The vulnerability arises from the Markdown rendering logic in md-fileserver, which accepts user‑supplied Markdown and directly injects any embedded raw HTML into the rendered page. Because no sanitization is performed, attackers can embed <script> tags or other malicious HTML, leading to arbitrary JavaScript execution in the victim’s browser context. This results in a classic stored or reflected XSS that can steal session cookies, deface pages, or perform phishing attacks.

Affected production environments include installations of commenthol/md-fileserver running any version older than v1.10.3. The product is a stand‑alone server that renders Markdown files locally, without the need for external authentication. All users who can view a Markdown file are at risk until the application is upgraded.

The CVSS score of 7.2 indicates high severity. With no reported EPSS score and the vulnerability not listed in the CISA KEV catalog, the public awareness may be lower. The local nature of the attack vector means that any user with access to the file viewer can trigger it. Attackers can supply malicious Markdown either by uploading a file, sharing a link, or, if the application allows, embedding malicious content within an existing file. The absence of built‑in sanitization means that arbitrary JavaScript will execute with the same privileges as the served domain.