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Stored XSS via Markdown media attribute() action in Grav CMS

Moderate
rhukster published GHSA-r7fx-8g49-7hhr Apr 27, 2026

Package

composer getgrav/grav (Composer)

Affected versions

< 2.0.0-beta.2

Patched versions

2.0.0-beta.2

Description

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
edycja - the JavaScript alert executing on the frontend page alert - browser DevTools showing the injected `onload` attribute in the rendered DOM inspect

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.

Discoverers

@K-Czaplicki
@morzelowski

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:

Thanks for the report.

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required High
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality High
Integrity Low
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity Low
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

CVE ID

CVE-2026-42841

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

Credits