| Package | Affected versions | Patched versions |
|---|---|---|
| mistune | ≥ 3.3.0, < 3.3.3 | 3.3.3 |
Mistune v3.3.2 is vulnerable to a Denial of Service (DoS) attack via uncontrolled recursion in the HTML rendering of deeply-nested emphasis tokens. By submitting Markdown containing approximately 1,000 consecutive asterisk characters, an attacker causes the Python process to crash with RecursionError.
The InlineParser's _process_emphasis_delimiters() creates deeply nested <strong> tokens from consecutive emphasis markers (every 2 asterisks add one nesting level). With 1,000 consecutive asterisks, approximately 500 levels of nesting are produced. The HTMLRenderer.render_token() method (src/mistune/renderers/html.py:40-57) renders these tokens recursively: when a token has children, line 48 calls self.render_tokens(token['children'], state), entering child rendering. Each nesting level produces ~2 stack frames, so 500 levels ≈ 1,000 frames, exceeding Python's default recursion limit (sys.getrecursionlimit() = 1000). The emphasis() and strong() methods (lines 80-84) wrap recursively rendered child content in <em> and <strong> tags, perpetuating the recursion. This vulnerability affects all mistune APIs including markdown() and html().
Core vulnerable code path:
# src/mistune/renderers/html.py:40-57
def render_token(self, token: Dict[str, Any], state: BlockState) -> str:
func = self._get_method(token["type"])
attrs = token.get("attrs")
if "raw" in token:
text = token["raw"]
elif "children" in token:
text = self.render_tokens(token["children"], state)
else:
if attrs:
return func(**attrs)
else:
return func()
if attrs:
return func(text, **attrs)
else:
return func(text)
The recursive call to render_tokens() on line 48 processes nested child tokens. With 500 levels of nested emphasis/strong tokens, this recursion exceeds Python's default recursion limit of 1000, causing a RecursionError.
# src/mistune/renderers/html.py:80-84
def emphasis(self, text: str) -> str:
return "<em>" + text + "</em>"
def strong(self, text: str) -> str:
return "<strong>" + text + "</strong>"
The emphasis() and strong() methods wrap their text content (which is itself the recursively-rendered output of nested child tokens) in HTML tags, creating the chain of recursion: each call to strong() includes rendered children that themselves call strong(), etc.
from mistune import html
payload = '*' * 1000
try:
result = html(payload)
print('No crash - recursion handled')
except RecursionError as e:
print(f'[VULN] RecursionError: {e} - Process would crash!')
except Exception as e:
print(f'Error: {type(e).__name__}: {e}')
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Scan my site freeCVSS 3.1: 7.5 (High) — AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. An attacker can crash any server process using mistune with approximately 2KB of Markdown input. In web applications, this can be triggered through user-generated content such as forum posts or comments, causing denial of service for all concurrent users sharing the same Python process. Both mistune.markdown() and mistune.html() are affected.