Attack Chain

1. An unauthenticated attacker identifies a Grav page using the Form plugin with a file upload field that accepts all file types (accept: ['*']).
2. The attacker crafts a malicious .md file containing YAML frontmatter designed to create a new administrator account (e.g., viaup.yaml as described in the PoC).
3. The attacker uploads the malicious .md file, setting the filename parameter in the POST request to match the target page’s content file name (e.g., form.md).
4. The Form::uploadFiles() function processes the upload, using the attacker-controlled filename to store the file in flash storage.
5. Upon form submission, Form::copyFiles() moves the uploaded file to its final destination, overwriting the original .md file of the target page.
6. The attacker accesses the target page via a GET request, causing Grav to parse the newly overwritten .md file and its injected YAML frontmatter.
7. The injected YAML frontmatter creates a new super-admin user.
8. The attacker uses the credentials of the newly created super-admin user to log in and gain administrative control of the Grav CMS instance.

Impact

Successful exploitation of this vulnerability allows unauthenticated attackers to overwrite existing page content, inject malicious code, and ultimately escalate their privileges to super-admin. This grants them complete control over the Grav CMS instance, potentially leading to data theft, website defacement, or further malicious activities. This vulnerability impacts any Grav page allowing file uploads with insufficiently restricted file types and can result in complete compromise of the Grav CMS.

Recommendation

• Apply the remediation patch described in the advisory by upgrading to Grav Form plugin version 9.1.0 or later to address CVE-2026-42845.
• Implement the provided code snippet within user/plugins/form/classes/Form.php to block uploads of sensitive page content file types (.md, .yaml, .yml, .json, .twig) to prevent page content overwrites.
• Add md, yaml, yml, json, twig, ini to the security.uploads_dangerous_extensions list in Grav’s configuration to prevent these file types from being processed.
• Deploy the Sigma rule “Detect Grav Form Plugin Page Content Overwrite Attempt” to identify potential exploitation attempts by monitoring for uploads of markdown or YAML files to the pages directory.
• Monitor web server logs for POST requests to form submission endpoints that contain the filename parameter with values matching page content filenames (e.g., form.md, default.md).