CVE-2026-42612: Getgrav/grav Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-9695-8fr9-hw5q",
  "modified" : "2026-05-13T13:52:24Z",
  "published" : "2026-05-05T21:27:15Z",
  "aliases" : [ "CVE-2026-42612" ],
  "summary" : "Grav Vulnerable to Publisher-Level Stored XSS via Unquoted Event Attributes",
  "details" : "### Summary\nA stored Cross-Site Scripting (XSS) vulnerability in `getgrav/grav` allows publisher-level accounts to execute arbitrary JavaScript. The issue arises from a blacklist bypass in the `detectXss()` function when handling unquoted HTML event attributes.\n\n### Details\nThe `detectXss()` function relies on a blacklist pattern to filter malicious attributes. The specific regex pattern used to match `on*` events is flawed:\n```php\n'on_events' => '#(<[^>]+[a-z\\x00-\\x20\\\"\\'\\/])(on[a-z]+|xmlns)\\s*=[\\s|\\'\\\"].*[\\s|\\'\\\"]>#iUu'\n```\nThis pattern fails to properly identify `on*` event handlers that are constructed without quotation marks. This allows an attacker to completely bypass the filter. *Note: It is highly recommended to replace this blacklist approach with a robust, established HTML sanitization library.*\n\n### PoC\nAn attacker with publisher-level access can reproduce this by injecting the following payload into any vulnerable content field:\n```html\n<img src=x onerror=eval(atob(/YWxlcnQoZG9jdW1lbnQuY29va2llKQ/.source))>\n```\n<img width=\"1889\" height=\"482\" alt=\"image1\" src=\"https://github.com/user-attachments/assets/0f1a339b-25a8-4b6e-91af-8c59e6a39297\" />\n<img width=\"3055\" height=\"920\" alt=\"image2\" src=\"https://github.com/user-attachments/assets/12680058-bbb3-4446-b58e-515533bb4e90\" />\n<img width=\"2909\" height=\"1339\" alt=\"image3\" src=\"https://github.com/user-attachments/assets/c7ed7e61-8dcf-402d-8589-98d18978c71a\" />\n\n\n**Execution Details:**\nThe `onerror` event is written without quotes to bypass the regex. Because unquoted attributes are restricted in their character usage (e.g., the `=` symbol cannot be used easily), the payload leverages `atob()` and regex `.source` to decode the base64 string `YWxlcnQoZG9jdW1lbnQuY29va2llKQ` (which translates to `alert(document.cookie)`). The `atob()` function conveniently auto-completes the necessary `=` padding for the base64 string.\n\n### Impact\n- **Vulnerability Type:** Stored Cross-Site Scripting (XSS)\n- **Impacted Parties:** Any user (including administrators) who views the compromised content published by the attacker.\n- **Consequences:** Attackers can execute malicious scripts in a victim's browser, leading to session hijacking (cookie theft), unauthorized actions.\n\n\n---\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:** the `on_events` regex in `Security::detectXss()` no longer requires quotes or whitespace around `=`. The previous form:\n\n```\n'on_events' => '#(<[^>]+[\\s\\x00-\\x20\\\"\\'\\/])(on\\s*[a-z]+|xmlns)\\s*=[\\s|\\'\\\"].*[\\s|\\'\\\"]>#iUu'\n```\n\nrequired `[\\s|'\"]` immediately after the `=`, so `<img src=x onerror=alert(1)>` slid past. The new regex drops the value-matching tail entirely and just flags the presence of an `on*=` attribute anywhere inside a tag:\n\n```\n'on_events' => '#<[^>]*?[\\s\\x00-\\x20\\\"\\'\\/](on\\s*[a-z]+|xmlns)\\s*=#iu'\n```\n\nDetecting the attribute name + `=` is enough for a tripwire — the trade-off is occasional false positives on legitimate attribute *values* containing `on*=` substrings, which the maintainer can hand-approve.\n\nThis same regex bypass was the detection-layer half of GHSA-c2q3-p4jr-c55f and GHSA-w8cg-7jcj-4vv2; the fix here knocks both down.\n\n**Files:**\n- [`system/src/Grav/Common/Security.php`](https://github.com/getgrav/grav/blob/2.0/system/src/Grav/Common/Security.php).\n- [`tests/unit/Grav/Common/Security/DetectXssTest.php`](https://github.com/getgrav/grav/blob/2.0/tests/unit/Grav/Common/Security/DetectXssTest.php) — 18 cases: unquoted PoCs, quoted-form regression, safe-content negatives.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:L/A: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-9695-8fr9-hw5q"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-42612"
  }, {
    "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" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-05T21:27:15Z",
    "nvd_published_at" : "2026-05-11T16:17:34Z"
  }
}