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This vulnerability, identified as CVE-2026-53653, is an unauthenticated denial of service (DoS) flaw affecting the Grav CMS. It resides in the image processing functionality, specifically within the `Grav::fallbackUrl()` method.
The core of the issue lies in how Grav handles URL query parameters for image manipulation. The `fallbackUrl()` function loops through all query parameters. When a parameter name matches one of the `ImageMedium::$magic_actions` (such as forceResize, resize, or crop), it directly passes the parameter’s value to the corresponding method on the image medium object.
An attacker can exploit this by requesting an image with excessively large dimensions, for example, GET /path/to/image.png?forceResize=20000,20000. The `forceResize` action, which runs with force=true, sets the output size to the attacker’s values without any clamping or validation against the source image’s dimensions or a system-defined maximum.
The Grav image library, which uses the GD adapter, then calls `imagecreatetruecolor($w, $h)` to allocate a buffer for the new image. Critically, libgd allocates this memory outside of PHP’s emalloc, meaning the `memory_limit` directive in `php.ini` does not cap the memory usage. A single request with dimensions of 20,000×20,000 can consume over 3 GB of RAM and 21.9 seconds of CPU time on a system with PHP 8.4.21 and a 128M memory limit. Even a request for an 8,000×8,000 image requires approximately 244 MB of memory.
Since the cache key for the generated image includes the requested dimensions, varying the dimensions on each request forces the server to perform fresh, resource-intensive work every time, bypassing any caching mechanisms. A few concurrent requests are enough to exhaust server resources and take the host down. This attack requires no authentication and works on any default Grav installation that serves images.
DailyCVE Form:
Platform: Grav CMS
Version: <1.7.53, <2.0.0-rc.8
Vulnerability: Unauthenticated DoS
Severity: High (CVSS: 8.7)
Date: 2026-07-10
Prediction: Already Patched (2026-07-10)
What Undercode Say:
The vulnerability stems from a lack of input validation, allowing unauthenticated users to trigger uncontrolled resource consumption (CWE-770). This can be exploited remotely with low attack complexity and no privileges. The fix introduces a configurable dimension cap (system.images.max_dimension) to limit the maximum allowed image size. The following analytics can be used to detect exploitation attempts:
Check for suspiciously large image dimension requests in access logs
grep -E "forceResize=[0-9]{4,}|resize=[0-9]{4,}|crop=[0-9]{4,}" /var/log/nginx/access.log
Monitor for high memory usage by PHP-FPM processes
while true; do ps aux | grep php-fpm | awk '{print $6}' | sort -n; sleep 5; done
Check for a high volume of image requests with varying query parameters
cat /var/log/nginx/access.log | awk '{print $7}' | grep -E "\?.=" | sort | uniq -c | sort -nr | head -20
Exploit: (Educational Purposes!)
An attacker can trigger the vulnerability with a simple HTTP GET request to any image URL on the target Grav site:
GET /path/to/any/image.png?forceResize=30000,30000 HTTP/1.1 Host: target-grav-site.com
The `forceResize` parameter can be replaced with other actions like resize, cropResize, cropZoom, zoomCrop, or crop. Varying the dimensions (e.g., 30001,30001, 30000,30001) ensures a cache miss and forces the server to reprocess the image each time.
Protection:
The primary protection is to upgrade Grav to version 1.7.53 or 2.0.0-rc.8 or later. These versions include the fix that clamps request dimensions.
If an immediate upgrade is not possible, the following mitigations can be applied:
1. Configure system.images.max_dimension: Set a reasonable cap (e.g., 2000) in your Grav `system.yaml` file to limit the maximum allowed image dimension.
2. Implement Rate Limiting: Apply rate limiting on image processing endpoints to reduce the impact of repeated malicious requests.
3. Web Application Firewall (WAF): Deploy a WAF rule to detect and block requests with suspiciously large numeric values in image manipulation query parameters.
Impact:
Successful exploitation leads to a complete denial of service. A single request can consume several gigabytes of RAM and tens of seconds of CPU time. Multiple concurrent requests can rapidly exhaust all available server resources, causing the web server to become unresponsive and crash. This affects the availability of the entire Grav site and potentially other services running on the same server. The attack requires no authentication, making it trivial for any external actor to execute.
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Sources:
Reported By: github.com
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