CVE-2026-115355 lepture CVE debrief

Who should care

Developers and administrators using the Mistune library, especially those who have enabled the Include plugin, should assess their exposure and verify their version of Mistune. They should also review and update documentation for the Include directive to ensure secure usage and apply patches or upgrades as necessary. Operators and security teams should be aware of the potential impact and plan for mitigations through normal change control.

Why it matters

CVE-2026-115355 is a vulnerability in the Mistune library's Include directive, allowing path traversal and XSS. Developers and administrators should verify their version and apply patches or upgrades as necessary.

• Path traversal could allow reading of arbitrary files, potentially exposing sensitive information.
• XSS vulnerability could lead to injection of malicious scripts, affecting user interactions.

Technical summary

The Include directive in `src/mistune/directives/include.py` does not validate that the resolved file path stays within the source directory, allowing path traversal via `../` sequences to read arbitrary files. Additionally, error messages and included file contents are rendered without HTML escaping, leading to XSS even when `escape=True`. The Include plugin must be explicitly enabled via the plugins parameter. Affected functions include path resolution, `block_error()` in `html.py`, and `render_html_include()`. Developers should review the code and apply patches or upgrades as necessary.

Defensive priority

High

Recommended defensive actions

• Verify the version of Mistune in use and check if it is vulnerable.
• Apply the patch or upgrade to a fixed version (v3.3.0 or later) if vulnerable.
• Review and update documentation for the Include directive to ensure secure usage.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
• Review compensating controls for exposed systems while remediation is scheduled and verified.

Evidence notes

The Include directive in `src/mistune/directives/include.py` does not validate that the resolved file path stays within the source directory, allowing path traversal via `../` sequences to read arbitrary files. Additionally, error messages and included file contents are rendered without HTML escaping, leading to XSS even when `escape=True`.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.