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Fencer Vulns
medium 6.9

CVE-2026-54888 json

Uncontrolled recursion over deeply nested Markdown crashes the BEAM in mdex

published 2026-06-29 4 mo ago modified 2026-07-30 3 mo ago also EEF-CVE-2026-54888 , GHSA-3w4f-53g2-f66p , EUVD-2026-40177

cvss
6.9
version 4.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 14%
public exploits
0
none known

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.

Patch / fix

  • hex mdex 0.12.3
  • hex mdex_native 0.2.3
  • leandrocp mdex 0.12.3
  • leandrocp mdex_native 0.2.3

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
4 mo ago

Exploit prediction

0.24% chance of exploitation in the next 30 days, higher than 14% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/leandrocp/mdex_native/security/advisories/GHSA-3w4f-53g2-f66p referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 4.0 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
CNA 4.0 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Affected packages

ecosystem package affected fixed in
hex mdex
>=0.3.0, <0.12.3
0.12.3
hex mdex_native
>=0.1.0, <0.2.3
0.2.3

Affected products

  • leandrocp mdex
  • leandrocp mdex_native

Classification

Weakness

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.