Grav: Blueprint dynamic-data bare-function branch is denylist-gated and omits error_log, giving arbitrary file write

## Affected versions and vulnerable location - Confirmed on grav core at `78ebfc1` (tag 2.0.13). - Sinks: - `system/src/Grav/Common/Data/Blueprint.php:455-458` `call_user_func_array($o, $params)` (bare-function dynamic-data provider). - Twin: `system/src/Grav/Framework/Flex/FlexDirectory.php:936-938` `call_user_func_array($function, $params)`. - Validation gate: `Blueprint::isSafeDynamicCall()` at `Blueprint.php:514-536`. - `Class::method` branch (`:514-527`) uses a strict positive allowlist `self::$allowedDynamicCallables`. - Bare-function branch (`:530-534`) uses only a denylist: `if (is_string($function) && Utils::isDangerousFunction($function)) return false; return !self::paramsContainDangerousCallable($params);`. - Denylist: `Utils::isDangerousFunction()` (`system/src/Grav/Common/Utils.php`, list around `:2020-2270`). ## Root cause GHSA-7pgq/CVE-2026-64850 hardened the `Class::method` half of the dynamic-callable validation to a positive allowlist because a page-edit account could otherwise name any static method as a provider and reach file/secret gadgets. The bare-function half was left on a denylist (`isDangerousFunction`). Any bare PHP function not on that list executes. `error_log` is not on the denylist (verified: no occurrence in `Utils.php`). `error_log($message, 3, $destination)` appends attacker-controlled `$message` to attacker-controlled file `$destination`, an arbitrary-file-append primitive. `paramsContainDangerousCallable()` (`:587-603`) only scans params for dangerous callable strings, so a PHP payload string and a destination path both pass. (`stream_socket_client`, `dl`, and `mb_send_mail` are likewise absent, giving SSRF/other primitives.) ## Attacker model The same surface the published dynamic-data advisories accept as reachable: a `data-*@` directive in a form blueprint the Form plugin assembles from page frontmatter (GHSA-fj2p), or a `data@` field in a Flex directory/pages/users blueprint (GHSA-c4wf). A page-edit / blueprint-config account, no shell. ## Reachability trace 1. Author a blueprint field with a bare-function data directive, e.g. `data-options@: ['error_log', '<?php system($_GET[0]); ?>', 3, 'user/data/x.php']`. 2. `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. 3. `call_user_func_array('error_log', ['<?php ...', 3, 'user/data/x.php'])` (`:455`) appends the PHP payload to `user/data/x.php`. 4. 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. ## Reproduction Executed 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: ```text [1] isSafeDynamicCall('error_log', [payload,3,dest]) => true # guard ACCEPTS error_log (bug) [2] isSafeDynamicCall('system', ['id']) => false # control isSafeDynamicCall('exec', ['id']) => false # control [3] call_user_func_array('error_log', ['<?php echo "PWNED"; ?>'.EOL, 3, '/tmp/grav_rce_proof.php']) file written: /tmp/grav_rce_proof.php (23 bytes) = <?php echo "PWNED"; ?> [4] php /tmp/grav_rce_proof.php => PWNED # arbitrary PHP executed (RCE) ``` The 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: ```bash rg -n "error_log|stream_socket_client|mb_send_mail" system/src/Grav/Common/Utils.php # no hits rg -n "isDangerousFunction|allowedDynamicCallables|call_user_func_array" system/src/Grav/Common/Data/Blueprint.php ``` `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. ## Suggested fix Convert 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. ## Severity and CVSS reasoning Suggested severity: High (same class and reach as GHSA-fj2p / CVE-2026-64850). Suggested 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. - `PR:L`: a blueprint/page-edit account, not super. - `C:H/I:H/A:H`: arbitrary file write leading to code execution. ## How I found it and a note on tooling I 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.