CVE-2026-42841: Getgrav/grav Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-r7fx-8g49-7hhr",
  "modified" : "2026-05-13T13:52:10Z",
  "published" : "2026-05-05T21:24:08Z",
  "aliases" : [ "CVE-2026-42841" ],
  "summary" : "Grav CMS vulnerable to stored XSS via Markdown media attribute() action",
  "details" : "### Summary\n\nAn 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.\n\nThe 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.\n\nFor example, this Markdown:\n\n```markdown\n![Quarterly market overview](market-overview.gif?attribute=onload,alert(document.domain))\n```\n\nis rendered as an image tag containing an executable `onload` handler:\n\n```html\n<img onload=\"alert(document.domain)\" alt=\"Quarterly market overview\" src=\"/user/pages/03.campaigns/market-overview.gif?...\">\n```\n\nThis 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.\n\nTested versions:\n\n- Grav CMS: 1.7.49.5\n- Admin Plugin: 1.10.49.1\n\nSuggested classification:\n\n- CWE-79: Improper Neutralization of Input During Web Page Generation\n- Stored Cross-Site Scripting\n- Suggested CVSS v4.0 score if page editing is considered high privilege: 6.9 Medium\n- 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`\n- Suggested CVSS v3.1 score if page editing is considered high privilege: 6.9 Medium\n- Suggested CVSS v3.1 vector: `CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:L/A:N`\n\n### Details\n\nThe issue appears to come from this source-to-sink flow:\n\n1. `ParsedownGravTrait::inlineImage()` processes Markdown images.\n2. `Excerpts::processImageExcerpt()` resolves the referenced media object.\n3. `Excerpts::processMediaActions()` parses the image URL query string into media actions.\n4. `call_user_func_array()` invokes the requested action method on the media object.\n5. `MediaObjectTrait::attribute()` stores the attacker-controlled attribute name and value.\n6. The media object returns a Parsedown element containing the injected attribute.\n7. Parsedown renders the attribute name into the final HTML.\n\nRelevant code paths:\n\n```text\nsystem/src/Grav/Common/Markdown/ParsedownGravTrait.php\nsystem/src/Grav/Common/Page/Markdown/Excerpts.php\nsystem/src/Grav/Common/Media/Traits/MediaObjectTrait.php\nsystem/src/Grav/Common/Page/Medium/StaticImageMedium.php\nsystem/src/Grav/Common/Page/Medium/ImageMedium.php\nvendor/erusev/parsedown/Parsedown.php\n```\n\nIn `system/src/Grav/Common/Markdown/ParsedownGravTrait.php`, Markdown image excerpts are passed into Grav-specific media handling:\n\n```php\nif (isset($excerpt['element']['attributes']['src'])) {\n    $excerpt = $this->excerpts->processImageExcerpt($excerpt);\n}\n```\n\nIn `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:\n\n```php\n$carry[] = ['method' => $parts[0], 'params' => $value];\n```\n\nThe requested method is later invoked dynamically:\n\n```php\n$medium = call_user_func_array([$medium, $action['method']], $args);\n```\n\nFor the payload:\n\n```text\nattribute=onload,alert(document.domain)\n```\n\nthe method is `attribute`, and the arguments are `onload` and `alert(document.domain)`.\n\nIn `system/src/Grav/Common/Media/Traits/MediaObjectTrait.php`, `attribute()` stores the caller-controlled attribute name directly:\n\n```php\npublic function attribute($attribute = null, $value = '')\n{\n    if (!empty($attribute)) {\n        $this->attributes[$attribute] = $value;\n    }\n    return $this;\n}\n```\n\nThe image media classes then return the collected attributes as attributes for an `img` element.\n\nIn `system/src/Grav/Common/Page/Medium/StaticImageMedium.php`:\n\n```php\nreturn ['name' => 'img', 'attributes' => $attributes];\n```\n\nThe non-static image path in `system/src/Grav/Common/Page/Medium/ImageMedium.php` also returns image attributes in the same way.\n\nFinally, in `vendor/erusev/parsedown/Parsedown.php`, the attribute value is escaped, but the attribute name is rendered as-is:\n\n```php\n$markup .= ' '.$name.'=\"'.self::escape($value).'\"';\n```\n\nAs a result, the attacker-controlled attribute name `onload` is emitted into the final HTML and executes as a browser event handler.\n\nThe Admin Plugin's save-time XSS detection does not appear to block this because the stored content is Markdown media syntax, not raw HTML:\n\n```markdown\n![Quarterly market overview](market-overview.gif?attribute=onload,alert(document.domain))\n```\n\nThe dangerous HTML is generated later during Markdown/media rendering.\n\n### PoC\n\nI reproduced this on a standard Grav CMS installation with the Admin Plugin enabled.\n\nConfiguration and prerequisites:\n\n- Grav CMS 1.7.49.5\n- Admin Plugin 1.10.49.1\n- Markdown processing enabled for pages\n- A user account with permission to create or edit pages\n- A page media file available in the edited page folder, for example `market-overview.gif`\n\nSteps to reproduce:\n\n1. Install Grav CMS with the Admin Plugin.\n2. Log in to the Admin panel as a user who can create or edit pages.\n3. Create a normal content page or edit an existing one.\n4. Add or reference a page media file named `market-overview.gif`.\n5. Insert the following Markdown into the page body:\n\n   ```markdown\n   ![Quarterly market overview](market-overview.gif?attribute=onload,alert(document.domain))\n   ```\n\n6. Save the page.\n7. Open the rendered frontend page in a browser.\n8. The JavaScript payload executes when the image loads.\n9. Inspect the generated DOM. The rendered image element contains the injected `onload` attribute.\n\nExpected result:\n\nThe 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.\n\nActual result:\n\nThe payload is accepted and rendered as an executable image event handler:\n\n```html\n<img onload=\"alert(document.domain)\" alt=\"Quarterly market overview\" src=\"/user/pages/03.campaigns/market-overview.gif?...\">\n```\n\nScreenshots:\n\n- the stored Markdown payload in the page editor\n<img width=\"1718\" height=\"1013\" alt=\"edycja\" src=\"https://github.com/user-attachments/assets/8f5e5275-e4ef-4d5e-a2cd-44683537b909\" />\n- the JavaScript alert executing on the frontend page\n<img width=\"1727\" height=\"1002\" alt=\"alert\" src=\"https://github.com/user-attachments/assets/6de81228-830c-49f2-ac41-b15658a8913d\" />\n- browser DevTools showing the injected `onload` attribute in the rendered DOM\n<img width=\"939\" height=\"539\" alt=\"inspect\" src=\"https://github.com/user-attachments/assets/7832c42d-6f3a-4ea2-b072-b837bd3913ed\" />\n\n### Impact\n\nThis is a stored cross-site scripting vulnerability.\n\nAn 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.\n\nIn 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.\n\nCVSS 4.0 rationale:\n\n- `AV:N`: the issue is exploitable through the web application.\n- `AC:L`: no special race condition or complex setup is required after page editing access is obtained.\n- `AT:P`: exploitation requires the malicious Markdown/media reference to be stored in page content and later rendered to a victim.\n- `PR:H`: the attacker needs page editing capability.\n- `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.\n- `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.\n- `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.\n\n## Maintainer note — fix applied (2026-04-24)\n\nFixed in Grav core on the `2.0` branch: commit [`5a12f9be8`](https://github.com/getgrav/grav/commit/5a12f9be8) — will ship in **2.0.0-beta.2**.\n\n**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:\n\n- any `on*` handler (case-insensitive)\n- `style` (inline CSS expression risk)\n- `xmlns` (XML namespace tricks)\n- `srcdoc` (iframe sandbox bypass)\n- `formaction` (form action override)\n\nInvalid 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.\n\n**Files:**\n- [`system/src/Grav/Common/Media/Traits/MediaObjectTrait.php`](https://github.com/getgrav/grav/blob/2.0/system/src/Grav/Common/Media/Traits/MediaObjectTrait.php) — new `isSafeAttributeName()` gate.\n- [`tests/unit/Grav/Common/Security/MediaAttributeSecurityTest.php`](https://github.com/getgrav/grav/blob/2.0/tests/unit/Grav/Common/Security/MediaAttributeSecurityTest.php) — 28 cases (14 dangerous-name rejections, 14 safe-name round-trips).\n\n### Discoverers\n\n@K-Czaplicki\n@morzelowski\n\n\n---",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "getgrav/grav"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.0.0-beta.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/getgrav/grav/security/advisories/GHSA-r7fx-8g49-7hhr"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-42841"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/getgrav/grav/commit/5a12f9be8314682c8713e569e330f11805d0a663"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/getgrav/grav"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-05T21:24:08Z",
    "nvd_published_at" : "2026-05-11T16:17:34Z"
  }
}