# CVE-2026-54888: Uncontrolled recursion over deeply nested Markdown crashes the BEAM in mdex

> Attackers can crash the entire BEAM node by submitting deeply nested Markdown, causing a stack overflow in the mdex library.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 29 Jun 2026
- **Updated:** 30 Jun 2026
- **Also known as:** EEF-CVE-2026-54888, GHSA-3w4f-53g2-f66p

## What is CVE-2026-54888?

CVE-2026-54888 is a medium-severity denial of service vulnerability in leandrocp mdex, published on 29 Jun 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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.

Weakness: CWE-674 (Uncontrolled Recursion)

## Which versions are affected by CVE-2026-54888?

2 open source packages ship an affected version, including mdex and mdex_native.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mdex` | Hex | From 0.3.0 | 0.12.3 |
| `mdex_native` | Hex | From 0.1.0 | 0.2.3 |

## How do I fix CVE-2026-54888?

Upgrade mdex to 0.12.3 or later.

## Is CVE-2026-54888 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54888 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54888 and fix it if it is affected.

What it is: CVE-2026-54888 is a medium-severity denial of service vulnerability in leandrocp mdex, published on 29 Jun 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54888 exists.

Affected packages and fixed versions:
- mdex (Hex): affected from 0.3.0; fixed in 0.12.3
- mdex_native (Hex): affected from 0.1.0; fixed in 0.2.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-54888/ (Markdown: https://insights.sbomify.com/CVE-2026-54888/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54888
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54888
- OSV: https://osv.dev/vulnerability/CVE-2026-54888

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-54888/
Summaries are generated and may contain mistakes; the official description above is the record.