Mistune CVE-2026-59927

The Include directive (src/mistune/directives/include.py) must be explicitly enabled and active in the target application - it is an optional Mistune feature, not enabled in all deployments. The attacker must be able to create or influence two distinct markdown files on the server's filesystem such that each references the other via Include syntax; this typically requires write access to the application's content store (e.g., wiki page creation, file upload capability), making PR:N in the vendor CVSS potentially optimistic for hardened deployments. Applications where the Include directive is disabled, where markdown content is entirely author-controlled (no user submissions), or where file-creation is restricted to privileged users are not practically exploitable. The scope is limited to availability (A:L) - no confidentiality or integrity impact applies. Python's built-in recursion limit bounds the damage to a single request crash rather than process or host compromise.

The vendor-assigned CVSS 5.3 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L) characterizes this as a low-complexity, unauthenticated network attack with limited availability impact. The PR:N and AV:N assignments may overstate ease of exploitation: the Include directive operates on server-side filesystem files, meaning an attacker typically must be able to write or influence two separate markdown files in the target application before a cycle can be triggered - suggesting PR:L and AC:H are more realistic in most deployments. No EPSS score was provided in the input; given the absence from CISA KEV and no reported POC code, real-world exploitation probability appears low. The practical blast radius is a single crashed rendering request rather than full service compromise, consistent with the A:L rating. Applications that expose the Include directive to unauthenticated user-supplied content (e.g., public wikis or documentation platforms) carry higher risk than those where markdown is author-controlled. Overall this is a genuine but low-urgency issue warranting a planned patch cycle rather than emergency response.

An attacker with the ability to create or upload markdown content to a Mistune-powered application constructs two files where file A contains an include reference to file B and file B contains an include reference back to file A. When any user or automated process triggers a render of either file, Mistune's Include directive recursively processes the chain without bound, exhausting the Python interpreter's call stack and raising a RecursionError that crashes the rendering request. No public exploit code has been identified at time of analysis.