Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
Priority HIGH 72CWE-1333Published · Updated
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Impact type
- Denial of service
- Confidentiality · integrity · availability
- None · None · High
- Fix status
- Partial Red Hat: 4 without a fix
Attack profile from CVSS 3.1 · CNA. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-1333.
Summary
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.
Why it matters now
Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00651 51st percentileScored 2026-11-07
Public exploit Yes 1 exploit referenceTagged by NVD
Fix Partial mistune (lepture): 3.3.0mistune (PyPI): 3.3.0Red Hat: 4 without a fix
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- Poc
- Automatable
- Yes
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| lepture | mistune | before 3.3.0 | 3.3.0 | Patch available | CNA |
| — | mistune (PyPI) | before 3.3.0 | 3.3.0 | Patch available | GHSA-ffq3-xpv3-j92q |
- mistune Vendorlepture Affected before 3.3.0 Fixed in 3.3.0 Patch available CNA
- mistune (PyPI) Vendor— Affected before 3.3.0 Fixed in 3.3.0 Patch available GHSA-ffq3-xpv3-j92q
Remediation
- Fix · CNA mistune (lepture) Fixed in 3.3.0
- Fix · GHSA-ffq3-xpv3-j92q mistune (PyPI) Fixed in 3.3.0 Advisory →
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Migration Toolkit for Applications 8 | Affected | No fix yet: Fix deferred |
| Red Hat OpenShift AI (RHOAI) | Affected | No fix yet: Fix deferred |
| Red Hat OpenShift Container Platform 4 | Affected | No fix yet: Fix deferred |
| Red Hat Satellite 6 | Affected | No fix yet: Fix deferred |
Timeline
- 2026-10-02 Threatist record created from CVE-2026-59928
- 2026-10-03 Alias added: GHSA-ffq3-xpv3-j92q
- 2026-10-14 EPSS moved to the 50th–90th percentile
References
More lepture Mistune vulnerabilities
- TI-2026-42730 Regular expression denial of service in lepture mistune CVSS 3.1HIGH 7.5
- TI-2026-42731 Mistune: XSS via percent-encoded javascript URI bypass in safe_url() CVSS 3.1MEDIUM 6.1
- TI-2026-32774 Mistune Image Directive CSS Injection Vulnerability CVSS 3.1MEDIUM 6.1
- TI-2026-32773 Mistune TOC Anchor Injection XSS CVSS 3.1MEDIUM 6.1
- TI-2026-32595 Mistune Math Plugin XSS Escape Bypass CVSS 3.1MEDIUM 6.1
- TI-2026-42738 Insufficient verification of data authenticity in lepture mistune CVSS 3.1MEDIUM 4.3
Sources
- CVE-2026-59928 · CVE Program
- CVE-2026-59928 · NVD
- FIRST EPSS
- GHSA-ffq3-xpv3-j92q · GitHub Advisory Database · CC BY 4.0
- CVE-2026-59928 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences