CVE Intelligence
Medium

CVE-2026-53428 — Unbounded memory allocation in highlight_lines range expansion in mdex

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.

Published Jun 29, 2026 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: EEF

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.
How the score was calculated
  • EPSS 0.2% (30-day exploitation probability)+1
  • Technical severity (medium)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation
Not confirmed
Not in CISA KEV — an absence of record, not proof it is unexploited.
EPSS FIRST
0.19%
ahead of 8% of scored CVEs · Oct 14, 2026
Affects
Leandrocp
Mdex and 1 more
CVSS base
6.9
Medium · 2 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation
None
none · proof-of-concept · active
Automatable
No
can an attacker script the kill-chain steps
Technical impact
Partial
partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

  1. Identify exposed assets running the affected vendor/product/version combinations (Leandrocp Mdex, Leandrocp Mdex Native).
  2. No fix documented in the sources: reduce exposure and monitor the vendor advisory.
  3. Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Leandrocp Mdex ≥ 0.11.0 and < 0.12.3≥ a8407611715d1ead35fbcba79c72cef1b7df387b and < 6ed94d905f97af188323f042698ae841c02293b4 Vulnerable CVE Program (cve.org) e CISA ADP
Leandrocp Mdex Native ≥ 0.1.0 and < 0.2.3≥ 956528c5e31746253347029e810a969ab916fd27 and < 798a363b4339f6f7162ec8437c4c9f9b5ae6fbf3 Vulnerable CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector
Local
Attack complexity
Low
Attack requirements
Present
Privileges required
None
User interaction
None
Scope
X
Confidentiality (vulnerable system)
None
Integrity (vulnerable system)
None
Availability (vulnerable system)
High
Confidentiality (subsequent)
None
Integrity (subsequent)
None
Availability (subsequent)
High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
6.9 CVSS 4.0 Medium — — 6b3ad84c-e1a6-4bf7-a703-f496b71e49db via NIST National Vulnerability Database
6.9 CVSS 4.0 Medium — — EEF via CVE Program (cve.org) e CISA ADP
CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H/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 (score shown at the top: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-789

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.19%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

  • Local/physical vector: focus on endpoint telemetry (EDR) from affected hosts.
endpoint telemetry (EDR)

Timeline

What happened to this CVE, newest first.

  • 2026
  • Affected products added · leandrocp/mdex, leandrocp/mdex native
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • SSVC assessment updated
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • Record published
    Oct 13, 2026
  • CVSS changed · 6.9
    Oct 13, 2026 · NIST National Vulnerability Database
  • Source status changed · → Deferred
    Oct 13, 2026 · NIST National Vulnerability Database
  • Record created in this database
    Oct 13, 2026 · NIST National Vulnerability Database
  • CVE published
    Jun 29, 2026 · EEF

Source activity

Repeated readings are counted rather than listed.

  • 1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

4 on the record

Elsewhere on this site

Leandrocp
every CVE for this vendor
CWE-789
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-53429 Unbounded native memory leak in mdex escaped-tag rendering enables unauthenticated denial of service Jun 29, 2026
  • CVE-2026-54889 Unsanitized URL schemes in MDEx Quill Delta output allow javascript: injection (XSS) Jun 29, 2026
  • CVE-2026-54888 Uncontrolled recursion over deeply nested Markdown crashes the BEAM in mdex Jun 29, 2026
  • CVE-2026-53426 Atom-table exhaustion denial-of-service via JSON parse_document in MDEx Jun 29, 2026
  • CVE-2026-53427 Cross-site scripting in MDEx via unescaped highlight_lines_class code-fence attribute Jun 29, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

  • No fixed version, patch or mitigation documented.
  • No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
  • No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
  • No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-53428 is "Unbounded memory allocation in highlight_lines range expansion in mdex", rated Medium. It affects Mdex (Leandrocp); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 6.9 describes technical severity. EPSS 0.19% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

Comments

Moderated technical discussion. Read the community guidelines.

No comments yet.

Sign in or create an account to comment and suggest corrections.