COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record
CAPEC-591
Reflected XSS
This type of attack is a form of Cross-Site Scripting (XSS) where a malicious script is "reflected" off a vulnerable web application and then executed by a victim's browser. The process starts with an adversary delivering a malicious script to a victim and convincing the victim to send the script to the vulnerable web application.
Abstraction
Detailed
Typical severity
Very High
Likelihood of attack
High
Understand the attack
What is this attack pattern?
This type of attack is a form of Cross-Site Scripting (XSS) where a malicious script is "reflected" off a vulnerable web application and then executed by a victim's browser. The process starts with an adversary delivering a malicious script to a victim and convincing the victim to send the script to the vulnerable web application.
Extended description
The most common method of this is through a phishing email where the adversary embeds the malicious script with a URL that the victim then clicks on. In processing the subsequent request, the vulnerable web application incorrectly considers the malicious script as valid input and uses it to creates a reposnse that is then sent back to the victim. To launch a successful Reflected XSS attack, an adversary looks for places where user-input is used directly in the generation of a response. This often involves elements that are not expected to host scripts such as image tags (<img>), or the addition of event attibutes such as onload and onmouseover. These elements are often not subject to the same input validation, output encoding, and other content filtering and checking routines.
Execution flow
How does the attack proceed, step by step?
Explore
Survey the application for user-controllable inputs
Using a browser or an automated tool, an adversary follows all public links and actions on a web site. They record all the links, the forms, the resources accessed and all other potential entry-points for the web application.
Techniques used
- Use a spidering tool to follow and record all links and analyze the web pages to find entry points. Make special note of any links that include parameters in the URL.
- Use a proxy tool to record all links visited during a manual traversal of the web application.
- Use a browser to manually explore the website and analyze how it is constructed. Many browsers' plugins are available to facilitate the analysis or automate the discovery.
Experiment
Probe identified potential entry points for reflected XSS vulnerability
The adversary uses the entry points gathered in the "Explore" phase as a target list and injects various common script payloads and special characters to determine if an entry point actually represents a vulnerability and to characterize the extent to which the vulnerability can be exploited.
Techniques used
- Use a list of XSS probe strings to inject script in parameters of known URLs. If possible, the probe strings contain a unique identifier.
- Use a proxy tool to record results of manual input of XSS probes in known URLs.
- Use a list of HTML special characters to inject into parameters of known URLs and check if they were properly encoded, replaced, or filtered out.
Craft malicious XSS URL
Once the adversary has determined which parameters are vulnerable to XSS, they will craft a malicious URL containing the XSS exploit. The adversary can have many goals, from stealing session IDs, cookies, credentials, and page content from the victim.
Techniques used
- Change a URL parameter to include a malicious script tag.
- Send information gathered from the malicious script to a remote endpoint.
Exploit
Get victim to click URL
In order for the attack to be successful, the victim needs to access the malicious URL.
Techniques used
- Send a phishing email to the victim containing the malicious URL. This can be hidden in a hyperlink as to not show the full URL, which might draw suspicion.
- Put the malicious URL on a public forum, where many victims might accidentally click the link.
Requirements
What does the attack require?
Prerequisites
- An application that leverages a client-side web browser with scripting enabled.
- An application that fail to adequately sanitize or encode untrusted input.
Skills required
- Medium: Requires the ability to write malicious scripts and embed them into HTTP requests.
Resources required
- None: No specialized resources are required to execute this type of attack.
Impact
What does a successful attack lead to?
Read Data
Affected scopes: Confidentiality
Gain Privileges
Affected scopes: Confidentiality, Authorization, Access Control
Execute Unauthorized Commands
Affected scopes: Confidentiality, Integrity, Availability
Modify Data
Affected scopes: Integrity
Defence
How is it prevented and mitigated?
1
Use browser technologies that do not allow client-side scripting.
2
Utilize strict type, character, and encoding enforcement.
3
Ensure that all user-supplied input is validated before use.
Real world
MITRE example instances
Example instance 1Detail
Consider a web application that enables or disables some of the fields of a form on the page via the use of a mode parameter provided on the query string.
http://my.site.com/aform.html?mode=full
The application’s server-side code may want to display this mode value in the HTML page being created to give the users an understanding of what mode they are in. In this example, PHP is used to pull the value from the URL and generate the desired HTML.
<?php
echo 'Mode is: ' . $_GET["mode"];
?>
Notice how the value provided on the URL is used directly with no input validation performed and no output encoding in place. A maliciously crafted URL can thus be formed such that if a victim clicked on the URL, a malicious script would then be executed by the victim’s browser:
http://my.site.com/aform.html?mode=<script>alert('hi');</script>
Example instance 2Detail
Reflected XSS attacks can take advantage of HTTP headers to compromise a victim. For example, assume a vulnerable web application called ‘mysite’ dynamically generates a link using an HTTP header such as HTTP_REFERER. Code somewhere in the application could look like:
<?php
echo "<a href="$_SERVER['HTTP_REFERER']">Test URL</a>"
?>
The HTTP_REFERER header is populated with the URI that linked to the currently executing page. A web site can be created and hosted by an adversary that takes advantage of this by adding a reference to the vulnerable web application. By tricking a victim into clicking a link that executes the attacker’s web page, such as:
"http://attackerswebsite.com?<script>malicious content</script>"
The vulnerable web application ('mysite') is now called via the attacker's web site, initiated by the victim's web browser. The HTTP_REFERER header will contain a malicious script, which is embedded into the page by the vulnerable application and served to the victim. The victim’s web browser then executes the injected script, thus compromising the victim’s machine.