Key points
- CVE-2026-115353 affects Mistune versions before 3.3.0 with a CVSS score of 9.2.
- The vulnerability enables arbitrary file read through path traversal in the RST include directive.
- Attackers can embed sensitive data like credentials into rendered HTML output.
Malicious actors can exploit a critical path traversal flaw in the Mistune markdown parser to read sensitive files from a server. This vulnerability allows attackers to access restricted data by injecting specific directives into markdown content. The flaw is rated CVSS 9.2, indicating severe risk.
The issue resides in how Mistune versions before 3.3.0 handle the RST include directive. Attackers submit markdown containing include statements that reference relative or absolute paths. This allows the parser to traverse directories and embed the contents of sensitive files, such as credentials or environment variables, directly into the resulting HTML output.
What we know
The National Vulnerability Database records this as CVE-2026-115353. The weakness is classified as CWE-22, which covers improper limitation of a pathname to a restricted directory. The affected component is the RST include directive within the Mistune library. The vendor, lepture, has identified the issue in all versions prior to 3.3.0.
This type of path traversal vulnerability mirrors other recent issues in web plugins. As Malware Chronicle reported on 2026-10-11, similar critical path traversal flaws in rtMedia versions through 4.7.13 allowed attackers to access restricted files on WordPress, BuddyPress, and bbPress installations. The pattern of abusing include or file-reference directives to leak data remains a persistent threat in content rendering libraries.
Defender's checklist
- Audit applications using Mistune versions earlier than 3.3.0 for exposure.
- Review server logs for unusual markdown submissions containing include directives.
- Monitor rendered HTML outputs for unexpected file contents or credential leaks.
- Verify that input sanitization does not bypass the underlying parser vulnerability.
What analysts watch for
Teams usually examine web server access logs for requests containing markdown payloads with relative path sequences like "../". Analysts check whether the application renders external files in the response body, which indicates successful traversal. They also inspect the RST include directive handling in the codebase to see if path resolution is restricted to a safe directory. Monitoring for unexpected file reads in system logs helps detect exploitation attempts before data exfiltration occurs.
What to do and how to stay safe: Mistune
- Update all instances of the Mistune library to version 3.3.0 or later immediately.
- Scan your codebase for any custom implementations that parse RST include directives.
- Review recent markdown submissions for any attempts to include external or absolute file paths.
- Once the vendor provides an update, verify that the patch correctly restricts path resolution.
Step-by-step guide: Security Advisories: Small Business Patching Strategy
General security guidance from the Malware Chronicle newsroom. It is not confirmed advice from the organisations named in this story.
Frequently asked questions
What is the CVSS score for CVE-2026-115353?
The National Vulnerability Database rates this vulnerability as CVSS 9.2, which is considered critical.
Which versions of Mistune are affected by this flaw?
All versions of Mistune before 3.3.0 are affected by this path traversal vulnerability.
How do attackers exploit this vulnerability?
Attackers submit markdown containing RST include directives with relative or absolute paths to read arbitrary files.
