Cross-site scripting (XSS) is a web security issue that happens when an attacker exploits vulnerabilities in a web application. This vulnerability allows attackers to bypass the security that separates different websites from each other and access users' data. They can then pretend to be the user and do things on their behalf.
Cross-site scripting manipulates a vulnerable website to return malicious JavaScript to users. When this malicious code executes within a victim's browser, the attacker can fully compromise their interaction with the application. This allows them to carry out unauthorized actions and retrieve sensitive data.
There are three main types of XSS attacks: Reflected XSS, Stored XSS, and DOM-based XSS.
Reflected XSS is the simplest form of XSS. It occurs when an application includes data from an HTTP request directly in its response without proper validation or sanitization. Attackers can exploit this vulnerability by injecting malicious scripts into the request data. When users interact with the vulnerable page and trigger the execution of the injected script, the attacker gains control over their session and can perform actions and access data on their behalf.
Stored XSS, or persistent or second-order XSS, arises when an application includes untrusted data in its responses from a database or another untrusted source. This can happen when users submit data, such as comments or messages, which are displayed to other users. If the application fails to properly sanitize this data, attackers can inject malicious scripts that are stored and executed whenever the vulnerable page is accessed by other users.
DOM-based XSS, also called DOM XSS occurs when an application's client-side JavaScript processes untrusted data in an unsafe manner, typically by directly manipulating the Document Object Model (DOM). Attackers can exploit this vulnerability by controlling the value of an input field, for example, and injecting a malicious script that gets executed when the vulnerable JavaScript code reads and writes to the DOM.
Read also: Common cyberattack vectors
An attacker who successfully exploits an XSS vulnerability can achieve various malicious objectives:
The impact of an XSS attack depends on various factors, such as the nature of the application, the sensitivity of the data it handles, and the privileges of the compromised user. While the impact may be minimal in some cases, it can be severe in applications dealing with sensitive data or those granting elevated privileges to compromised users.
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Preventing XSS vulnerabilities involves a combination of measures to ensure the security of user-controllable data:
See also: HIPAA Compliant Email: The Definitive Guide