The vulnerable code path is only reachable when MDEx is configured with a syntax-highlighting formatter (for example syntax_highlight: [formatter: {:html_inline, ...}] or {:html_linked, ...}) and with full info-string forwarding enabled (render: [full_info_string: true]). Full info-string forwarding is required for comrak to hand the highlight_lines_class attribute to the highlighter, so any application that uses the line-highlighting attributes already has it enabled.
" } ], "value": "The vulnerable code path is only reachable when MDEx is configured with a syntax-highlighting formatter (for example syntax_highlight: [formatter: {:html_inline, ...}] or {:html_linked, ...}) and with full info-string forwarding enabled (render: [full_info_string: true]). Full info-string forwarding is required for comrak to hand the highlight_lines_class attribute to the highlighter, so any application that uses the line-highlighting attributes already has it enabled." } ], "cpeApplicability": [ { "nodes": [ { "cpeMatch": [ { "criteria": "cpe:2.3:a:leandrocp:mdex:*:*:*:*:*:*:*:*", "versionEndExcluding": "0.12.3", "versionStartIncluding": "0.11.3", "vulnerable": true } ], "negate": false, "operator": "AND" }, { "cpeMatch": [ { "criteria": "cpe:2.3:a:leandrocp:mdex_native:*:*:*:*:*:*:*:*", "versionEndExcluding": "0.2.3", "versionStartIncluding": "0.1.0", "vulnerable": true } ], "negate": false, "operator": "AND" } ], "operator": "OR" } ], "credits": [ { "lang": "en", "type": "finder", "value": "Peter Ullrich" }, { "lang": "en", "type": "remediation developer", "value": "Leandro Pereira" }, { "lang": "en", "type": "analyst", "value": "Jonatan Männchen / EEF" } ], "descriptions": [ { "lang": "en", "supportingMedia": [ { "base64": false, "type": "text/html", "value": "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: [full_info_string: true]) are enabled, the Lumis adapter copies the value of a code fence's highlight_lines_class info-string attribute, unescaped, into the class attribute of every rendered line. comrak_nif::lumis_adapter::LumisAdapter::parse_custom_attributes in native/comrak_nif/src/lumis_adapter.rs shlex-parses the info string and stores each key=value pair verbatim, highlight_lines_config pulls highlight_lines_class 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/comrak_nif/src/lumis_adapter.rs) and was later extracted into the separate mdex_native package (native/mdex_native_nif/src/lumis_adapter.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.
" } ], "value": "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.\n\nWhen syntax highlighting and full info-string forwarding (render: [full_info_string: true]) are enabled, the Lumis adapter copies the value of a code fence's highlight_lines_class info-string attribute, unescaped, into the class attribute of every rendered line. comrak_nif::lumis_adapter::LumisAdapter::parse_custom_attributes in native/comrak_nif/src/lumis_adapter.rs shlex-parses the info string and stores each key=value pair verbatim, highlight_lines_config pulls highlight_lines_class into the per-line class value, and write_highlighted interpolates that value directly into the class attribute of the per-lineDo not enable full info-string forwarding (render: [full_info_string: true]) when rendering untrusted Markdown, which prevents the highlight_lines_class attribute from reaching the highlighter. Alternatively, restrict highlight_lines_class values to a safe character set (for example [A-Za-z0-9_- ]) before rendering.
" } ], "value": "Do not enable full info-string forwarding (render: [full_info_string: true]) when rendering untrusted Markdown, which prevents the highlight_lines_class attribute from reaching the highlighter. Alternatively, restrict highlight_lines_class values to a safe character set (for example [A-Za-z0-9_- ]) before rendering." } ], "x_generator": { "engine": "cvelib 1.8.0" } }, "adp": [ { "metrics": [ { "other": { "type": "ssvc", "content": { "timestamp": "2026-06-29T19:18:13.166991Z", "id": "CVE-2026-53427", "options": [ { "Exploitation": "none" }, { "Automatable": "no" }, { "Technical Impact": "partial" } ], "role": "CISA Coordinator", "version": "2.0.3" } } } ], "title": "CISA ADP Vulnrichment", "providerMetadata": { "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2026-06-29T19:19:28.028Z" } } ] } }