How to Safely Decode HTML Entities in JavaScript to Prevent XSS Vulnerabilities?

Decoding HTML Entities in JavaScript

In this threaded discussion, a JavaScript developer encountered an issue where HTML entities returned from an XML-RPC backend were rendering literally in the browser rather than being parsed as HTML. This article explores the provided solutions and delves into the potential pitfalls and considerations when unescaping HTML entities in JavaScript.

The accepted answer presented a function for decoding HTML entities, but it contained a significant flaw. By not validating the input string, it left the application vulnerable to cross-site scripting (XSS) attacks. Consider the following example:

htmlDecode("<img  src="dummy" onerror="alert(/xss/)" alt="How to Safely Decode HTML Entities in JavaScript to Prevent XSS Vulnerabilities?" >")

In this case, the function would decode the HTML entity, but it would also execute the JavaScript code embedded within it, leading to a potential XSS vulnerability.

To address this issue, the discussion introduced the use of DOMParser, which provides a more reliable method for parsing HTML strings. By utilizing DOMParser, the unescaped HTML entities can be accurately decoded without the risk of introducing malicious code.

function htmlDecode(input) {
  var doc = new DOMParser().parseFromString(input, "text/html");
  return doc.documentElement.textContent;
}

This solution effectively parses the HTML string and extracts the decoded plaintext content, preventing XSS vulnerabilities and ensuring secure handling of untrusted data.

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