CVE-2026-53427

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') vulnerability in leandrocp MDEx allows stored or reflected cross-site scripting via attacker-controlled Markdown.

When syntax highlighting and full info-string forwarding (render: [fullinfostring: true]) are enabled, the Lumis adapter copies the value of a code fence's highlightlinesclass info-string attribute, unescaped, into the class attribute of every rendered line. comraknif::lumisadapter::LumisAdapter::parsecustomattributes in native/comraknif/src/lumisadapter.rs shlex-parses the info string and stores each key=value pair verbatim, highlightlinesconfig pulls highlightlinesclass into the per-line class value, and write_highlighted interpolates that value directly into the class attribute of the per-line <div>. A single-quoted shell token preserves an inner double quote through shlex parsing, so a value such as '"><script>alert(1)</script>' terminates the class attribute early and the markup that follows is emitted as live HTML.

An attacker who can submit Markdown (through comments, posts, wiki pages, documentation, or any user-generated content) can inject arbitrary HTML and JavaScript that runs in the browser of every user who views the rendered output, enabling session theft, account takeover, and other client-side attacks. No authentication or special privileges are required.

The vulnerable native code originally shipped inside mdex (in native/comraknif/src/lumisadapter.rs) and was later extracted into the separate mdexnative package (native/mdexnativenif/src/lumisadapter.rs), where it remains unpatched.

This issue affects mdex from 0.11.3 before 0.12.3, and mdex_native from 0.1.0 before 0.2.3.