Weakness Type (CWE)

CWE-79 — CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

CAPEC

CVSS 4.0 Score

2.3

LOW

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

Vulnerability description

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.

Affected

pkg:hex/mdex

Module Source File Routine
MDEx native/comrak_nif/src/lumis_adapter.rs comrak_nif::lumis_adapter::LumisAdapter::parse_custom_attributes
comrak_nif comrak_nif::lumis_adapter::LumisAdapter::highlight_lines_config
comrak_nif::lumis_adapter::LumisAdapter::write_highlighted
MDEx.to_html/2
Status Type Version Changes / Fixed in
affected semver ⓘ 0.11.3 < 0.12.3

pkg:github/leandrocp/mdex

Module Source File Routine
MDEx native/comrak_nif/src/lumis_adapter.rs comrak_nif::lumis_adapter::LumisAdapter::parse_custom_attributes
comrak_nif comrak_nif::lumis_adapter::LumisAdapter::highlight_lines_config
comrak_nif::lumis_adapter::LumisAdapter::write_highlighted
MDEx.to_html/2
Status Type Version Changes / Fixed in
affected git ⓘ 0d7ffc84ea < 6ed94d905f

pkg:hex/mdex_native

Module Source File Routine
MDExNative.Comrak native/mdex_native_nif/src/lumis_adapter.rs mdex_native_nif::lumis_adapter::LumisAdapter::parse_custom_attributes
mdex_native_nif mdex_native_nif::lumis_adapter::LumisAdapter::highlight_lines_config
mdex_native_nif::lumis_adapter::LumisAdapter::write_highlighted
MDExNative.Native.document_to_html_with_options/2
MDExNative.Comrak.document_to_html/2
Status Type Version Changes / Fixed in
affected semver ⓘ 0.1.0 < 0.2.3

pkg:github/leandrocp/mdex_native

Module Source File Routine
MDExNative.Comrak native/mdex_native_nif/src/lumis_adapter.rs mdex_native_nif::lumis_adapter::LumisAdapter::parse_custom_attributes
mdex_native_nif mdex_native_nif::lumis_adapter::LumisAdapter::highlight_lines_config
mdex_native_nif::lumis_adapter::LumisAdapter::write_highlighted
MDExNative.Native.document_to_html_with_options/2
MDExNative.Comrak.document_to_html/2
Status Type Version Changes / Fixed in
affected git ⓘ 956528c5e3 < 798a363b43

Configurations

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.

Workarounds

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.

References

Credits

  • Finder: Peter Ullrich
  • Remediation developer: Leandro Pereira
  • Analyst: Jonatan Männchen / EEF

CVE record as JSON:  GET /cves/CVE-2026-53427.json
OSV record as JSON:  GET /osv/EEF-CVE-2026-53427.json