CVE-2026-53427
LowCVSS 2.3Summary
A cross-site scripting (XSS) vulnerability was found in the MDEx library due to improper input neutralization in Markdown processing. An attacker can inject arbitrary HTML/JavaScript that executes in the browser of every user viewing the rendered output.
Risk Assessment
The risk includes session theft, account takeover, and other client-side attacks. No authentication or special privileges are required – only the ability to submit Markdown content (e.g., in comments, posts, wiki pages).
Recommendation
Immediately update mdex to version 0.12.3 or later, and mdex_native to version 0.2.3 or later. If an update is not possible, disable syntax highlighting and full info-string forwarding (render: [full_info_string: true]).
Other vulnerabilities in MDEx
See all- CVE-2026-53428Medium
A vulnerability in the mdex and mdex_native libraries allows an unauthenticated attacker to cause a denial of service via unbounded memory allocation. The parse_highlight_lines function in the LumisAdapter component expands a user-controlled line range from a fenced code block without an upper bound, allocating huge vectors. A payload with a range like 1-2000000000 can exhaust host memory and abort the BEAM process.
- CVE-2026-54889Medium
XSS vulnerability in the mdex library for Elixir allows an attacker to inject JavaScript code via improper validation of URL schemes in Markdown to Quill Delta processing. An attacker can supply a malicious link with javascript: scheme in Markdown text, which is copied verbatim to the Delta attribute and executed in the victim's browser when rendered to HTML.
- CVE-2026-54888Medium
A vulnerability in the mdex library (and mdex_native) allows a DoS attack by supplying a deeply nested Markdown document. Recursive conversion functions lack a maximum depth check, causing a C stack overflow and crashing the entire BEAM node.
- CVE-2026-53429Medium
A missing release of memory after effective lifetime vulnerability in mdex and mdex_native allows an attacker who controls a rendered document to cause a denial of service through unbounded native memory exhaustion. The native rendering code permanently leaks memory when rendering a document containing escaped-tag nodes, leading to unlimited memory consumption and process crash.
- CVE-2026-53426High
A vulnerability in the MDEx library for Elixir/Erlang allows exhaustion of the atom table by processing a specially crafted JSON document. The json_to_node/1 function creates new atoms for each unique node_type value, which are never garbage collected, leading to a crash of the entire BEAM virtual machine.
Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS