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What is a Cross-Site Scripting (XSS) Attack?

Computer security system vulnerability creates a perfect opportunity for a threat agent to compromise an entirely secure network. The National Vulnerability Database (NVD) recorded 8,000 vulnerabilities in the first quarter of 2022. One of the attacks cybercriminals unleash against organizations due to system vulnerabilities is the XSS attack. 

Cross-site scripting attack happens when attackers inject a malicious payload into a computer system remotely. Web applications are easy targets for XSS attacks accounting for half of all attacks. The malicious code injection is done as a side browser script which is sent to a different end user. 

XSS attack overview

XSS attacks happen when a malicious script is injected into a non-harmful and trusted website. The attacker inserts malicious code into a web application, mostly as a side browser script, to another end user. Attackers leverage a wide range of web application vulnerabilities that can result from user input. An unsuspecting user can receive a malicious script from an attacker and execute it because they are not aware it should not be trusted. 

Since the user believes the script is from a trusted source, they will do nothing about it. In return, the script will access all sensitive information within the browser, including session tokens and cookies. The script may even rewrite all information contained in the HTML page. Such scripts can be blocked from accessing user applications through the browser isolation technique. The key XSS concepts to take into consideration are as follows. 

  • XSS attacks are based on the web and are executed on vulnerable web apps
  • When XSS attacks happen, the web application is not the target but the user.
  • In most cases, all XSS attacks deliver malicious codes to users through JavaScript. It does have an impact on Flash, CSS, HTML, VBScript, and several other programming languages. 
  • The attacks occur after an incoming web app input fails to be validated, and the output is not fully escaped before it is displayed. 
  • An unsanitized input by users upon any web application can be affected if exposed to an XSS attack. 

How XSS attacks impact a business

XSS attackers disguise themselves as genuine users through session cookies. These permit them to execute every activity permitted by a user. The attack poses a great danger to a company’s reputation. The role of marketing in business growth is to ensure clients become aware of company products by engaging them in productive communication. If the system is attacked through XSS operation, business growth is put at risk. XSS attacks have serious consequences on organizations. 

  • Defaced website: XSS attacks can deface company websites. It alters website content which can lead to damaging a business's reputation. 
  • Confusing information: Attackers can alter information and instructions to give misdirecting information to users. This can significantly change user behavior, such as purchases, shopping, and other actions. If a government website is attacked, the user can give wrong directives to key government employees and cripple an entire government sector. 
  • Loss of data: XSS attacks can lead to the loss of sensitive data. The cost of recovery can be damaging to a company’s financial health. 

Types of XSS attacks

Cross-site scripting attacks are categorized into three major groups. 

Reflected (non-persistent) XSS attack

This is the commonest XSS scripting used to attack a wide range of web applications. The attacker injects a malicious script into the web but is reflected away from the web server. Its response comprises the entire input request to the server or a section of it. The malicious code moves to the target website and then reflects in the web browser of the victim. This is the point where the browser executes the code because it is coming from a trusted server. Reflected XSS attack mainly targets errors contained in web servers or search outcomes. 

Persistent (stored) XSS

This is a highly damaging XSS attack because attackers leverage stored XSS to execute a malicious code, also known as payload, into a web application. They mainly use JavaScript code, although they can target other programming languages. If validation is lacking, the attacker permanently stores the code in the target location. The locations can be a forum post, database, application, blog comment field, and many others. Any time the victim opens the targeted location, the XSS payload is activated in the browser as an HTML inclusion and will look legitimate. That means the affected person will execute the malicious code immediately after they view the page in the browser. 

DOM-based XSS

This is a highly advanced XSS attack and only happens when a client-side script writes information given by a user to the DOM (Document Object Model). The web app then reads the information in the DOM and outputs it to the browser. Once in the browser, the attacker waits for a loophole to inject a payload. The payload is then stored as an inclusion of the DOM, and once the information is read back, the code is immediately executed. In all three main types of attacks, cross-site scripting is executed in two main ways.

  • Client XSS: Data supplied comes from an untrusted user, which updates the DOM with malicious JavaScript. When JavaScript is used to inject unsafe code into DOM, it is regarded as unsafe. 
  • Server XSS: Data is supplied by an untrusted user and then added to an HTTP reply coming from the server. The data sources can either be from a database or a request. If data comes from both, the result is a stored server XSS and a reflected server XSS. This means both sides suffer vulnerability and may effectively execute any malicious code injected into it. 

How to discover XSS and prevent it

Open Web Security Project mentions XSS attacks in the top 10 web-related security attacks. Its detection is done by running an automated web scan using the latest XSS scanning tools. The best way to prevent XSS attacks is to sanitize all inputs, strictly use the HTTPOnly cookie flag, and have a strong content security policy. Always be sensitive and use the right API output techniques. Avoid manual output encoding when displaying information generated by users. 

Conclusion

The vulnerability of computer systems creates excellent opportunities for cybercriminals to compromise the systems. XSS is affected through the injection of malicious code into a trusted website. Unsuspecting recipients execute the code allowing it to perform its purpose. The infected website begins to send confusing information to users, which can significantly change client actions or behavior. 

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