Grav CMS vulnerable to stored XSS via Markdown media attribute() action

Summary

An authenticated user with page editing permissions can inject an executable JavaScript event-handler attribute into rendered image HTML through Grav's Markdown media action syntax.

The issue is caused by Markdown image query parameters being converted into callable media actions. The public attribute() media method can be reached this way, allowing an editor to set an arbitrary HTML attribute name and value on the generated image element.

For example, this Markdown:

![Quarterly market overview](market-overview.gif?attribute=onload,alert(document.domain))

is rendered as an image tag containing an executable onload handler:

<img onload="alert(document.domain)" alt="Quarterly market overview" src="/user/pages/03.campaigns/market-overview.gif?...">

This results in stored XSS when another user views the affected page. In a multi-user Grav installation, a lower-privileged page editor could use this to target administrators or reviewers who preview or view editor-controlled content.

Tested versions:

• Grav CMS: 1.7.49.5
• Admin Plugin: 1.10.49.1

Suggested classification:

• CWE-79: Improper Neutralization of Input During Web Page Generation
• Stored Cross-Site Scripting
• Suggested CVSS v4.0 score if page editing is considered high privilege: 6.9 Medium
• Suggested CVSS v4.0 vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:L/VA:N/SC:H/SI:L/SA:N
• Suggested CVSS v3.1 score if page editing is considered high privilege: 6.9 Medium
• Suggested CVSS v3.1 vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:L/A:N

Details

The issue appears to come from this source-to-sink flow:

1. ParsedownGravTrait::inlineImage() processes Markdown images.
2. Excerpts::processImageExcerpt() resolves the referenced media object.
3. Excerpts::processMediaActions() parses the image URL query string into media actions.
4. call_user_func_array() invokes the requested action method on the media object.
5. MediaObjectTrait::attribute() stores the attacker-controlled attribute name and value.
6. The media object returns a Parsedown element containing the injected attribute.
7. Parsedown renders the attribute name into the final HTML.

Relevant code paths:

system/src/Grav/Common/Markdown/ParsedownGravTrait.php
system/src/Grav/Common/Page/Markdown/Excerpts.php
system/src/Grav/Common/Media/Traits/MediaObjectTrait.php
system/src/Grav/Common/Page/Medium/StaticImageMedium.php
system/src/Grav/Common/Page/Medium/ImageMedium.php
vendor/erusev/parsedown/Parsedown.php

In system/src/Grav/Common/Markdown/ParsedownGravTrait.php, Markdown image excerpts are passed into Grav-specific media handling:

if (isset($excerpt['element']['attributes']['src'])) {
    $excerpt = $this->excerpts->processImageExcerpt($excerpt);
}

In system/src/Grav/Common/Page/Markdown/Excerpts.php, query string parameters are converted into media action calls. The query parameter name becomes the method name:

$carry[] = ['method' => $parts[0], 'params' => $value];

The requested method is later invoked dynamically:

$medium = call_user_func_array([$medium, $action['method']], $args);

For the payload:

attribute=onload,alert(document.domain)

the method is attribute, and the arguments are onload and alert(document.domain).

In system/src/Grav/Common/Media/Traits/MediaObjectTrait.php, attribute() stores the caller-controlled attribute name directly:

public function attribute($attribute = null, $value = '')
{
    if (!empty($attribute)) {
        $this->attributes[$attribute] = $value;
    }
    return $this;
}

The image media classes then return the collected attributes as attributes for an img element.

In system/src/Grav/Common/Page/Medium/StaticImageMedium.php:

return ['name' => 'img', 'attributes' => $attributes];

The non-static image path in system/src/Grav/Common/Page/Medium/ImageMedium.php also returns image attributes in the same way.

Finally, in vendor/erusev/parsedown/Parsedown.php, the attribute value is escaped, but the attribute name is rendered as-is:

$markup .= ' '.$name.'="'.self::escape($value).'"';

As a result, the attacker-controlled attribute name onload is emitted into the final HTML and executes as a browser event handler.

The Admin Plugin's save-time XSS detection does not appear to block this because the stored content is Markdown media syntax, not raw HTML:

![Quarterly market overview](market-overview.gif?attribute=onload,alert(document.domain))

The dangerous HTML is generated later during Markdown/media rendering.

PoC

I reproduced this on a standard Grav CMS installation with the Admin Plugin enabled.

Configuration and prerequisites:

• Grav CMS 1.7.49.5
• Admin Plugin 1.10.49.1
• Markdown processing enabled for pages
• A user account with permission to create or edit pages
• A page media file available in the edited page folder, for example market-overview.gif

Steps to reproduce:

1. Install Grav CMS with the Admin Plugin.

2. Log in to the Admin panel as a user who can create or edit pages.

3. Create a normal content page or edit an existing one.

4. Add or reference a page media file named market-overview.gif.

5. Insert the following Markdown into the page body:

   ![Quarterly market overview](market-overview.gif?attribute=onload,alert(document.domain))
   

6. Save the page.

7. Open the rendered frontend page in a browser.

8. The JavaScript payload executes when the image loads.

9. Inspect the generated DOM. The rendered image element contains the injected onload attribute.

Expected result:

The Markdown media action should not be able to generate executable HTML attributes. The payload should be rejected, sanitized, or rendered without the dangerous event-handler attribute.

Actual result:

The payload is accepted and rendered as an executable image event handler:

<img onload="alert(document.domain)" alt="Quarterly market overview" src="/user/pages/03.campaigns/market-overview.gif?...">

Screenshots:

• the stored Markdown payload in the page editor <img width="1718" height="1013" alt="edycja" src="https://github.com/user-attachments/assets/8f5e5275-e4ef-4d5e-a2cd-44683537b909" />
• the JavaScript alert executing on the frontend page <img width="1727" height="1002" alt="alert" src="https://github.com/user-attachments/assets/6de81228-830c-49f2-ac41-b15658a8913d" />
• browser DevTools showing the injected onload attribute in the rendered DOM <img width="939" height="539" alt="inspect" src="https://github.com/user-attachments/assets/7832c42d-6f3a-4ea2-b072-b837bd3913ed" />

Impact

This is a stored cross-site scripting vulnerability.

An authenticated user with page editing permissions can store a malicious Markdown image reference. When the affected page is rendered, the payload executes in the browser of any user who views that page.

In multi-user Grav installations, this may allow a lower-privileged editor to target administrators, reviewers, or other privileged users who preview or view editor-controlled content. Depending on the victim's privileges and deployed plugins, successful exploitation may allow JavaScript execution in the site origin, access to same-origin page data available to the victim, and same-origin actions performed as the victim.

CVSS 4.0 rationale:

• AV:N: the issue is exploitable through the web application.
• AC:L: no special race condition or complex setup is required after page editing access is obtained.
• AT:P: exploitation requires the malicious Markdown/media reference to be stored in page content and later rendered to a victim.
• PR:H: the attacker needs page editing capability.
• UI:P: a victim must view the affected page. The demonstrated onload payload executes on passive page rendering, without requiring a click or form submission by the victim.
• VC:H/VI:L/VA:N: confidentiality impact can be high when the victim is an administrator or reviewer; integrity impact is limited; no direct availability impact was demonstrated.
• SC:H/SI:L/SA:N: the injected script executes in the browser/application context and may affect subsequent same-origin interactions available to the victim.

Maintainer note — fix applied (2026-04-24)

Fixed in Grav core on the 2.0 branch: commit 5a12f9be8 — will ship in 2.0.0-beta.2.

What changed: MediaObjectTrait::attribute() — the sink reached by Markdown like ![alt](img.gif?attribute=onload,alert(1)) — now gates the attribute name through an allowlist regex (^[A-Za-z][A-Za-z0-9_:.\-]*$) plus an explicit denylist of script-context names:

• any on* handler (case-insensitive)
• style (inline CSS expression risk)
• xmlns (XML namespace tricks)
• srcdoc (iframe sandbox bypass)
• formaction (form action override)

Invalid names are silently dropped — the attribute isn't stored, so it doesn't survive into the rendered <img>. src/href/data-*/aria-*/standard media attributes are unaffected.

Files:

• system/src/Grav/Common/Media/Traits/MediaObjectTrait.php — new isSafeAttributeName() gate.
• tests/unit/Grav/Common/Security/MediaAttributeSecurityTest.php — 28 cases (14 dangerous-name rejections, 14 safe-name round-trips).

Discoverers

@K-Czaplicki @morzelowski