CVE-2026-58657: Grav Stored CSS Injection Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-ffmg-hfvg-jhg9",
  "modified" : "2026-09-16T22:14:53Z",
  "published" : "2026-09-16T22:14:53Z",
  "aliases" : [ "CVE-2026-58657" ],
  "summary" : "Grav: Stored CSS injection via Markdown image resize() bypasses prior media style sanitizers in Grav",
  "details" : "## Summary\n\nGrav 2.0.0-rc.9 and the current 2.0 branch still allow stored CSS injection through Markdown image media actions. The prior media hardening rejects direct `?style=` payloads and unsafe `attribute()` fallbacks, but the adjacent `resize()` action still writes caller-controlled values directly into `styleAttributes`.\n\nA publisher who can edit page Markdown can store a crafted image URL that renders additional CSS declarations in the final `<img style=...>` attribute. This crosses the same lower-privileged publisher to higher-privileged reviewer/admin rendered-content boundary as the earlier media style and attribute advisories.\n\n## Impact\n\nA lower-privileged content editor can persist CSS declarations that are rendered when a higher-privileged user views the page or admin preview. The demonstrated payload creates a full-viewport fixed overlay by injecting `position:fixed`, viewport dimensions, background color, and z-index declarations.\n\nThis does not require JavaScript execution. The impact is stored CSS injection in rendered content, with UI redress/overlay and content-manipulation risk in higher-privileged sessions.\n\n## Reproduction\n\nTested versions:\n\n- Grav 2.0 branch commit `6582166173bb8eb5869d96aea384e0e73777c94c`\n- Grav `2.0.0-rc.9` commit `e03d29aa0d3ece16d73c1ffccfa78df8bf5f28b8`\n\nMinimal Markdown payload:\n\n```markdown\n![logo](image.png?resize=100;position:fixed;top:0;left:0;width:100vw;height:100vh;background:white;z-index:9999,200)\n```\n\nA minimal PHPUnit-style reproducer can drive the same parser path directly:\n\n```php\n$m = new class {\n    use \\Grav\\Common\\Media\\Traits\\MediaObjectTrait;\n    use \\Grav\\Common\\Media\\Traits\\StaticResizeTrait;\n\n    public function addMetaFile($filepath) {}\n    public function __toString(): string { return ''; }\n    public function url($reset = true) { return '/img.png'; }\n    public function get($name, mixed $default = null, $separator = null) { return $default; }\n    public function set($name, mixed $value, $separator = null) { return $this; }\n    protected function createThumbnail($thumb) { return null; }\n    protected function createLink(array $attributes) { return null; }\n    protected function getItems(): array { return []; }\n};\n\n$excerpts = new \\Grav\\Common\\Page\\Markdown\\Excerpts(null, ['markdown' => [], 'images' => []]);\n$m = $excerpts->processMediaActions(\n    $m,\n    'image.png?resize=100;position:fixed;top:0;left:0;width:100vw;height:100vh;background:white;z-index:9999,200'\n);\n$element = $m->parsedownElement('', '', '', '', false);\nvar_dump($element['attributes']['style']);\n```\n\nObserved style attribute:\n\n```text\nwidth: 100;position:fixed;top:0;left:0;width:100vw;height:100vh;background:white;z-index:9999px;height: 200px;\n```\n\nThe appended `px` lands on the final `z-index` value, but the preceding injected declarations remain syntactically valid CSS.\n\n## Root Cause / Technical Details\n\n`system/src/Grav/Common/Page/Markdown/Excerpts.php::processMediaActions()` parses the image query string into media actions and invokes the requested public media method with `call_user_func_array([$medium, $action['method']], $args)`.\n\nFor `resize()`, `system/src/Grav/Common/Media/Traits/StaticResizeTrait.php::resize()` stores width and height directly into style attributes:\n\n```php\n$this->styleAttributes['width'] = $width . 'px';\n$this->styleAttributes['height'] = $height . 'px';\n```\n\nIt does not verify that the values are numeric, length-only, or free of CSS declaration delimiters. Later, `system/src/Grav/Common/Media/Traits/MediaObjectTrait.php::parsedownElement()` serializes keyed style attributes as raw CSS declarations:\n\n```php\n$style .= $key . ': ' . $value . ';';\n```\n\nThe sanitizer added for direct `style()` inputs is not reached for values introduced by `resize()`. As a result, `resize=100;position:fixed;...,200` breaks out of the intended `width:` value and injects additional declarations.\n\n## PoC Evidence\n\nOn both current 2.0 and 2.0.0-rc.9, the targeted regression test produced the injected style string above. Existing tests still confirm the direct `style()` and `attribute()` paths are rejected; the bypass is specific to the adjacent `resize()` styleAttributes path.\n\n## Remediation\n\nSanitize or type-normalize all values before they enter `styleAttributes`, not only values passed through `MediaObjectTrait::style()`. For `resize()`, cast or validate width and height as numeric values before appending `px`, or use a shared CSS declaration builder that rejects semicolons, colons, property names, and other declaration-breaking characters. Add regression coverage for `resize=100;position:fixed;top:0,200` and any other media action that writes to `styleAttributes` directly.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "getgrav/grav"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.0.0-rc.9"
      }, {
        "fixed" : "2.0.0"
      } ]
    } ],
    "versions" : [ "2.0.0-rc.9" ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/getgrav/grav/security/advisories/GHSA-ffmg-hfvg-jhg9"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-58657"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/getgrav/grav/commit/6582166173bb8eb5869d96aea384e0e73777c94c"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/getgrav/grav/commit/e03d29aa0d3ece16d73c1ffccfa78df8bf5f28b8"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/getgrav/grav"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/grav-stored-css-injection-via-markdown-image-resize-action"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-16T22:14:53Z",
    "nvd_published_at" : null
  }
}