What is Cross Site Scripting (XSS)?

Among the present security threats affecting web applications, cross-site scripting has been the most persistent and prevalent. Also, it continues to be a constant challenge for businesses because XSS vulnerabilities are commonly exploited by assaulters to inject malicious scripts into trusted websites. The consequences of cross-site scripting might be very serious, including data breaches, unauthorized access, or even the compromising of user privacy. Such security mishaps can have disastrous implications for organizations, leading to loss of customer confidence, revenue, and possible litigation.

In this blog, we will aim to answer what is cross-site scripting (XSS), discuss various cross-site scripting types, how they occur, and most importantly, how businesses can protect their website against these threats. The article will further cover some examples of cross-site scripting attack vectors and present actionable best practices for mitigation of those risks.

What is Cross-Site Scripting (XSS)?

Cross-site scripting also referred to as XSS, is a security vulnerability that allows an attacker to inject malicious scripts into web pages viewed by clients. Recent security reports state that more than 55% of all XSS vulnerabilities were associated with applications used by IT companies, followed by 39% for the public sector. This makes it among the most common high-risk issues businesses are facing today. XSS vulnerabilities usually come through a lack of proper validation of input, enabling attackers to execute scripts in users’ browsers, thus bypassing access controls.

These include vulnerabilities that can literally affect any web application, from an e-commerce website to a social networking one. Therefore, their fixation has become a matter of urgency for any business organization.

What Can XSS be Used For?

Since the time cross-site scripting was first discovered, various methods of bypassing filters and firewalls have made XSS attacks progressively sophisticated. As a result, security teams have to be proactive with respect to security practices. Attacks via XSS do not result in damage from one type but serve a variety of malicious purposes that could have devastating consequences for businesses and users alike.

Some of these include:

1. Session Hijacking: One of the most damaging uses of XSS is session hijacking. In such an attack, the malicious script steals a user’s session cookies, which allows the attacker to impersonate the user. Once the attacker accesses the user account, he can execute financial transactions or modify user settings or some other malicious activity on behalf of a valid user.
2. Identity Theft: Another dangerous possibility with XSS is identity theft. This could be performed by injecting a script that captures sensitive user information, such as login credentials or other personal data, where the attacker needs to collect usernames and passwords. Later on, this information can be sold on the dark web or used in even more focused kinds of attacks, like phishing or account takeovers.
3. Web Site Defacement: Attackers perform site defacement by using XSS to change the contents and appearance of a web page, whereby some obscenity images and text may be injected, or even divert users to another malicious site. This kind of attack might lead to reputation degradation for the business, leading to decreased users’ trust in the website’s credibility.
4. Phishing Attacks: Another very common application of XSS is phishing. The scripts injected allow attackers to create fake login forms or capture sensitive information. They can also even forward the users to a phishing page, which seems to be valid in all aspects but has the sole intention of stealing information. By hijacking the interface of a website in such a case, the attackers are able to trick users into entering their personal information, believing they are interacting with a legitimate site.
5. Exploiting Browser Vulnerabilities: Sometimes, XSS attacks are used to exploit specific browser vulnerabilities. By injecting specific scripts, an attacker may take advantage of security weaknesses in the browser or plugins of the victim, which will open it up to even more serious kinds of attacks, such as malware installation or gaining access to the user’s machine.

The Impact of Cross-Site Scripting on Web Security

If a website gets infected with cross-site scripting viruses, there can be a number of damaging effects on its security. The effects do not just remain mere immediate financial and operational challenges but rather pose serious long-term problems for businesses. Following are some of the major impacts of cross-site scripting on web security:

1. Data Leak: XSS attacks have some of the most frequent but serious repercussions stemming from data breaches. When an attacker successfully performs an XSS attack, they will be in a position to steal sensitive user data in the form of PII, credit card numbers, and login credentials. These kinds of breaches will cost them a great deal of financial loss for both the company and the customer, not to mention a loss of integrity in user trust.
2. Legal Outcomes: If an XSS attack turns as serious as data leakage or unauthorized access, it may also have legal repercussions. In respect of the General Data Protection Regulation and other regulations, a business must ensure proper security for user data and, upon any breach, notify parties involved within a certain timeframe. Non-compliance can lead to significant fines, along with severe damage to the company’s image.
3. Service Downtime: Another result of XSS attacks is service downtime. For example, if an attack occurs, businesses are bound to take the website or application offline to investigate the incident and fix the vulnerability. This can lead to lost revenue for companies whose operations are highly enabled online, such as e-commerce and subscription-based services.
4. Reputation Damages: One of the biggest assets a company can have is its reputation, and a cross-site scripting attack clearly undermines this. If there is even a data breach or if customers see that some sort of compromise has taken place on a site, they may begin to distrust the brand. As security and privacy are increasing concerns for every consumer, after such damage occurs, it might take a long time to regain lost credits.
5. Operational Disruption: There is a possibility that an XSS attack might cause operational hiccups, especially if the attack has compromised vital systems or platforms. The employees may find themselves shut out of internal systems where they execute their jobs. In this case, resources have to be redeployed to fix that security breach, which slows down productivity and causes a dent in usual business operations.

Types of Cross-Site Scripting Attacks

There are several cross-site scripting types, each with different ways of execution and potential impact. These kinds of attacks enable the attackers to inject malicious scripts through the web application vulnerability, compromise the private data of users, hijack the session, or change the content of the site. So, the knowledge of these forms of XSS is significant for safeguarding your applications and reducing security risks.

1. Stored XSS (Persistent XSS): Stored XSS is one of the most hazardous types of cross-site scripting attacks as it actually lives on the affected server. An attacker injects the malicious script, which gets saved into a database or server-side storage. Each time a user goes to that compromised page, the script runs within his browser. In the case of this attack, all users who visit the page may be affected, which makes it particularly dangerous for websites that have huge user databases.
2. Reflected XSS: This happens when a malicious script is reflected off a web server, usually delivered through an email or URL. An attacker crafts a specially prepared URL that contains the script, and when a victim accesses the URL, the script executes within their browser. Unlike stored XSS, which is not a one-time attack, once the attacker uses reflected XSS, he cannot abuse the vulnerability anymore, though it might be useful for serious consequences such as session hijacking or phishing.
3. DOM-Based XSS: DOM-based XSS attacks take place completely on the client side, with no malicious script processed by the server. The attack will manipulate the Document Object Model of the webpage to have the malicious script execute on the user’s browser. This will make the DOM-based XSS attack more difficult to choose or prevent since this exploits client-side JavaScript and not one that relies on server-side vulnerabilities.
4. Self-XSS: In Self-XSS, the users are socially engineered (manipulated) to execute malicious scripts in their own browsers. The user can be tricked by the attackers to copy and paste malicious code into the browser developer console by convincing them with false authority. Although this kind of XSS depends on user action, it is still harmful because a user may not know the level of risk.
5. Mutated XSS-mXSS: This occurs when input sanitization routines, in an effort to block injected code, turn that code in such a way that the resultant attack pattern can get past traditional defenses. This form of XSS is particularly dangerous because it manages to avoid the traditional filtering and encoding mechanisms set up to block script execution.

How Cross Site Scripting Works?

To fully understand how cross-site scripting (XSS) operates, it’s essential to explore the stages of an attack, from identifying the vulnerability to executing the malicious payload. Understanding these stages provides insight into how attackers bypass security measures and compromise sensitive data.

1. Identifying the Vulnerability: The first step in exploiting a web application via Cross-Site Scripting (XSS) is identifying input fields or parameters that fail to sanitize or validate user-supplied data. This happens most frequently in search bars, form fields, and URL parameters when special characters like <, >, or quotes (‘, “) are not properly checked. If left unchecked, these might lead to a trigger to enable the attacks through the injection of malicious code (like JavaScript) into the application’s HTML. Hence, it will permit harmful scripts to run in users’ browsers and the ability to perform data theft or session hijacking.
2. Script Injection: Once a vulnerability is found, an attacker creates a malicious payload, usually in the form of a piece of JavaScript code that will exploit the vulnerable input. This code is injected into form fields, search queries, or URL parameters that the web application doesn’t sanitize properly. An example could be something like <script>alert(‘XSS’)</script> inserted into a text field or URL. If the website doesn’t sanitize this input, then this script gets included in the page’s HTML and is executed upon the user loading the page.
3. Execution of Malicious Payload: This malicious script executes when the user visits the compromised web page. Since the browser trusts the domain of the website, it executes the script like any other part of the page content. At this stage, the attacker can carry out several activities like cookie theft, redirection of the user to a phishing page, or defacement of the website. Importantly, it also means the user can have no idea that the browser is executing bad code, as it happens behind the scenes without actually any visible signs.
4. Abusing Browser Trust: XSS is an attack that abuses the implicit trust a browser has in a website. Since browsers believe that content from a trusted site is safe, they automatically execute any scripts originating from that domain. In that respect, an attacker abuses this trust by running his code without the knowledge of the user. In gaining control through XSS of the web page’s contents, he can hijack user sessions, steal sensitive data, or run additional malicious scripts.
5. User Impact and Data Theft: An XSS attack leaves attackers with sensitive data in the form of stranded session tokens, cookies, or even credentials stored in local storage. For example, an attacker accessing an eCommerce platform or a social media site can steal the session cookie that identifies the user and hijack his account. That is dangerous, considering different types of applications handling sensitive financial or personal data.
6. Further Manipulation: In more sophisticated attacks, XSS can be the first stage in a multi-step attack. Attackers can inject additional scripts that download malware or redirect users to other vulnerable systems. An attacker might chain vulnerabilities together, moving from client-side control via XSS to server-side exploits, thus escalating an attack and reaching more critical systems.

How to Prevent Cross-Site Scripting Attacks

Prevention of XSS attacks can demand anything from strong coding practices to security policies or proactive mechanisms for detection. So, here are a few strategies that may assist in reducing the risk of XSS vulnerabilities.

1. Input validation: Input validation is one of the best practices in preventing attacks against XSS. It consists of validating user input to ensure that data conforms to known or allowed formatting. For example, form fields accepting email address input should only permit characters allowed in typical email format. Such techniques help in lowering the risk of arbitrary code execution being injected into form fields, search boxes, or URL parameters.
2. Output Encoding: Output encoding can be very critical when including user-supplied data in a web page. The web application should replace special characters such as <, >, and & by using their HTML-encoded equivalents (&lt;, &gt;, and &amp;) to avoid any browser interpretation of special characters as executable code. This enables the web application to display user input as text, which will not be executed as a script.
3. Content Security Policy – CSP: The Content Security Policy, or simply CSP, is a security standard that helps to avoid XSS in web pages by only allowing resources from a given white list to execute scripts. Specifically, in the implementation of CSP, it sets a specific header in which a developer can specify what domains are allowed to serve executable code. By doing so, it blocks an attacker from injecting and then executing malicious scripts from unauthorized sources. It is considered a best practice in terms of preventing XSS attacks that a good CSP be implemented.
4. Sanitizing Inputs: All user input must be sanitized, meaning previously hazardous characters must be removed or neutralized prior to being processed. This can be done using the help of sanitization libraries or some intrinsic functions in the programming language being used. In PHP, a function like htmlspecialchars() can be employed to sanitize an input field so that it will ensure the removal of harmful scripts before storing or displaying the data.
5. Avoid Inline JavaScript: Where possible, inline JavaScript, which includes scripts written directly into HTML elements, should be avoided. Inline scripts are very prone to XSS attacks because that’s usually the method through which such scripts are injected by an attacker. As an alternative, references to external scripts should be made using tags and event handlers attached via JavaScript files so that the layers of HTML and scripting are kept separate. This makes the management of code easier and more secure.
6. Education of developers: The best ways of preventing XSS vulnerabilities lie in the morale of developers to stick to secure coding practices. Developers need to be taught best practices, which mainly relate to input validation, output encoding, and proper use of libraries in order to prevent XSS. Regular training programs and security audits will also help find potential vulnerabilities and fix them before being exploited.

Detecting and Mitigating XSS Vulnerabilities

Early detection of XSS vulnerabilities greatly reduces the chances of a successful attack. Various tools and methods are available to assist developers and security groups or businesses to detect and mitigate XSS vulnerabilities. Some of these are:

1. Automatic Security Scanners: XSS vulnerability scanners, such as SentinelOne’s Singularity™ Vulnerability Management,  could turn into some of the best available alternatives for businesses to identify and respond to XSS vulnerabilities. These automated security scanners crawl a web application, performing attack scenario simulations and trying to identify weak spots in the system. Automated scanners can have the largest coverage and point to XSS vulnerabilities that may be missed upon manual testing.
2. Penetration Testing: Penetration testing is a manual testing activity whereby security professionals, playing the role of an attacker, conduct real-world attacks to expose application vulnerabilities. A penetration tester will use the techniques that might be employed by an attacker, which in the case of XSS means trying to inject scripts into any input field that looks potentially vulnerable. Regularly conducted penetration testing ensures that possible vectors of attack are identified and patched before an attacker can take advantage of them.
3. Code Reviews: Periodic code reviews are absolutely needed to identify XSS vulnerabilities. This constitutes a code review, where developers or security personnel study the source code for portions dealing with users’ input and active rendering of output. Particular attention should be given to processes involving input validation and input encoding since these are by far the most common sources of XSS vulnerabilities.
4. WAFs or Web Application Firewalls: A WAF adds an extra layer of defense against XSS attacks with its filtering and monitoring of HTTP requests. WAFs can detect and block malicious traffic intending to exploit cross-site scripting issues and act like a safety net for applications that do not have in-depth security measures. WAFs work very effectively together with other XSS prevention mechanisms.
5. Monitoring and Logging: Comprehensive logging and monitoring are required to identify attempts of cross-site scripting. The thin line between identifying and responding effectively by security teams is created through the logging of all user inputs while closely monitoring this input for suspected activities. For example, unexpected spikes in one particular input field can show an ongoing attack of XSS.

Best Practices for Protecting Against XSS

The key to reducing the consequences of XSS attacks in web applications lies in following best practices. Some effective strategies that businesses and developers can easily consider include the following:

1. Keep Software Up to Date: Regularly update all software and libraries, including frameworks. Most XSS vulnerabilities result from the use of older web frameworks that have been patched up in newer versions. By keeping them updated regularly, this ensures that all the latest security fixes are put into operation.
2. Enforce HTTP-Only Cookies: Setting cookies as HTTP-Only will prevent access via JavaScript and thus will prevent session hijacking through XSS attacks: because of the nature of HTTP-only cookies, they are only transmitted over HTTP requests and aren’t accessible from client-side scripts, making them more secure than traditional cookies​.
3. Disable Inline JavaScript: Disabling inline JavaScript in web applications can greatly reduce the risk of XSS. Ensuring that all JavaScript executes from an external source via trusted script tags or proper encoding can protect websites against risks imposed by injected malicious code.
4. Utilize Security Libraries: Many security libraries, such as Google’s Caja, are designed to sanitize user input and output in such a way as to prevent XSS vulnerabilities. These libraries are updated regularly to handle emerging threats, and they can be very effective at greatly reducing the risk of cross-site scripting if implemented in web applications.
5. Continuous Security Training: Web application security awareness and training can also be considered as one of the best practices to prevent XSS attacks. Developers should be continuously trained on the various latest techniques of preventing XSS through proper validation of user inputs, encoding output, and implementation of necessary security headers. At the same time, security groups should keep updating their knowledge with timely updates about newly discovered vulnerabilities or threat vectors.

Cross-Site Scripting XSS Attack Examples

Cross-site scripting attacks can be carried out in many different forms, and it becomes very important to understand how attacks leverage vulnerabilities to inject malicious code. By taking a closer look at some specific attack scenarios, developers and security teams can better avoid XSS vulnerabilities.

1. Reflected XSS Example

Reflected XSS are those where malicious script data sent to the server gets reflected back in the server’s response. For instance, imagine a search form whose user inputs are directly reflected in the results page without encoding. An attacker injects a script in a search query like <script>alert(‘XSS’)</script>, and the server returns this on the HTML without any escaping. This will mean that when the user opens the page, the malicious script will then be executed in their browser-which provides means for session hijacking or phishing attacks. Hence, it is a pretty significant threat.

2. Stored XSS Example

Stored XSS is considered a more hazardous form of XSS attack because the payload that is malicious in nature has, momentarily or permanently, been stored on the server, perhaps within a database or message board. An example could be an attacker submitting a harmful script via a comment form on a blog. Every time a different user comes along and views the infected comment, it runs that script again. In such a way, an attacker could steal cookies, hijack the page, or even forward users to other malicious sites. For instance, with an injected script, such as <script>document.cookie</script>, one would give an attacker access to all cookies for anyone who views the implanted page.

3. DOM-Based XSS Example

DOM-Based XSS attacks happen on the client side, whereby the attacker manipulates the DOM environment to execute his malicious scripts. A good example could be a website that reflects URL parameters onto the page content without proper sanitization. Think of a URL like http://example.com/page?user=<script>alert(1)</script>, whereby a web application generates the HTML content of the page dynamically, using unencoded user input. In such a case, it will run the alert script in the victim’s browser. Because this operates directly with scripts on the client side, this attack bypasses server-side controls.

4. Error Page XSS Attack Example

Error pages are also susceptible to XSS when they provide unescaped user input, such as query parameters in the URL. Consider an example of a 404 error page displaying a user-supplied URL that might inadvertently execute JavaScript code unless properly escaped. Perhaps an attacker can attempt using something like: http://example.com/404?error=<script>alert(‘XSS’)</script>. This will trigger the script into a user’s browser if reflected back into the page without proper sanitization. That makes error pages among the most common yet overlooked XSS vulnerability vectors.

Conclusion

In summary, we learned how cross-site scripting (XSS) remains among the most pervasive and dangerous threats to web security for organizations of any size. Being capable of injecting malicious scripts into otherwise trusted websites, inspires data theft, identity fraud, and significant reputational damage. Although most organizations are not that aware of the XSS vulnerability, preventive measures such as input validation, output encoding, and using security policies like Content Security Policy may play a great role in mitigation.

Such vulnerabilities, when patched, will prevent XSS attacks and ensure the security of users for organizations. Routine code audits, automated security testing, and penetration testing are a great way to figure out such threats well in time and should form an integral part of any organizational security strategy. For businesses aiming at strengthening their security posture, solutions providing holistic approaches toward a wide range of vulnerabilities, including XSS become highly relevant.

FAQs

1. How do you find and test for XSS vulnerabilities?

Testing for XSS vulnerabilities can be done automatically using tools such as SentinelOne’s Singularity™ Vulnerability Management or can be manually tested using testing techniques. Most security analysts test form fields with simple scripts such as <script>alert(1)</script> just to see if any input is sanitized. Regular security audits and/or penetration testing has to be done in order to find the XSS bug before an attacker does.

2. What is the difference between XSS and SQL injection?

XSS and SQL injection are web application vulnerabilities, though the manner of the attack differs. XSS involves the injection of malicious scripts into web pages; they then affect users who visit those web pages. SQL injection targets the database and gives an attacker the capability to manipulate SQL queries, enabling them to access sensitive data, such as usernames and passwords.

3. How do I prevent XSS?

The best action to take in order to avoid XSS attacks altogether is to validate and sanitize all user inputs. Proper encoding should be used on web-page rendering of users’ data, along with a good Content Security Policy that helps restrict what scripts run. Besides that, periodic security audits can be carried out, and/or web application firewalls might be used to support detecting and blocking such XSS attacks.

4. How do I prevent XSS in PHP?

Always sanitize user input. For that, you can use htmlspecialchars() or htmlentities() or any similar function escaping special characters such as <, >, and &. Make sure that, at the end, user inputs are validated and filtered to be sure about the type of expected data.

5. How do I prevent XSS in Java?

Validate user input through a whitelist that allows only safe characters, and HTML encodes your output in the proper contexts- namely HTML and JavaScript. Libraries such as Google Guava provide methods to HTML encode output, and JavaScript Unicode escapes will protect the contexts of JavaScript.

6. What are the most common consequences of an XSS attack?

The most common consequences of an XSS attack include hijacking of sessions, whereby attackers steal session cookies and impersonate users, theft of data, and unauthorized access to user accounts. Other minor cases involve the defacement of websites or the redirection of users to malicious sites that create reputation damage in affected businesses.