Also known as non-persistent attacks, reflect XSS is one of the most uncomplicated cross-site scripting attacks. This article delves into a Reflected XSS, typical examples of such attacks, and best practices to prevent the underlying vulnerabilities.
What are Cross-Site Scripting Vulnerabilities?
Cross-site scripting is a client-side application security vulnerability that allows the adversary to manipulate user interactions with the webserver/application. In violation of the same-origin policy, attackers exploit insecure input interfaces and client-side scripting to execute malicious code. While doing so, the victim is tricked into visiting the page with a malicious script so that the executable code is delivered to the victim’s browser. In such attacks, hackers also commonly masquerade as compromised users to access the victim’s data and carry out actions with their permissions. In instances where the victim is a privileged-access user, attackers can fully compromise the application’s data and functionality. Any application not validating the user input when generating script outputs is vulnerable to XSS attacks.
XSS attacks are generally categorized as:
- Stored/Persistent XSS
- Reflected Attacks
- DOM-based XSS
What is a Reflective Cross-Site Scripting Attack?
Reflected (Non-Persistent) XSS attacks occur when the malicious payload is included in the request sent to the vulnerable web application and is then reflected such that the server’s HTTP response consists of the payload. Attackers leverage social engineering techniques such as phishing attacks to make the victim include the malicious script in their request to the webserver. The victim’s browser then executes the malicious script as the HTTP response.
Reflected XSS attacks are non-persistent such that each victim needs to send the request with a malicious payload. As a result, attackers tend to trick as many users as possible to succeed in the attack. Such attacks are often aimed at message forum form submissions, error messages, or search engine results pages, as it is easier to craft a malicious e-mail message that many users can click on. As one of the most common types of XSS attacks, reflected XSS does not require the attacker to locate and access a vulnerable web application that would allow them to inject malicious scripts permanently.
How to Prevent Reflected XSS and its Importance in Security
A reflected XSS attack is a commonly exploited form of XSS vulnerability. Organizations should leverage common prevention mechanisms to prevent the reflected cross-site scripting vulnerabilities in web applications.
Reflected Cross-Site Scripting Prevention
Protection against reflected XSS primarily involves avoiding using dynamic malicious content from HTTP requests to embed scripts on a vulnerable application. Some approaches to achieve this include:
Validate user input
User input validation/content filtering forms the first line of defense against most XSS attacks, including reflected XSS. It is essential to treat a user input from any source as untrusted and formulate mechanisms to check against semantic and grammatical requirements. Security teams should also apply whitelists/blacklists for data patterns that should be accepted or rejected. Additionally, it is recommended that QA and administration teams treat data from both authenticated and public users as untrusted inputs while applying the same input sanitization techniques across all users.
Escape dynamic content
Suppose the application relies on user-controllable data as part of its HTTP responses. In that case, the output data should be encoded so that the server does not interpret it as active content. This ensures that any special characters from the application’s data store are treated as HTML tag content, not raw HTML. It is recommended to replace any significant dynamic characters with HTML entity encoding schemes to be interpreted safely. Developers should use dedicated tools to enable a safe stylesheet and script encoding if dynamic content is inserted into the <script> and <style> tags.
Implement a content security policy
A robust Content Security Policy lets web administrators/developers control where the web page can load and execute scripts from. Since reflected XSS attack relies on the attacker embedding malicious content on a web page, CSPs prevent attacks by specifying the sources of inline scripts. Developers instruct browsers using a content-security-policy header or meta element, including directives for sources and sinks of specific resources and actions. Web developers can whitelist allowed sources of trusted scripts using a directive of the form:
Content-Security-Policy: script-src 'self' https://darwin.com
This directive allows for the trusted execution of scripts from the server hosting the current page and those originating from https://darwin.com.
Most modern browsers support CSPs, making them one of the most crucial aspects of preventing reflected XSS attacks.
Utilize a vulnerability scanning tool
Why Preventing Reflected XSS Vulnerabilities is Important for Your Business?
XSS Prevention Guide
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Reflected XSS Attacks Examples
Below are some common approaches attackers leverage to carry out reflected XSS attacks.
1. Reflected XSS Attack on a Search query
Assume a web app accepts a search string from users through a search parameter within a query string:
In the event the application server uses PHP to show the user-supplied value on the results page by pulling it from the URL and then generating the resulting HTML:
<?php echo 'You Searched:' .$_GET["search"];?>
The web server directly parses the user-supplied value in the URL to HTML with no input validation or output encoding. In such instances, attackers can craft arbitrary code that is executed in the browser when the victim clicks on the URL:
http://darwin.com/aform.html?search=<script>alert('XSS by Product1');</script>
This script gives hackers access to the user’s session cookie and allows them to assume a legitimate user’s identity.
2. Reflected XSS Attack on an Error Message
Suppose a web page accepts an input parameter that contains the text displayed in an error message and displays it directly within its response. In that case, attackers use such vulnerabilities to orchestrate XSS attacks.
Assuming the URL that returns the error message as:
The returned error page directly copies the values of the URL’s message parameter then displays it suitably within the page:
<p>Sorry, some error message</>
Without sanitization or appropriate validation, attackers can create a malicious payload to generate a pop-up screen:
http://darwin.com/error/5/Error.ashx?message=<script>alert("XSS by Hacker1")</script>
If an unsuspecting user requests this page, the browser executes the malicious script, redirecting the user to an HTML page with the following content in place of the actual content:
<p><scrRipt>alert("XSS by Hacker1");</script></p>
Reflected XSS Video Explanation
Reflected XSS Vulnerability – FAQs
Differences: Stored XSS VS Reflected XSS
In persistent/stored cross-site scripting, the web application stores the invalid input, which is then unsafely executed later in the client’s browser. In this attack, the adversary must identify potential vulnerabilities in the web application and inject the malicious payload into the server. Reflected/non-persistent XSS involves embedding the malicious code in an HTTP request, sending it back to be executed by the client browser as part of the server’s HTTP response.
How do I test for reflected XSS vulnerabilities?
While it is possible to test such vulnerabilities manually, using automated scanners such as the Crashtest Security Suite is recommended to find and fix reflected XSS vulnerabilities. Developers and QA teams should first identify user input vectors, typically using a web proxy or HTML editor. They can then use the OWASP-provided list of test input data to analyze the input vectors for reflected XSS vulnerabilities. These analysis results are used to determine whether the website is vulnerable and which vulnerabilities may have the highest impact.