1.
What Is XSS?
Among the most common cybersecurity vulnerabilities on the internet, XSS is defined as a “cross-site scripting flaw.” By continuing to read, you can find answers to questions such as what is XSS, what types it has, how attacks can be prevented, and what their business impacts are.
The term cross-site scripting is abbreviated as XSS to avoid confusion with CSS. It is a cross-site scripting attack carried out by malicious actors. These attacks are performed by injecting malicious code, most commonly through browser-executable commands such as JavaScript; HTML serves as the markup structure into which the script injection is placed. In other words, XSS can be accurately described as a security vulnerability that arises when user input is returned to the browser without proper validation or encoding. As a result of these attacks, not only can user sessions be hijacked, but risks such as cookie theft, form data harvesting, redirecting users to fraudulent pages, or performing various manipulations in the browser can also occur. XSS attacks are categorized into three types—stored, reflected, and DOM-based. The goal is to execute arbitrary commands in the browser to hijack user sessions, steal sensitive data, or perform unauthorized client-side actions. In most scenarios, exploiting this vulnerability does not require direct access to the server. However, in the case of stored XSS, access to user input points is sufficient, as the malicious payload is stored on the server side. The attacker can simply execute the attack by running client-side code in the browser. For this reason, XSS represents a highly critical threat for both users and site owners. Below you can see the detailed steps of an XSS attack.
XSS Attack Steps
- Lack of input validation: Entry points such as forms, search boxes, or comment fields on the website do not properly filter and sanitize incoming data.
- Injection of malicious code: The attacker injects script-based malicious code into this unvalidated input field. This code can be placed inside the HTML structure or at any point operating on the DOM.
- Reflection of the injected code on the page: The malicious command is included in the page output and, when the user views the page, is processed by the browser as if it were legitimate code.
- Execution of the code in the browser: The browser executes the malicious command, enabling the attacker to carry out the intended actions. For example, reading cookies, collecting form data, or redirecting the user to a phishing page.
- Manipulation of the user session: Through the executed code, the attacker can access user sessions, take over privileges, or perform actions on behalf of the user.
- Persistence of the attack (in stored XSS scenarios): If the vulnerability is stored on the server side, the malicious code is automatically triggered for every user visiting the page, making the attack persistent.
2.
What Are the Types of XSS Attacks?
- Reflected XSS: A reflected cross-site scripting (XSS) attack occurs when unsanitized user input is sent via an HTTP request and directly reflected in the server response. Typically carried out through URL parameters or search boxes, this attack often involves sending customized links to users to lure them to a vulnerable page.
- DOM XSS: This is a type of XSS where the attack takes place entirely on the client side, within the page’s Document Object Model. In DOM-based XSS, the vulnerability arises not from the server response but from client-side JavaScript manipulating the DOM; the server’s response remains unchanged, while the page’s DOM structure can be altered by the attacker.
- Stored XSS: Triggered when malicious input is stored persistently in a database, logs, or any server-side storage and returned to users with each request. It is the most dangerous type of XSS. It is commonly exploited through comment sections, profile information, or other fields that store user input.
3.
How to Prevent XSS Attacks?
Those who are curious about the answer to the question what is cross-site scripting also want to know how these attacks can be mitigated. Below is a list of potential methods to prevent XSS attacks.
- Inventory your web assets: Keeping an inventory of all pages, forms, and user input fields in your application or website helps you identify areas that may be at risk for XSS. Considering that any field accepting user input can be a potential XSS target, this approach is extremely valuable.
- Sanitize inputs: Filtering, cleaning, and stripping unnecessary characters from user inputs on both the front end and back end significantly reduces XSS risk. Special characters, HTML tags, and script expressions that could be used for injection must be sanitized. In addition to sanitizing user input, encoding data for the appropriate context (HTML, attribute, JS context, etc.) when returning it to the browser greatly reduces XSS exposure.
- Regularly scan the client side: Conducting vulnerability scans on browser-executed code structures (particularly JavaScript files and DOM manipulations) helps detect DOM-based XSS attacks. Regular scans also identify new vulnerabilities that may arise after updates.
- Use automated monitoring: By using security scanning tools, log analysis systems, and intrusion detection and prevention systems (IDS/IPS) for your application, you can monitor suspicious activities in real time. These tools help you detect XSS attempts before they fully materialize.
- Validate form inputs: Validating all form data submitted by users on both the client and server sides prevents unexpected input from reaching the system. Ensure that only allowed data types and formats are accepted.
- Set secure cookie policies: Enabling HttpOnly, Secure, and SameSite flags on cookies significantly reduces XSS-driven session hijacking. This is a critical step, especially for protecting session cookies.
4.
Business Impact of XSS Vulnerabilities
XSS attacks have the potential to cause serious damage to businesses due to their wide-ranging effects and associated risks. Below are some of the ways cross-site scripting attacks can harm brands.
Loss of Customer Information
No website owner wants session cookies to be stolen, user accounts to be compromised, form data to be leaked, or sensitive customer information to be misused by third parties. However, XSS attacks expose websites to exactly these risks. Therefore, taking measures to protect your website against XSS attacks is critically important for safeguarding personal data such as customer information—for both you and your users.
Revenue Loss
Negative customer experiences and service interruptions are directly linked to revenue loss for businesses. Especially when loyal customers are affected by such incidents, they may hesitate to continue using your website, leading to uncertainty about purchasing your products and services. For this reason, it is essential to protect your website as effectively as possible against XSS attacks.
Legal Issues
Thanks to the laws and regulations in the country where the website is operated, user data is placed under legal protection. If customer data is exposed as a result of an XSS attack, this may indicate that the website in question has violated KVKK, which can lead to fines, legal proceedings, and long-term compliance costs for businesses.
Erosion of Trust
When user data is put at risk due to a security vulnerability, the brand’s credibility is damaged. Users become hesitant to return to a platform perceived as insecure. Such a loss of trust can reduce brand equity, weaken customer loyalty, and cause long-term reputational damage. It should be remembered that in the digital world, rebuilding trust is extremely difficult.
You may also be interested in our blog post titled What Is Data Leakage? How to Prevent Data Leaks in Businesses.
5.
Frequently Asked Questions
What is XSS?
XSS is a security vulnerability that allows malicious commands injected into a website to execute in users’ browsers and typically stems from unvalidated user input.
How does an XSS attack occur?
The attacker injects malicious code into an unfiltered user input field. When the page is displayed, this code runs in the browser and puts user data at risk.
What information can XSS target?
Session cookies, user login credentials, form data, personal information, and user interactions can all be compromised in XSS attacks.
What are the types of XSS?
There are three main categories: Reflected XSS, Stored XSS, and DOM-based XSS. Each exploits a different type of vulnerability.
How can XSS attacks be prevented?
Sanitizing user inputs, performing data validation, applying secure coding practices, enabling HttpOnly and Secure flags on cookies, and conducting regular security scans are effective methods.
How do XSS vulnerabilities harm businesses?
Customer data loss, financial damage, reputational harm, service disruptions, and legal sanctions are among the primary risks that XSS vulnerabilities pose to businesses.
