Grav has Unauthenticated Path Traversal & Arbitrary File Write in its FormFlash component
# Vulnerability Report: Grav CMS Unauthenticated Path Traversal & Arbitrary File Write
[ZERO-DAY] Unauthenticated Path Traversal leading to Arbitrary Directory Creation and Configuration Injection
Summary
Grav CMS (v1.7.49.5 and latest development source) is vulnerable to a Zero-Day Path Traversal vulnerability within the FormFlash core component. By manipulating the session_id (passed as __form-flash-id in POST requests), an unauthenticated attacker can traverse the filesystem to create arbitrary directories and write an index.yaml file containing attacker-controlled data.
This vulnerability can lead to unauthorized modification of application behavior, potential data integrity issues, and service disruption in production environments.
Affected Component
- Versions: Confirmed in Grav v1.7.49.5 (latest stable) and the latest development source (March 2026).
- Class:
Grav\Framework\Form\FormFlash - Method:
__construct() / getTmpDir() - Parameter:
session_id (Mapped to __form-flash-id in POST requests)
Vulnerability Details
The FormFlash class is used to persist form data across redirects. It constructs a temporary storage path using the provided session_id. The path construction logic in the latest source:
```php
$folder = $config['folder'] ?? ($this->sessionId ? 'tmp://forms/' . $this->sessionId : '');
$this->folder = $folder && $locator->isStream($folder) ? $locator->findResource($folder, true, true) : $folder;
```
Lack of sanitization on the sessionId (the raw session identifier) allows the use of ../ sequences. When findResource resolves the stream, it allows escape into any writable directory within the webserver's scope (typically user/config/, cache/, logs/, and tmp/).
Affected Versions & Zero-Day Status
- Tested Version: v1.7.49.5 (Latest Stable Release as of Nov 2025).
- Development Branch Status: Vulnerable. The latest source code in the GitHub develop branch (March 2026) remains unpatched.
- Affected Range: All Grav CMS versions utilizing the FormFlash component (v1.7.x and potentially older v1.6.x versions).
- CVE Status: Zero-Day (Non-Registered). Extensive research confirmed no existing CVE addresses this specific core FormFlash session-based traversal.
Steps to Reproduce
1. Identify any page containing a Grav Form (e.g., /contact).
2. Intercept the POST request during form submission.
3. Modify the __form-flash-id parameter to include a traversal sequence targeting a writable directory (e.g., ../../user/config/proof_dir).
4. Submit the request.
5. Observe that a new directory (poc/) and file (index.yaml) have been created at the traversed path.
Request Example
```http
POST /contact HTTP/1.1
Host: target.grav.cms
Content-Type: application/x-www-form-urlencoded
__form-name-=contact&__form-flash-id=../../user/config/proof_dir&form-data[name]=Attack&form-data[message]=Payload
```
Response / Result
- HTTP/1.1 302 Found (Standard redirect)
- Filesystem Modification:
- Directory Created:
/var/www/html/user/config/proof_dir/poc/ - File Created:
/var/www/html/user/config/proof_dir/poc/index.yaml
Proof of Concept Evidence (Before/After)
Before Exploitation
- Status: Directory does not exist.
- Evidence:
```bash
$ ls -la /var/www/html/user/config/proof_dir/
ls: cannot access '/var/www/html/user/config/proof_dir/': No such file or directory
```
After Exploitation
- Status: Arbitrary directory and
index.yaml created. - Evidence:
```bash
$ ls -la /var/www/html/user/config/proof_dir/poc/index.yaml
-rw-rw-r-- 1 www-data www-data 158 Mar 23 22:15 /var/www/html/user/config/proof_dir/poc/index.yaml
$ cat /var/www/html/user/config/proof_dir/poc/index.yaml
form: ''
id: ''
unique_id: poc
...
data:
poc_status: confirmed
```
Impact
- Clarified Cross-User Attack: By controlling the session identifier, an attacker can overwrite or interfere with other users temporary form data, breaking session isolation.
- Configuration Injection: Writing
index.yaml into plugin/theme configuration subdirectories can alter application behavior or inject malicious settings. - Data Integrity: Unauthorized modification of configuration subfolders can lead to widespread site corruption or logical bypasses.
- Denial of Service (DoS): Recursive directory creation enables attackers to exhaust disk space or inodes (inode exhaustion).
Attack Requirements
- Authentication: None (Unauthenticated)
- Configuration: Standard Grav installation with at least one form-enabled page (e.g., Contact, Login, Registration)
Exploitability Assessment
- Complexity: Low. Requires only basic HTTP POST parameters.
- Reliability: 100% (Deterministically reproducible in vulnerable versions).
- Severity: Critical / High. The vulnerability requires no authentication and allows filesystem manipulation and session data corruption.
Remediation
1. Sanit
