GHSA-2hm2-hc3v-44h9: Mistune toc / TableOfContents directive: heading IDs use predictable `toc_N` numbering with no slugification, allowing collision with attacker-controlled `id="toc_N"` content

Summary Type: Predictable identifier generation. The toc plugin and TableOfContents directive both default to generating heading IDs of the form toc_1, toc_2, toc_3, ... with no input-derived component. An attacker who can place a heading anywhere in the document can predict which toc_N ID it will receive, and can inject HTML elsewhere (in a non-heading context) that uses the same id="toc_N" to either (a) shadow the legitimate heading anchor, breaking same-page navigation, or (b) collide with CSS or JavaScript that targets #toc_N selectors, redirecting click handlers and styling to attacker-chosen content. File: src/mistune/toc.py line 36-37 (heading_id = lambda token, index: "toc_" + str(index + 1)); src/mistune/directives/toc.py line 33 (same default). Root cause: the default heading_id callback ignores the heading's text content and uses only the headings's order in the document. Two documents rendered together (or one document with attacker-influenced headings spliced into trusted content) produce overlapping toc_N IDs. Because id attribute uniqueness is required by HTML, browsers behaviour on duplicate IDs is undefined; document.getElementById('toc_1') returns the first match, getElementsByTagName + querySelector semantics differ across paths, and CSS rules targeting #toc_1 apply to whichever element matches first in tree order. Affected Code File: src/mistune/toc.py, lines 33-39. def add_toc_hook(md, min_level=1, max_level=3, heading_id=None): if heading_id is None: def heading_id(token, index): return "toc_" + str(index + 1) # <-- BUG: index-only ID, no slug derived from heading text File: src/mistune/directives/toc.py, lines 32-33. class TableOfContents(DirectivePlugin): def init(self, min_level=1, max_level=3): ... def generate_heading_id(self, token, index): return "toc_" + str(index + 1) # <-- BUG: same predictable scheme Why it's wrong: the standard markdown-engine convention (used by GitHub-flavoured Markdown, Sphinx, MkDocs, pandoc, every mod