What are Cross-Site Scripting (XSS) Attacks?

What is Cross-Site Scripting?

The technique of inserting malicious code on a legitimate website in order to capture user information for nefarious reasons is known as cross-site scripting (XSS).

  • By inserting malicious code in a genuine web page or web application, the attacker attempts to execute harmful scripts on the victim's web browser.

  • The real attack occurs when the victim visits a website or uses a web application containing malicious code. The malicious script is delivered to the user's browser via the web page or application.

  • Forums, message boards, and online pages with commenting capabilities are typical targets for Cross-site Scripting attacks.

When a web page or a web application produces output that incorporates unsanitized user input, it is known as XSS. This user input must be parsed by the victim's browser.

  • XSS attacks can be carried out in VBScript, ActiveX, Flash, and even CSS. They are, nevertheless, most ubiquitous in JavaScript because JavaScript is essential to most browser experiences.

  • The security of that vulnerable website or online application, as well as its users, has been compromised if an attacker can exploit an XSS vulnerability on a web page to run arbitrary JavaScript in a user's browser.

  • Cross-site scripting can be used to deface a website. The attacker can use injected scripts to alter the website's content or even redirect the browser to another website, such as one that includes malicious code.

Types of Cross-site Scripting (XSS) Attacks

XSS attacks come in a variety of forms. They are classified into the following categories −

Persistent XSS

This sort of vulnerability, also known as stored XSS, happens when untrusted or unverified user input is stored on a target server. Message boards, comment sections, and visitor logs are all common targets for persistent XSS—any feature where other users, both authenticated and unauthenticated, would see the attacker's malicious text.

A good example of a suitable target for persistent XSS is publicly available profile pages, such as those seen on social media sites and in membership groups. When other users view the profile, their browsers will automatically execute the code entered by the attacker in the profile boxes.

Reflective XSS

Reflected or non-persistent cross-site scripting, on the other hand, entails the immediate return of user input. An attacker must mislead the user into transferring data to the target site to exploit a reflected XSS, which is generally done by deceiving the user into clicking a maliciously designed link.

Reflective XSS attacks frequently rely on phishing emails or URLs that have been truncated or otherwise hidden before being transmitted to the target user. When the victim clicks on the link, the script runs in the victim's browser.

Reflected XSS commonly targets search results and error message sites. They frequently send unaltered user input as part of the response without appropriately escaping the data so that it can be safely shown in the browser.


DOM-based cross-site scripting, also known as client-side XSS, is similar to reflected XSS in that it is frequently provided via a rogue URL containing a harmful script. The attack is carried out entirely within the browser rather than including the payload in a trusted site's HTTP response by changing the DOM (Document Object Model). This is aimed at legal JavaScript already on the page and fails to sanitize user input properly.

What are the dangers of XSS?

The impact of an XSS exploit on a website is determined by the application or site targeted and the data and compromised user. The following is generally true about an XSS attack's potential impact −

  • If sensitive data is involved, such as bank transactions or medical records, the consequences could be severe.

  • The more privileges a compromised user has in an application, the more serious the attack's consequences are likely to be.

  • The consequences of users entering sensitive, personally identifiable information can be severe.

  • Cross-site scripting can have a significant impact on a whole company. If an e-commerce website is discovered to be the source of an XSS attack, it might harm the company's reputation and customer trust.

How to look for and test for XSS flaws?

Burp Suite's web vulnerability scanner can quickly and accurately detect most XSS vulnerabilities.

Manually testing for reflected and stored XSS typically involves entering some simple, unique input (such as a short alphanumeric string) into each entry point in the application while it identifies every location where the submitted input is returned in HTTP responses and testing each location individually to see if suitably crafted input can be used to execute arbitrary JavaScript. This allows you to identify the context in which the XSS happens and choose an appropriate payload to exploit it.

Manually testing for DOM-based XSS in URL parameters follows a similar pattern −

  • Enter some simple, unique input in the parameter.

  • Use the browser's developer tools to search the DOM for this input.

  • Test each place to see if it's exploitable.

Other varieties of DOM XSS, on the other hand, are more challenging to detect. There is no replacement for studying JavaScript code, which can be incredibly time-consuming, to uncover DOM-based vulnerabilities in non-URL-based input (such as a document. cookie) or non-HTML-based sinks (such as setTimeout). The web vulnerability scanner in Burp Suite combines static and dynamic JavaScript analysis to find DOM-based vulnerabilities successfully.

How can you protect yourself from XSS attacks?

With so many different types of cross-site scripting assaults, businesses need to know how to protect themselves and avoid future difficulties. Due to their increasing complexity, websites are becoming more challenging to monitor than ever before strictly. As time goes on, the frequency of attacks is likely to increase.

The following recommendations can help protect your users from XSS attacks −

  • User input should be sanitized. Validate user input to catch possibly harmful input. Encode output to avoid a browser's automated load-and-execute behavior triggered by potentially dangerous user-provided data.

  • Limit the usage of data provided by users.

  • Use the Content Security Policy to your advantage. Additional layers of protection and mitigation against XSS attacks are provided. To detect XSS vulnerabilities in your software, regularly utilize a web application vulnerability scanning tool.

While XSS attacks remain a popular (and successful) vector of attack, you can protect yourself by implementing careful design and testing to ensure that your website or web application is not vulnerable (and keeping your users protected).

Updated on: 30-May-2022


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