked altogether, then you will need to test alternative payloads and techniques that might deliver a working XSS attack based on the context of the reflection and the type of input validation that is being performed. For more details, see cross-site scripting contexts