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CVE-2026-44899 — Mistune Image Directive CSS Injection Vulnerability

Affected: lepture mistune

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.

GHSA-ccfx-mfmx-2fx9 — Mistune Image Directive CSS Injection Vulnerability

Affected: PyPI: mistune

## 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…

PYSEC-2026-2209

Affected: PyPI: mistune