CVE-2026-42845

Grav Form Plugin has an Anonymous Page Content Overwrite via Form File Upload filename Override

Grav pages that allows user to uploads any file (besides the ones in the blocklist) with the default self@ configuration is able to upload a malicious markdown file to overwrite the existing markdown file. In this case, unauthenticated users were able to escalate their privileges to super-admin.

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

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Block sensitive page-content filenames at upload

In user/plugins/form/classes/Form.php, after Utils::checkFilename() succeeds, add a content-area-aware check:

// Block files that would overwrite Grav page content if uploaded into
// a page directory. Page templates are .md (Markdown) and .yaml/.yml
// (frontmatter overrides). Block both for safety.
$ext = strtolower(pathinfo($filename, PATHINFO_EXTENSION));
$pageContentExtensions = ['md', 'yaml', 'yml', 'json', 'twig'];
if (in_array($ext, $pageContentExtensions, true)) {
    return [
        'status'  => 'error',
        'message' => 'File type not allowed for upload (page content files are blocked)',
    ];
}

Add md, yaml, yml, json, twig, ini to the global security.uploads_dangerous_extensions list, these all carry executable semantics in Grav's runtime even though they are not "PHP".

1. What is CVE-2026-42845? CVE-2026-42845 is a high-severity improper input validation vulnerability in getgrav/grav-plugin-form (composer), affecting versions < 9.1.0. It is fixed in 9.1.0. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. Which versions of getgrav/grav-plugin-form are affected by CVE-2026-42845? getgrav/grav-plugin-form (composer) versions < 9.1.0 is affected.
3. Is there a fix for CVE-2026-42845? Yes. CVE-2026-42845 is fixed in 9.1.0. Upgrade to this version or later.
4. Is CVE-2026-42845 exploitable, and should I be worried? Whether CVE-2026-42845 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-42845 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-42845? Upgrade getgrav/grav-plugin-form to 9.1.0 or later.