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.
```python
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.
```python
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 modern markdown renderer in production) is to slugify the heading TEXT for the ID — `
Introduction
` — with a numeric suffix appended only when slug collisions occur. mistune's default punts the slugification entirely and produces purely positional IDs that an attacker can predict in O(1).
The downstream impacts:
- Same-page links with `[click](#toc_1)` go to whichever element with `id="toc_1"` appears first in tree order. If the attacker can land any HTML element with `id="toc_1"` before the real heading (via inline_html with `escape=False`, via the include-directive HTML branch, via attacker-supplied content earlier in the document), navigation is hijacked.
- CSS rules targeting `#toc_1` apply to the wrong element.
- JavaScript bound to `document.getElementById('toc_1')` operates on the wrong element.
- The TOC's own `
` link in the rendered TOC list points to whichever element wins the duplicate-ID race.
## Exploit Chain
1. Application uses mistune with `add_toc_hook(md)` or `TableOfContents` directive enabled (the documented setup for sites with TOC support).
2. Application renders an attacker-supplied document, or splices attacker content into a trusted document. With `escape=False` (or via the include-directive `.html` branch covered by my prior advisory), the attacker can place `...` anywhere in the document.
3. mistune assigns `id="toc_1"` to the first heading. Now there are two elements with `id="toc_1"` in the page.
4. The rendered TOC contains `
First heading`. Clicking it navigates to whichever element with `id="toc_1"` appears first in tree order. If the attacker placed their `
` BEFORE the heading, navigation is hijacked.
5. Same-page CSS / JS / aria-described references to `#toc_1` similarly redirect.
## Security Impact
**Severity:** sec-low. Not a direct XSS or RCE; the issue is identifier confusion that enables UI-redirection / navigation-hijack attacks. The realistic attacker capability is "make an internal anchor link go to attacker content instead of the real heading", or "make a CSS selector apply to attacker content", or "break aria/screen-reader associations".
**Attacker capability:** with the ability to plant any HTML element with `id="toc_N"` in the document, hijack `
` navigation and any CSS/JS targeting that ID. With `escape=False`, this is straightforward. With `escape=True`, the attacker needs another vector to land a raw `id` attribute (one of the include-directive branches, a sibling tooling pipeline that lets HTML through, etc.).
**Preconditions:** applicati
