Cross-site scripting (XSS) vulnerabilities continue to pose significant threats to web applications, with DOM-based XSS presenting a unique challenge due to its reliance on client-side manipulation. A critical vulnerability was identified in GitLab’s Markdown text fields, where a clipboard DOM-based XSS flaw allows attackers to execute arbitrary JavaScript under a user’s credentials. This article provides an in-depth exploration of the vulnerability, its technical underpinnings, reproduction steps, impact, and mitigation strategies, offering valuable insights for developers and security professionals.

DOM-based XSS occurs when client-side scripts dynamically manipulate the Document Object Model (DOM) based on unsanitized user input, such as data from the clipboard. Unlike server-side XSS, the attack vector is processed entirely within the browser, making it harder to detect through traditional security measures. In GitLab’s case, the vulnerability leverages the clipboard’s text/x-gfm-html MIME type, enabling malicious payloads to be injected into Markdown text fields…