EEF-CVE-2026-54888

## Summary

Uncontrolled Recursion vulnerability in leandrocp mdex allows denial of service via deeply nested Markdown input.

mdex converts between an Elixir %MDEx.Document{} struct and Comrak's internal AST using two mutually recursive Rust functions, ex\_document\_to\_comrak\_ast and comrak\_ast\_to\_ex\_document, in the NIF source file document.rs. Neither function enforces a maximum nesting depth, so the recursion depth is bounded only by the structure of the input. An attacker who can get a Markdown document rendered (for example through MDEx.parse\_document!/1 or MDEx.to\_html/1) can supply a document with thousands of nested block quotes, which drives unbounded recursion across the NIF boundary and exhausts the native C stack.

Because the resulting stack overflow is an uncatchable SIGSEGV raised inside a NIF, it cannot be contained by the Erlang runtime. It terminates the operating system process running the BEAM, killing every Elixir and Erlang process on the node, not just the caller that triggered the render. No authentication or special privileges are required.

The vulnerable conversion code was extracted from mdex into the separate mdex\_native package starting in mdex 0.12.3. This issue affects mdex from 0.3.0 before 0.12.3 and mdex\_native from 0.1.0 before 0.2.3.