Mistune directives/include: mutual `.. include::` recursion crashes the renderer with `RecursionError`, denial of service via two attacker-controlled markdown files

Why it's wrong: the cycle-detection check is one level deep. Multi-file cycles slip through trivially. Python's default recursion limit is 1000 frames, so a cycle of length 2 trips after a few hundred mutual includes; the exception is uncaught by the directive, propagating out of Markdown.__call__() and crashing whatever called it.

Exploit Chain

1. Application uses mistune with the Include directive enabled. Application accepts user-supplied markdown files (CMS, wiki, multi-user documentation platform, note-taking app, CI/CD doc renderer).
2. Attacker uploads two markdown files:
   • a.md: .. include:: b.md
   • b.md: .. include:: a.md
3. Renderer is invoked on a.md (or any markdown that references this pair). Include directive includes b.md, which includes a.md, which includes b.md, ... Each recursion adds Python frames.
4. After ~340 cycle iterations (depending on default sys.setrecursionlimit(1000) and the per-include frame depth), Python raises RecursionError: maximum recursion depth exceeded.
5. The exception is not caught by the directive. It propagates through block.parse, through Markdown.__call__, and into the application's request handler. If the application doesn't catch it explicitly, the request errors out (HTTP 500 in web contexts, crash in CLI tools).

Security Impact

Attacker capability: crash the rendering engine on demand by submitting any markdown that triggers the cycle. Repeated requests deny service. If the renderer is used in a hot path (per-page-view docs rendering, search-index regeneration, scheduled doc-export jobs), the cycle persists across the whole pipeline. Preconditions: application uses mistune with the Include directive enabled and renders user-supplied markdown that can reference other user-uploaded files. Attacker needs write access to two .md files in the include search path (or a single file including a known-recurring pair). Differential: PoC-verified against [email protected]:

import os, mistune
from mistune.directives import RSTDirective, Include

os.makedirs('/tmp/mistune-recur', exist_ok=True)
with open('/tmp/mistune-recur/a.md', 'w') as f:
    f.write('A\n\n.. include:: b.md')
with open('/tmp/mistune-recur/b.md', 'w') as f:
    f.write('B\n\n.. include:: a.md')

md = mistune.create_markdown(plugins=[RSTDirective([Include()])])
state = md.block.state_cls()
state.env['__file__'] = '/tmp/mistune-recur/a.md'
md.parse('.. include:: b.md', state=state)
# RecursionError: maximum recursion depth exceeded

The patched build (with the suggested fix below) returns a block_error token like the existing self-include check, instead of recursing forever.

Suggested Fix

Track included paths in state.env and reject any include that would re-enter a path already on the include stack:

--- a/src/mistune/directives/include.py
+++ b/src/mistune/directives/include.py
@@ -28,8 +28,18 @@ class Include(DirectivePlugin):
         relpath = self.parse_title(m)
-        dest = os.path.join(os.path.dirname(source_file), relpath)
-        dest = os.path.normpath(dest)
+        base = os.path.realpath(os.path.dirname(source_file))
+        dest = os.path.realpath(os.path.join(base, relpath))
+
+        # Track include stack across recursive parses to detect cycles.
+        include_stack = state.env.setdefault("__include_stack__", [])
+        if dest in include_stack or dest == os.path.realpath(source_file):
+            return {
+                "type": "block_error",
+                "raw": "Could not include (cycle): " + relpath,
+            }

-        if os.path.abspath(dest) == os.path.abspath(source_file):
-            return {
-                "type": "block_error",
-                "raw": "Could not include self: " + relpath,
-            }
@@ ... in the markdown-include branch ...
+        include_stack.append(dest)
+        try:
+            new_state = block.state_cls()
+            new_state.env["__file__"] = dest
+            new_state.env["__include_stack__"] = include_stack
+            new_state.process(content)
+            block.parse(new_state)
+            return new_state.tokens
+        finally:
+            include_stack.pop()

This catches cycles of any length (a → b → a, a → b → c → a, etc.). Pair this with the path-containment fix from the LFI advisory and the HTML-extension fix from the include-XSS advisory; together those three patches make the Include directive safe to enable on user-supplied markdown.

Add a regression test asserting that a 2-cycle and a 3-cycle both produce block_error rather than RecursionError.