Grav: Unauthenticated denial of service via unbounded image derivative dimensions

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:

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.

--- 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);
                     }
                 }