EEF-CVE-2026-53428

## Summary

Memory Allocation with Excessive Size Value vulnerability in leandrocp mdex allows an unauthenticated attacker to cause a denial of service through unbounded memory allocation.

comrak\_nif::lumis\_adapter::LumisAdapter::parse\_highlight\_lines in native/comrak\_nif/src/lumis\_adapter.rs eagerly expands a user-controlled inclusive line range from a fenced code block's highlight\_lines decorator into a Vec<usize>, pushing one element per integer in the range with no upper bound on the range size. An attacker who can supply Markdown that an application renders with MDEx.to\_html/2 (for example a comment, chat message, or wiki page) can embed a code block whose info string is rust highlight\_lines="1-100000000", forcing the native adapter to allocate roughly 8 bytes per line in the range.

A payload that differs by only a few bytes can therefore allocate hundreds of megabytes, and a sufficiently large range (for example 1-2000000000) exhausts host memory and aborts the BEAM, denying service to every user of the rendering process. The per-line write loop additionally tests membership with a linear scan over the same vector, degrading rendering to a quadratic cost even for ranges that do not immediately exhaust memory.

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 0.11.0 before 0.12.3, and mdex\_native from 0.1.0 before 0.2.3.

## Workaround

Do not enable code-block decorators: leave the github\_pre\_lang and full\_info\_string render options unset, or avoid configuring an inline syntax-highlight formatter, so that highlight\_lines specifications are never parsed.

## Configuration

Exploitation requires the application to enable code-block decorators. Decorators are active only when the render options github\_pre\_lang and full\_info\_string are both set and an inline syntax-highlight formatter (for example {:html\_inline, ...}) is configured. Applications that render Markdown with the default options do not parse highlight\_lines specifications and are not affected.