Mistune: Denial of Service — RecursionError via Excessive Emphasis Markers in Markdown

Summary

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.

Details

The InlineParser's _process_emphasis_delimiters() creates deeply nested 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 and 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["c…