19 Фев, 2024

Reflected XSS

Reflected XSS

Reflected Cross-Site Scripting (Reflected XSS) is a type of web application attack in which an attacker embeds a malicious script in a request to a website, which is then reflected and executed in the victim’s browser. This is achieved by embedding the script in elements such as URLs, search strings or data forms, which are then directly displayed on the web page without proper filtering or sanitisation by the web application.

Unlike stored XSS attacks, where the malicious script is stored on the server and can affect multiple users, the impact of Reflected XSS is limited to a single session and often requires the victim to click on a link pre-prepared by the attacker. This can be achieved through phishing emails, manipulating links on trusted sites, or other social engineering techniques.

A Reflected XSS attack poses a significant risk to privacy and data security as it can steal credentials, sessions, personal information, and allows attackers to manipulate the victim by performing actions on their behalf.

Примеры эксплуатации

To better understand the reflected XSS vulnerability, let’s look at one of the labs from PortSwigger, a well-known web security company. This lab demonstrates a reflected XSS vulnerability where some SVG markup elements are allowed to be exploited. To successfully pass this lab work, we need to implement cross-site scripting that calls the alert() function.

Let’s start by looking at the functionality of the site, and try typing <svg> into a site search.

Let’s send this request and see how the lab reacts.

As we can see the SVG has rendered on the page. Let’s pick up some HTML tags from the XSS cheat sheet and run burp intruder to see what tag XSS works with.

As we can see the lab only missed 4 HTML tags, let’s now pick up the payload for our Reflected XSS.

Let’s use this payload and make sure the vulnerability is actually on the service!

Scanners that detect vulnerabilities

  1. OWASP ZAP (Zed Attack Proxy): An open-source web тестирования приложений scanner. It can automatically find security vulnerabilities in your web applications while you are developing and testing applications.

  2. Burp Suite: This integrated platform is used for performing AMAZON WEB SERVICES of web applications. It has various tools that work together to support the entire testing process, from initial mapping and analysis of an application’s attack surface to finding and exploiting security vulnerabilities, including Reflected XSS.

  3. Nessus: A widely used vulnerability scanner, Nessus can scan for thousands of known vulnerabilities including various forms of XSS.

  4. Qualys Web Application Scanning (WAS): A cloud service that provides automated crawling and testing of custom web applications to identify vulnerabilities, including Reflected XSS.

  5. Acunetix: A fully automated web vulnerability scanner that detects and reports on over 4500 web application vulnerabilities including all variants of XSS.

  6. Veracode: Offers an automated cloud-based service for SECURING WEB, mobile, and third-party enterprise applications. It can detect Reflected XSS vulnerabilities as well.

  7. Detectify: A web security scanner that performs fully automated tests to identify security issues on your website. It checks for a broad range of vulnerabilities, including Reflected XSS.

  8. IBM Security AppScan: Provides static and dynamic application security testing to identify vulnerabilities in applications, including XSS.

  9. WebInspect: A dynamic security application scanning tool by Micro Focus that simulates real-world hacking techniques and attacks to provide a comprehensive dynamic analysis of complex web applications and services.

  10. Netsparker: Claims to be a false-positive free web application security scanner that can identify Reflected XSS and other security flaws.

Average CVSS score for Reflected XSS

The Common Vulnerability Scoring System (CVSS) provides a way to capture the principal characteristics of a security vulnerability and produce a numerical score reflecting its severity. The numerical score can then be translated into a qualitative representation (such as low, medium, high, and critical) to help organizations properly assess and prioritize their vulnerability management processes.

For Reflected Cross-Site Scripting (XSS) vulnerabilities, the CVSS score can vary significantly based on the context in which the vulnerability exists. Factors that might affect the scoring include the complexity of the exploit, the required user interaction, the impact on data confidentiality, integrity, and availability, and other potential mitigations in place.

Generally, Reflected XSS vulnerabilities tend to have a moderate CVSS score. They typically require user interaction, such as clicking a malicious link, and do not directly compromise the server itself. As a result, these vulnerabilities often receive CVSS base scores in the range of 4.0 to 6.5.

However, this is a rough estimate and the actual score can be higher or lower based on the specific details of each case. For instance, if an XSS vulnerability is present in a highly sensitive application, like online banking, and could potentially lead to significant exposure of personal information or financial loss, the score might be higher.

To study Reflected XSS

Understanding the Basics

  1. Knowledge of HTML and JavaScript is crucial, as XSS exploits often involve injecting malicious scripts into web pages.

  2. Understand how web applications work, including the HTTP protocol, client-server architecture, and the role of web browsers.

  3. Familiarize yourself with basic security concepts such as the same-origin policy, session management, and authentication mechanisms.

Theoretical Study

  1. Read articles, white papers, and official documentation that explain how Reflected XSS vulnerabilities arise.

  2. Study the Common Vulnerability Scoring System (CVSS) to understand how the severity of vulnerabilities is assessed.

  3. Review the Open Web Application Security Project (OWASP) resources, especially the OWASP Top Ten, which frequently includes XSS.

  4. Analyze vulnerability reports and case studies where Reflected XSS was exploited to understand the real-world impact.

Practical Skills

  1. Practice with tools like OWASP ZAP, Burp Suite, and other security scanners to detect XSS vulnerabilities.

  2. Engage in hands-on practice with platforms like Академия веб - безопасности PortSwigger, Взломайте Коробку, or OWASP WebGoat, which provide labs for testing XSS exploits.

  3. Understand how to review code for XSS vulnerabilities by looking for inputs that are reflected in web page output without proper sanitization.

  4. Learn to write scripts that exploit XSS vulnerabilities, understanding the process of crafting payloads that bypass security filters.

Defensive Techniques

  1. Study techniques for validating and sanitizing user input to prevent malicious data from being rendered in a browser.

  2. Learn how to implement CSP to reduce the risk of XSS attacks by specifying which dynamic resources are allowed to load.

  3. Understand secure coding practices that can prevent Reflected XSS, such as using frameworks that automatically escape XSS by design.

Continuing Education

  1. Follow security blogs, forums, and news sites to stay current with the latest XSS vulnerabilities and defenses.

  2. Attend security conferences and participate in workshops to learn from experts in the field.

  3. Join security communities and forums where you can ask questions, share knowledge, and collaborate on security projects.

How to be protected from Reflected XSS

  1. Always escape special characters in user input before rendering them in the browser to prevent them from being interpreted as code.

  2. Implement strict validation of input data to ensure that it conforms to expected formats and types.

  3. Use APIs that prevent direct insertion of HTML or JavaScript. This includes APIs that automatically escape user input.

  4. CSP helps reduce the risk of XSS by limiting the sources from which scripts can be loaded and executed.

  5. Conduct penetration tests regularly to identify potential vulnerabilities.

  6. Hold regular cybersecurity training sessions for developers to raise awareness about XSS and other threats.

  7. Use templating systems and frameworks that perform necessary escaping by default.

  8. Keep all systems and applications up-to-date with the latest versions to fix known vulnerabilities.

  9. Perform regular code audits and monitoring of web applications for vulnerabilities.

  10. Educate users about the risks associated with clicking on suspicious links and how to recognize phishing attempts.


Reflected Cross-Site Scripting (XSS) remains a persistent threat to web security, exploiting the dynamic nature of web applications to execute malicious scripts on the client side. This attack vector can lead to data theft, session hijacking, and other security breaches that compromise both user privacy and system integrity.

However, with the correct defensive strategies in place, such as input sanitization, validation, the use of secure coding practices, and the implementation of robust Content Security Policies (CSP), the risks associated with Reflected XSS can be significantly mitigated. Regular security training for developers, along with user education on the dangers of phishing and malicious links, also plays a crucial role in fortifying web applications against such attacks.

Adopting a holistic approach to web security that incorporates both proactive and reactive measures will ensure a resilient posture against Reflected XSS attacks. Continuous monitoring, vulnerability scanning, and staying updated with the latest security patches are indispensable practices that contribute to the ongoing security of web environments.

Ultimately, vigilance and adherence to best practices in web development and security are the best defense against Reflected XSS and other evolving cyber threats. By fostering a culture of security mindfulness and maintaining a commitment to regular assessments and updates, organizations can safeguard their digital assets and user data against these pervasive attacks.

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