CVE-2026-53428 Unbounded memory allocation in highlight_lines range expansion in mdex
An unauthenticated attacker can cause a denial of service by supplying Markdown with a large highlight_lines range, triggering excessive memory allocation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-53428?
CVE-2026-53428 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
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.
Weakness: CWE-789
Which versions are affected by CVE-2026-53428?
2 open source packages ship an affected version, including mdex and mdex_native.
| Package | Affected | Fixed in |
|---|---|---|
mdex Hex | From 0.11.0 | 0.12.3 |
mdex_native Hex | From 0.1.0 | 0.2.3 |
How do I fix CVE-2026-53428?
Upgrade mdex to 0.12.3 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-53428 being exploited?
There is no known exploitation of CVE-2026-53428 in the wild and no public exploit code.
Sources
Also known as EEF-CVE-2026-53428, GHSA-j93q-9cvj-rxfm.