{"id":"GHSA-r7fx-8g49-7hhr","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\u003cimg onload=\"alert(document.domain)\" alt=\"Quarterly market overview\" src=\"/user/pages/03.campaigns/market-overview.gif?...\"\u003e\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 = $this-\u003eexcerpts-\u003eprocessImageExcerpt($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' =\u003e $parts[0], 'params' =\u003e $value];\n```\n\nThe requested method is later invoked dynamically:\n\n```php\n$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        $this-\u003eattributes[$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' =\u003e 'img', 'attributes' =\u003e $attributes];\n```\n\nThe non-static image path in `system/src/Grav/Common/Page/Medium/ImageMedium.php` also returns image attributes in the same way.\n\nFinally, in `vendor/erusev/parsedown/Parsedown.php`, the attribute value is escaped, but the attribute name is rendered as-is:\n\n```php\n$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\u003cimg onload=\"alert(document.domain)\" alt=\"Quarterly market overview\" src=\"/user/pages/03.campaigns/market-overview.gif?...\"\u003e\n```\n\nScreenshots:\n\n- the stored Markdown payload in the page editor\n\u003cimg width=\"1718\" height=\"1013\" alt=\"edycja\" src=\"https://github.com/user-attachments/assets/8f5e5275-e4ef-4d5e-a2cd-44683537b909\" /\u003e\n- the JavaScript alert executing on the frontend page\n\u003cimg width=\"1727\" height=\"1002\" alt=\"alert\" src=\"https://github.com/user-attachments/assets/6de81228-830c-49f2-ac41-b15658a8913d\" /\u003e\n- browser DevTools showing the injected `onload` attribute in the rendered DOM\n\u003cimg width=\"939\" height=\"539\" alt=\"inspect\" src=\"https://github.com/user-attachments/assets/7832c42d-6f3a-4ea2-b072-b837bd3913ed\" /\u003e\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 `\u003cimg\u003e`. `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---","aliases":["CVE-2026-42841"],"modified":"2026-09-10T03:50:47.418344786Z","published":"2026-05-05T21:24:08Z","database_specific":{"severity":"MODERATE","github_reviewed":true,"github_reviewed_at":"2026-05-05T21:24:08Z","nvd_published_at":"2026-05-11T16:17:34Z","cwe_ids":["CWE-79"]},"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"}],"affected":[{"package":{"name":"getgrav/grav","ecosystem":"Packagist","purl":"pkg:composer/getgrav/grav"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.0.0-beta.2"}]}],"versions":["0.8.0","0.9.0","0.9.1","0.9.10","0.9.11","0.9.12","0.9.13","0.9.14","0.9.15","0.9.16","0.9.17","0.9.18","0.9.19","0.9.2","0.9.20","0.9.21","0.9.22","0.9.23","0.9.24","0.9.25","0.9.26","0.9.27","0.9.28","0.9.29","0.9.3","0.9.30","0.9.31","0.9.32","0.9.33","0.9.34","0.9.35","0.9.36","0.9.37","0.9.38","0.9.39","0.9.4","0.9.40","0.9.41","0.9.42","0.9.43","0.9.44","0.9.45","0.9.5","0.9.6","0.9.7","0.9.8","0.9.9","1.0.0","1.0.0-rc.1","1.0.0-rc.2","1.0.0-rc.3","1.0.0-rc.4","1.0.0-rc.5","1.0.0-rc.6","1.0.1","1.0.10","1.0.2","1.0.3","1.0.4","1.0.5","1.0.6","1.0.7","1.0.8","1.0.9","1.1.0","1.1.0-beta.1","1.1.0-beta.2","1.1.0-beta.3","1.1.0-beta.4","1.1.0-beta.5","1.1.0-rc.1","1.1.0-rc.2","1.1.0-rc.3","1.1.1","1.1.10","1.1.11","1.1.12","1.1.13","1.1.14","1.1.15","1.1.16","1.1.17","1.1.2","1.1.3","1.1.4","1.1.5","1.1.6","1.1.7","1.1.8","1.1.9","1.1.9-rc.1","1.1.9-rc.2","1.1.9-rc.3","1.2.0","1.2.0-rc.1","1.2.0-rc.2","1.2.0-rc.3","1.2.1","1.2.2","1.2.3","1.2.4","1.3.0","1.3.0-rc.1","1.3.0-rc.2","1.3.0-rc.3","1.3.0-rc.4","1.3.0-rc.5","1.3.1","1.3.10","1.3.2","1.3.3","1.3.4","1.3.5","1.3.6","1.3.7","1.3.8","1.3.9","1.4.0","1.4.0-beta.1","1.4.0-beta.2","1.4.0-beta.3","1.4.0-rc.1","1.4.0-rc.2","1.4.1","1.4.2","1.4.3","1.4.4","1.4.5","1.4.6","1.4.7","1.4.8","1.5.0","1.5.0-beta.1","1.5.0-beta.2","1.5.0-rc.1","1.5.1","1.5.10","1.5.2","1.5.3","1.5.4","1.5.5","1.5.6","1.5.7","1.5.8","1.5.9","1.6.0","1.6.0-beta.1","1.6.0-beta.2","1.6.0-beta.3","1.6.0-beta.4","1.6.0-beta.5","1.6.0-beta.6","1.6.0-beta.7","1.6.0-beta.8","1.6.0-rc.1","1.6.0-rc.2","1.6.0-rc.3","1.6.0-rc.4","1.6.1","1.6.10","1.6.11","1.6.12","1.6.13","1.6.14","1.6.15","1.6.16","1.6.17","1.6.18","1.6.19","1.6.2","1.6.20","1.6.21","1.6.22","1.6.23","1.6.24","1.6.25","1.6.26","1.6.27","1.6.28","1.6.29","1.6.3","1.6.30","1.6.31","1.6.4","1.6.5","1.6.6","1.6.7","1.6.8","1.6.9","1.7.0","1.7.0-beta.1","1.7.0-beta.10","1.7.0-beta.2","1.7.0-beta.3","1.7.0-beta.4","1.7.0-beta.5","1.7.0-beta.6","1.7.0-beta.7","1.7.0-beta.8","1.7.0-beta.9","1.7.0-rc.1","1.7.0-rc.10","1.7.0-rc.11","1.7.0-rc.12","1.7.0-rc.13","1.7.0-rc.14","1.7.0-rc.15","1.7.0-rc.16","1.7.0-rc.17","1.7.0-rc.18","1.7.0-rc.19","1.7.0-rc.2","1.7.0-rc.20","1.7.0-rc.3","1.7.0-rc.4","1.7.0-rc.5","1.7.0-rc.6","1.7.0-rc.7","1.7.0-rc.8","1.7.0-rc.9","1.7.1","1.7.10","1.7.12","1.7.13","1.7.14","1.7.15","1.7.16","1.7.17","1.7.18","1.7.19","1.7.20","1.7.21","1.7.22","1.7.23","1.7.24","1.7.25","1.7.26","1.7.26.1","1.7.27","1.7.27.1","1.7.28","1.7.29","1.7.29.1","1.7.3","1.7.30","1.7.31","1.7.32","1.7.33","1.7.34","1.7.35","1.7.36","1.7.37","1.7.37.1","1.7.38","1.7.39","1.7.39.1","1.7.39.2","1.7.39.3","1.7.39.4","1.7.4","1.7.40","1.7.41","1.7.41.1","1.7.41.2","1.7.42","1.7.42.1","1.7.42.2","1.7.42.3","1.7.43","1.7.44","1.7.45","1.7.46","1.7.47","1.7.48","1.7.49","1.7.49.1","1.7.49.2","1.7.49.3","1.7.49.4","1.7.49.5","1.7.5","1.7.51","1.7.52","1.7.53","1.7.53.1","1.7.53.2","1.7.53.3","1.7.6","1.7.7","1.7.8","1.7.9","1.8.0-beta.1","1.8.0-beta.10","1.8.0-beta.11","1.8.0-beta.12","1.8.0-beta.13","1.8.0-beta.14","1.8.0-beta.15","1.8.0-beta.16","1.8.0-beta.17","1.8.0-beta.18","1.8.0-beta.19","1.8.0-beta.2","1.8.0-beta.20","1.8.0-beta.21","1.8.0-beta.22","1.8.0-beta.23","1.8.0-beta.24","1.8.0-beta.25","1.8.0-beta.26","1.8.0-beta.27","1.8.0-beta.28","1.8.0-beta.29","1.8.0-beta.3","1.8.0-beta.4","1.8.0-beta.5","1.8.0-beta.6","1.8.0-beta.7","1.8.0-beta.8","1.8.0-beta.9","2.0.0-beta.1"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/05/GHSA-r7fx-8g49-7hhr/GHSA-r7fx-8g49-7hhr.json"}}],"schema_version":"1.9.0","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"}]}