What Are Cross-Site Scripting (XSS) Attacks And How To Deal With Them

What Are Cross-Site Scripting (XSS) Attacks

Cross-site scripting (XSS) attacks are a type of cyber attack that targets web applications by injecting malicious scripts into web pages. They are typically used to: 

  • Steal sensitive information (PII, passwords, credit card numbers, etc.)
  • Deface websites
  • Redirect users to malicious websites
  • Launch other malicious activities

Attack Types

There are three main types of XSS attacks:

Type Attack Method Targets Storage and Execution
Stored/Persistent XSS Attacker injects malicious code into a website, which is then stored and served to all users who view the affected page Publicly visible profile pages Permanently stored on the server; can potentially be served to thousands of victims
Reflected/non-persistent XSS Attacker sends a specially crafted URL to a victim that reflects the malicious code back to the user’s browser Search results and error message pages Not stored on the server; executed in real-time in the user’s browser
DOM-based XSS Attacker sends a specially crafted URL to a victim that reflects the malicious code back to the user’s browser DOM without proper sanitization Executed in the victim’s browser; does not reach the server

Prevention and Mitigation

Effective prevention of XSS attacks is crucial in maintaining the integrity and security of web applications. Developers can implement two primary courses of action to prevent such attacks, namely validation and sanitization. When executing an XSS attack, malicious actors aim to insert and run harmful content on a web page. Therefore, protecting each variable in a web application is essential. Any unprotected variable can become a potential vulnerability waiting to be exploited.

Web developers can implement various measures to safeguard against XSS attacks. However, there is no universal solution to mitigate the risk. Reducing the risk of cyberattacks requires a multi-pronged approach. It’s not a matter of if, but when an attack will occur. Therefore, combining best practices, utilizing existing web standards, and education are crucial. The more tools and strategies developers have, the better prepared they are.

We will break these practices down into these three categories: Development, Standards, and Initiative.

DevSecOps Pipeline


“Development” refers to items that developers can undertake to mitigate the risk of XSS attacks. 

One essential aspect of secure web development is input validation and sanitization. It involves verifying and refining user inputs before incorporating them into any dynamic content. The primary goal is to accept only specific values, rejecting all others – this is commonly known as “allowlisting.” For instance, a phone number field may only accept a set of numbers and reject any other characters like letters. Input validation can be performed either client-side, such as using the “required” attribute, or server-side using frameworks like OWASP ESAPI. However, input validation is just the first step of defense, as client-side validation can be easily bypassed by intercepting the request before it reaches the server.

The second aspect pertains to output encoding, which is closely linked to the first one. When user-generated data is involved, encoding is employed to convert the input into secure content before it is displayed. The process of escaping is typically combined with encoding, whereby a special character is added before a character or string to prevent any misinterpretation. Output encoding is generally executed just before output, and its values are not usually saved in a database.

Lastly, it is highly recommended to conduct regular testing for XSS vulnerabilities on all applications. This can be achieved through a combination of automated tools and manual testing. While automated tools can help identify potential vulnerabilities, they have limitations and may not catch all issues. On the other hand, manual testing takes more time but can reveal vulnerabilities that automated tools may miss. To ensure optimal security, it is best to employ both approaches rather than relying solely on one or the other.


“Standards” pertains to established frameworks, products, and guidelines that developers should prioritize and incorporate whenever feasible. These standards necessitate minimal effort and provide an additional layer of protection.

The implementation of the Content-Security-Policy (CSP) HTTP response header plays a crucial role in minimizing cross-site scripting (XSS) vulnerabilities on contemporary web browsers. Originally intended to curtail the attack surface of XSS exploits, subsequent versions of the CSP specification encompass protection against other types of attacks such as clickjacking and data injections. The CSP operates by establishing a set of regulations that determine which resources are permissible to load on the website, such as scripts, images, and stylesheets. It is compatible with all major, contemporary web browsers and can also be utilized as a meta tag.

Another critical aspect of website security is the implementation of HTTP Strict Transport Security (HSTS). By adding this header to the HTTP response from a web server, HSTS ensures that web browsers always connect to the server using HTTPS, thereby preventing attackers from exploiting unencrypted HTTP connections and injecting malicious JavaScript code. This feature is especially crucial in preventing malicious requests and scripts from executing on the client side, as any attempt to send a malicious request using HTTP will automatically switch to HTTPS.

In addition, it is prudent for organizations to incorporate a Web Application Firewall (WAF) as an additional layer of protection for their applications at the network level. WAFs effectively filter, monitor, and analyze the traffic between the internet and the web application. Furthermore, WAFs provide an added advantage of mitigating the risks associated with other types of threats such as SQL injection and distributed denial of service (DDoS) attacks.


“Initiative” pertains to tasks that do not fall under the previous two categories but are still crucial in mitigating XSS risk. These activities may appear optional but require a proactive approach and additional time investment to analyze and follow the latest XSS trends.

Maintaining up-to-date software is a crucial step in ensuring the security and reliability of your systems. Regular updates are an integral part of the software development pipeline, but it is important to extend this practice beyond just the development stage. This includes conducting regular audits and updating all actively used components such as the web server, CMS, and third-party plugins. However, it is crucial to balance this with stability, as new releases may have unforeseen issues that could impact existing features. Therefore, it is recommended to have a collaborative effort between DevOps and developers to ensure a smooth transition and minimize the risk of negative user experiences.

The next essential consideration is the education of users and developers. An informed public is less likely to fall prey to XSS attacks, as they will exercise caution in managing their digital presence. On the other hand, developers should stay up-to-date on the latest XSS vulnerabilities in the industry, enabling them to identify and address potential risks in their applications. It is essential to note that these two groups require different approaches: the former requires more straightforward content to inform and educate, while the latter demands more technical guidance that they can apply and implement to protect against XSS attacks. Rather than providing different sources for each audience, a credible voice in the AppSec space can cater to both by delivering simplified yet authoritative information that is accessible to the layman.


As a recap, below are the following steps to take when it comes to minimizing the risk of XSS attacks:

Input validation (and sanitization) Validating and cleaning user inputs (type, length, and format) before using them in any dynamic content
Output encoding Rendering a user input to be viewable as content and not as HTML source
Escaping untrusted data Escaping user inputs before rendering them on a page
Regularly test for XSS vulnerabilities Manual testing and automated tools can help identify and address potential vulnerabilities (refer to OWASP’s XSS Filter Evasion Cheat Sheet for a litany of tests)
Content Security Policy (CSP) A block/allowlisting mechanism for resources loaded or run, mitigating the risk of content injection vulnerabilities and reducing the privilege with which applications execute
HTTPS and HTTP Strict Transport Security An opt-in security enhancement; informs browsers that the site should only be accessed using HTTPS, and that any future attempts to access it using HTTP should automatically be converted to HTTPS
Web Application Firewall (WAF) Monitors incoming requests and blocks those that contain malicious payloads; can be network-based, host-based, or cloud-based
Keep software up-to-date Updating includes web server, CMS, and any other third-party plugins or components
Educate developers and users Keeping abreast with current AppSec trends, recent XSS attacks, framework security, and best practices

XSS attacks pose a significant risk to website security and user data integrity. It is crucial to adopt robust detection and prevention measures to mitigate the probability of a successful attack. Given the growing reliance on technology, it is essential to be proactive and alert in identifying and addressing this potential threat. In today’s digital-first economy, maintaining trust has become a critical currency. By prioritizing the protection of our users, we can establish trust in our brand. 

Putting the Sec in DevSecOps

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