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Grav has Unauthenticated Path Traversal & Arbitrary File Write in its FormFlash component

High severity GitHub Reviewed Published Apr 27, 2026 in getgrav/grav

Package

composer getgrav/grav (Composer)

Affected versions

< 2.0.0-beta.2

Patched versions

2.0.0-beta.2

Description

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:

$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

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:
$ 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:
$ 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. Sanitize Session IDs: Apply basename() or a strict alphanumeric regex to the session_id in FormFlash before path construction.
  2. Filesystem Hardening: Ensure user/config/ and other sensitive directories have restrictive permissions preventing the webserver from creating new subdirectories.
  3. Update Grav: Monitor for patches addressing FormFlash sanitization.

Maintainer note — fix applied (2026-04-24)

Fixed in Grav core on the 2.0 branch: commit d904efc33 — will ship in 2.0.0-beta.2.

What changed: FormFlash::__construct() now sanitizes session_id, unique_id, and id through a strict [A-Za-z0-9,_-]{1,64} allowlist before any path is constructed from them. Invalid values collapse to '', which causes save()/delete()/getTmpDir() to no-op — so a __form-flash-id=../../user/config/proof_dir POST simply does nothing on disk.

Files:

References

@rhukster rhukster published to getgrav/grav Apr 27, 2026
Published to the GitHub Advisory Database May 5, 2026
Reviewed May 5, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:P

EPSS score

Weaknesses

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. Learn more on MITRE.

CVE ID

CVE-2026-42608

GHSA ID

GHSA-hmcx-ch82-3fv2

Source code

Credits

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