EEF-CVE-2026-54889

## Summary

Improper Neutralization of Input During Web Page Generation (XSS) vulnerability in leandrocp mdex allows cross-site scripting via unsanitized URL schemes in Quill Delta output.

'Elixir.MDEx':to\_delta/2 converts Markdown into a Quill Delta. 'Elixir.MDEx.DeltaConverter':default\_convert\_node/3 in lib/mdex/delta\_converter.ex copies the URL of a link, wikilink, or image node directly from the parsed Markdown into the Delta "link" or "image" attribute without applying a scheme allowlist or any normalization.

An attacker who controls the Markdown text can supply a javascript: URL (for example \[click\](javascript:alert(document.cookie))) that survives verbatim into the Delta attribute. When the Delta is rendered to HTML by a downstream renderer (such as quill-delta-to-html or the Quill client), the attribute becomes an <a href> or <img src>, and the javascript: scheme executes in the browser of anyone who views the rendered content. The link and wikilink cases are the strongest vectors because javascript: in an href executes on click; the image case is lower impact because javascript: in <img src> generally does not execute in modern browsers.

This issue affects mdex: from 0.8.3 before 0.13.2.

## Workaround

Sanitize the Quill Delta produced by 'Elixir.MDEx':to\_delta/2 before rendering it: drop or blank any "link" or "image" value whose URL scheme is not in a safe allowlist (http, https, mailto, tel).

## Configuration

The application must pass untrusted Markdown to 'Elixir.MDEx':to\_delta/2 and then render the resulting Quill Delta to HTML with a renderer that maps the "link" and "image" attributes to href and src without applying its own URL scheme sanitization (for example quill-delta-to-html or the Quill client).