PatchSiren cyber security CVE debrief
CVE-2026-67422 facelessuser CVE debrief
The pymdown-extensions library, a collection of extensions for the Python Markdown library, is vulnerable to a denial of service attack. Versions up to and including 11.0 contain four inline processors (caret, tilde, betterem, and magiclink) that use regular expressions which can cause catastrophic backtracking. This results in exponentially increasing CPU usage when rendering specially crafted Markdown lines under 50 bytes. The issue is fixed in version 11.0.1. Web applications rendering user-supplied Markdown are exposed, including those using pymdownx.extra which bundles the vulnerable betterem default. Affected product deployments should be inventoried and assessed for exposure, and owners should be assigned for follow-up. The vulnerability has a CVSS score of 7.5 and is considered HIGH severity. The CVE record was published on 2026-08-06T22:18:21.600Z and has not been modified since then.
- Vendor
- facelessuser
- Product
- pymdown-extensions
- CVSS
- HIGH 7.5
- CISA KEV
- Not listed in stored evidence
- Original CVE published
- 2026-08-06
- Original CVE updated
- 2026-08-07
- Advisory published
- 2026-08-06
- Advisory updated
- 2026-08-07
Who should care
Web application developers and administrators using pymdown-extensions or pymdownx.extra, especially those rendering user-supplied Markdown in comments, wikis, issue bodies, or live previews.
Technical summary
The pymdown-extensions library, a collection of extensions for the Python Markdown library, is vulnerable to a denial of service attack. Versions up to and including 11.0 contain four inline processors (caret, tilde, betterem, and magiclink) that use regular expressions which can cause catastrophic backtracking. This results in exponentially increasing CPU usage when rendering specially crafted Markdown lines under 50 bytes. The issue is fixed in version 11.0.1.
Defensive priority
Denial of service vulnerability in pymdown-extensions, a collection of extensions for the Python Markdown library, allows unauthenticated remote attackers to cause CPU usage issues via specially crafted Markdown lines.
Recommended defensive actions
- Inventory and assess exposure of web applications and services that render user-supplied Markdown using pymdown-extensions, especially those using pymdownx.extra.
- Apply the patch by updating pymdown-extensions to version 11.0.1 or later.
- Implement compensating controls such as rate limiting or monitoring for suspicious Markdown rendering activity.
- Review and update documentation and CI/CD pipelines that process untrusted Markdown.
- Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
- Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
- Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.
Evidence notes
The CVE-2026-67422 vulnerability in pymdown-extensions allows unauthenticated remote attackers to cause denial of service via specially crafted Markdown lines. The issue is fixed in version 11.0.1. Web applications rendering user-supplied Markdown are exposed, including those using pymdownx.extra which bundles the vulnerable betterem default.
Official resources
AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-08-06T22:18:21.600Z and has not been modified since then.