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:
```markdown
)
```
is rendered as an image tag containing an executable onload handler:
```html
<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:
```text
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:
```php
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:
```php
$carry[] = ['method' => $parts[0], 'params' => $value];
```
The requested method is later invoked dynamically:
```php
$medium = call_user_func_array([$medium, $action['method']], $args);
```
For the payload:
```text
attribute=onload,alert(document.domain)
```
the method is attribute, and the arguments are onload and alert(document.domain).
In system/src/Grav/Common/Media/Traits/MediaObjectTrait.php, attribute() stores the caller-controlled attribute name directly:
```php
public function attribute($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:
```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:
```php
$markup .= ' '.$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:
```markdown
)
```
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
