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Public proof of conceptFix available

CVE-2026-33079

Mistune ReDoS in LINK_TITLE_RE allows denial of service with crafted Markdown titles

A public proof of concept exists; no exploitation attested.

  • Published2026-05-06
  • Fixed in3.2.1
  • WeaknessCWE-1333

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

In versions 3.0.0a1 through 3.2.0 of Mistune, there is a ReDoS (Regular Expression Denial of Service) vulnerability in `LINK_TITLE_RE` that allows an attacker who can supply Markdown for parsing to cause denial of service. The regular expression used for parsing link titles contains overlapping alternatives that can trigger catastrophic backtracking. In both the double-quoted and single-quoted branches, a backslash followed by punctuation can be matched either as an escaped punctuation sequence or as two ordinary characters, creating an ambiguous pattern inside a repeated group. If an attacker supplies Markdown containing repeated ! sequences with no closing quote, the regex engine explores an exponential number of backtracking paths. This is reachable through normal Markdown parsing of inline links and block link reference definitions. A small crafted input can therefore cause significant CPU consumption and make applications using Mistune unresponsive.

Timeline

dates in UTC
  1. 2026-05-0618:16CVE 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-2026-33079?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-33079?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

OSV
mistune
BranchAffectedFixed in
3.x≥ 3.0.0a1, < 3.2.13.2.1

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-2026-33079 Red Hat Product Security41 products fixed · 1 known affected

    mistune: Mistune: Regular Expression Denial of Service (ReDoS) via crafted Markdown input

    rhoai/odh-workbench-jupyter-pytorch-llmcompressor-cuda-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI)

  • RHSA-2026:60520 Red Hat Product Security39 products fixed

    Red Hat Security Advisory: RHOAI 3.4.4 - Red Hat OpenShift AI

  • RHSA-2026:43038 Red Hat Product Security2 products fixed

    Red Hat Security Advisory: Migration Toolkit for Applications

Full record on NVD

Fix

  1. Update mistune to 3.2.1
  2. Vendor fix Red Hat Product Security RHSA-2026:60520 · 39 productsFor Red Hat OpenShift AI 3.4.4 see the following documentation, which will be updated shortly for this release, for important instructions on how to upgrade your cluster and fully apply this errata update: https://docs.redhat.com/en/documentation/red_hat_openshift_ai/
  3. Vendor fix Red Hat Product Security RHSA-2026:43038 · 2 productsBefore applying this update, make sure all previously released errata relevant to your system have been applied.
  4. Workaround from the vendor Red Hat Product Security RHSA-2026:60520 · 321 productsMitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
  5. No fix available, says the vendor Red Hat Product Security CVE-2026-33079 · 1 productAffected

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
CVSS 4.0 vector
CVSS:4.0/AV:N/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
Published
2026-05-06
Last Modified
2026-08-28
NVD status
Deferred
MSRC Title
Mistune ReDoS in LINK_TITLE_RE allows denial of service with crafted Markdown titles

Frequently asked questions

What is CVE-2026-33079?

In versions 3.0.0a1 through 3.2.0 of Mistune, there is a ReDoS (Regular Expression Denial of Service) vulnerability in `LINK_TITLE_RE` that allows an attacker who can supply Markdown for parsing to cause denial of service. The regular expression used for parsing link titles contains overlapping alternatives that can trigger catastrophic backtracking. In both the double-quoted and single-quoted branches, a backslash followed by punctuation can be matched either as an escaped punctuation sequence or as two ordinary characters, creating an ambiguous pattern inside a repeated group. If an attacker supplies Markdown containing repeated ! sequences with no closing quote, the regex engine explores an exponential number of backtracking paths. This is reachable through normal Markdown parsing of inline links and block link reference definitions. A small crafted input can therefore cause significant CPU consumption and make applications using Mistune unresponsive.

Is CVE-2026-33079 actively exploited?

A proof-of-concept exploit exists for CVE-2026-33079, but active exploitation has not been confirmed at this time.

What is the CVSS score for CVE-2026-33079?

CVE-2026-33079 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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