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Public proof of conceptFix available

CVE-2026-59928

A public proof of concept exists; no exploitation attested.

  • Published2026-07-08
  • Fixed in3.3.0
  • WeaknessCWE-1333 · CWE-407

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0.

Timeline

dates in UTC
  1. 2026-07-0817:17CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-59928?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-59928?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD · OSV
Mistune project mistune
BranchAffectedFixed in
3.3< 3.3.03.3.0

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Vendor advisories

  • CVE-2026-59928 Red Hat Product Security25 known affected

    mistune: Mistune: Denial of Service via crafted Markdown document with reference-link definitions

    mta/mta-solution-server-rhel9 as a component of Migration Toolkit for Applications 8 · rhoai/odh-pipeline-runtime-datascience-cpu-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI) · rhoai/odh-pipeline-runtime-minimal-cpu-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI) · +22 more

Full record on NVD

Fix

  1. Update Mistune project mistune to 3.3.0
  2. Workaround from the vendor Red Hat Product Security CVE-2026-59928 · 25 productsTo mitigate this issue, avoid processing untrusted Markdown documents with affected Red Hat products. Restricting the input sources to trusted content can reduce the risk of a denial of service attack.
  3. No fix available, says the vendor Red Hat Product Security CVE-2026-59928 · 25 productsFix deferred

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Published
2026-07-08
Last Modified
2026-07-09
NVD status
Analyzed
GitHub Advisory
GHSA-ffq3-xpv3-j92q

Frequently asked questions

What is CVE-2026-59928?

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0.

Is CVE-2026-59928 actively exploited?

A proof-of-concept exploit exists for CVE-2026-59928, but active exploitation has not been confirmed at this time.

What is the CVSS score for CVE-2026-59928?

CVE-2026-59928 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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