What are trusted types and how do they address DOM-based XSS vulnerabilities in web applications?
Trusted types are a modern platform feature that addresses DOM-based Cross-Site Scripting (XSS) vulnerabilities in web applications. DOM-based XSS is a type of vulnerability where an attacker injects malicious code into a web page, which is then executed by the victim's browser. This can lead to various security risks, such as stealing sensitive information, performing unauthorized actions on behalf of the user, or spreading malware.
To understand how trusted types work, let's first consider the concept of the Document Object Model (DOM). The DOM is a programming interface for HTML and XML documents, representing the structure of a web page as a tree-like structure. Each element in the DOM tree can be manipulated using JavaScript, allowing developers to dynamically update the content and behavior of a web page.
Trusted types introduce a mechanism that restricts the types of values that can be assigned to certain DOM properties. By enforcing a strict type policy, trusted types prevent the injection of untrusted code into the DOM, thereby mitigating the risk of XSS attacks. This is achieved through a combination of input validation, output encoding, and code execution policies.
To enable trusted types in a web application, developers need to define a policy that specifies the allowed types for certain DOM properties. This policy is then enforced by the browser, preventing any assignments of untrusted values to those properties. For example, if a developer specifies that only trusted HTML strings can be assigned to the innerHTML property of an element, any attempt to assign an untrusted value (e.g., a string containing a script tag) will be blocked.
Trusted types can be used to address both reflected and stored XSS vulnerabilities. Reflected XSS occurs when user-supplied data is immediately reflected back to the user without proper validation or encoding. By enforcing strict type policies, trusted types ensure that only trusted values are assigned to DOM properties, preventing the injection of malicious code.
Stored XSS, on the other hand, involves the persistence of malicious code in a web application's database or other storage mechanisms. When the stored data is later retrieved and rendered in a web page, the code is executed in the victim's browser. Trusted types can be used to sanitize the stored data by enforcing type policies during the retrieval and rendering process, thereby preventing the execution of any injected code.
Trusted types also provide a mechanism for developers to extend the default type policies provided by the browser. This allows for custom validation and encoding rules to be applied to specific DOM properties, further enhancing the security of web applications.
Trusted types are a powerful feature that helps mitigate DOM-based XSS vulnerabilities in web applications. By enforcing strict type policies for DOM properties, trusted types prevent the injection of untrusted code, reducing the risk of XSS attacks. This mechanism can be used to address both reflected and stored XSS vulnerabilities, providing an additional layer of security to web applications.
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