GHSA-4x9g-vw65-vvf9

### Summary An unauthenticated visitor exhausts server memory and CPU by requesting an image with oversized resize dimensions. One request drives a worker to several gigabytes of RAM and tens of seconds of CPU. A few concurrent requests take the host down.

### Details `Grav::fallbackUrl()` (system/src/Grav/Common/Grav.php:800-804) loops over every query parameter and, when the name matches `ImageMedium::$magic_actions`, calls that method on the medium with the comma-split value as arguments:

```php foreach ($uri->query(null, true) as $action => $params) { if (in_array($action, ImageMedium::$magic_actions, true)) { call_user_func_array([&$medium, $action], explode(',', $params)); } } ```

`forceResize` runs with `force=true`, so it sets the output size to the attacker's values with no clamp against the source or any ceiling. The `getgrav/image` GD adapter then calls `imagecreatetruecolor($w, $h)`. libgd allocates that buffer outside PHP's `emalloc`, so `memory_limit` does not cap it. Grav exposes no `system.images.max_width`/`max_height` setting.

### PoC Any page that serves an image works. With a 200x150 source image:

``` GET /home/test.png?forceResize=20000,20000 ```

Measured on PHP 8.4.21 with `memory_limit=128M`:

- peak worker RSS 3,109 MB - 21.9 s CPU - HTTP 200, 1.6 MB response

`8000x8000` already needs ~244 MB. The cache key includes the dimensions, so varying them forces fresh work on every request.

### Impact Unauthenticated denial of service against any Grav site that serves images. No account, plugin, or non-default config required.

## Fix Clamp the request-derived dimensions before dispatch, behind a configurable cap. The image library is the wrong layer; bound the arguments at the request boundary.

```diff --- a/system/src/Grav/Common/Grav.php +++ b/system/src/Grav/Common/Grav.php @@ public function fallbackUrl($path) foreach ($uri->query(null, true) as $action => $params) { if (in_array($action, ImageMedium::$magic_actions, true)) { - call_user_func_array([&$medium, $action], explode(',', $params)); + $args = explode(',', $params); + $max = (int) $config->get('system.images.max_dimension', 8000); + if ($max > 0 + && in_array($action, ['resize', 'forceResize', 'cropResize', 'cropZoom', 'zoomCrop', 'crop'], true)) { + foreach ($args as $a) { + if (is_numeric($a) && (int) $a > $max) { + return false; // reject oversized derivative request + } + } + } + call_user_func_array([&$medium, $action], $args); } } ```

Document `system.images.max_dimension` (default 8000) so operators can tune it. A total-pixel ceiling (`width * height`) is a stricter alternative.