GHSA-hmcx-ch82-3fv2

Source

https://github.com/advisories/GHSA-hmcx-ch82-3fv2

Import Source

https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/05/GHSA-hmcx-ch82-3fv2/GHSA-hmcx-ch82-3fv2.json

JSON Data

https://api.osv.dev/v1/vulns/GHSA-hmcx-ch82-3fv2

Aliases

CVE-2026-42608

Published

2026-05-05T21:34:58Z

Modified

2026-09-10T03:51:05Z

Severity

8.8 (High) CVSS_V4 - 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 CVSS Calculator

Summary

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

Details

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

Identify any page containing a Grav Form (e.g., /contact).
Intercept the POST request during form submission.
Modify the __form-flash-id parameter to include a traversal sequence targeting a writable directory (e.g., ../../user/config/proof_dir).
Submit the request.
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

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

system/src/Grav/Framework/Form/FormFlash.php
tests/unit/Grav/Common/Security/FormFlashSecurityTest.php — 32 test cases covering the PoC + variants.

Database specific

{
    "cwe_ids": [
        "CWE-22"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-05T21:34:58Z",
    "nvd_published_at": "2026-05-11T16:17:33Z",
    "severity": "HIGH"
}

References

Affected packages

Packagist / getgrav/grav

Package

Name: getgrav/grav
Purl: pkg:composer/getgrav/grav

Affected ranges

Type: ECOSYSTEM
Events: Introduced

0 Unknown introduced version / All previous versions are affected

Fixed

2.0.0-beta.2

Affected versions

0.*

0.8.0

0.9.0

0.9.1

0.9.2

0.9.3

0.9.4

0.9.5

0.9.6

0.9.7

0.9.8

0.9.9

0.9.10

0.9.11

0.9.12

0.9.13

0.9.14

0.9.15

0.9.16

0.9.17

0.9.18

0.9.19

0.9.20

0.9.21

0.9.22

0.9.23

0.9.24

0.9.25

0.9.26

0.9.27

0.9.28

0.9.29

0.9.30

0.9.31

0.9.32

0.9.33

0.9.34

0.9.35

0.9.36

0.9.37

0.9.38

0.9.39

0.9.40

0.9.41

0.9.42

0.9.43

0.9.44

0.9.45

1.*

1.0.0-rc.1

1.0.0-rc.2

1.0.0-rc.3

1.0.0-rc.4

1.0.0-rc.5

1.0.0-rc.6

1.0.0

1.0.1

1.0.2

1.0.3

1.0.4

1.0.5

1.0.6

1.0.7

1.0.8

1.0.9

1.0.10

1.1.0-beta.1

1.1.0-beta.2

1.1.0-beta.3

1.1.0-beta.4

1.1.0-beta.5

1.1.0-rc.1

1.1.0-rc.2

1.1.0-rc.3

1.1.0

1.1.1

1.1.2

1.1.3

1.1.4

1.1.5

1.1.6

1.1.7

1.1.8

1.1.9-rc.1

1.1.9-rc.2

1.1.9-rc.3

1.1.9

1.1.10

1.1.11

1.1.12

1.1.13

1.1.14

1.1.15

1.1.16

1.1.17

1.2.0-rc.1

1.2.0-rc.2

1.2.0-rc.3

1.2.0

1.2.1

1.2.2

1.2.3

1.2.4

1.3.0-rc.1

1.3.0-rc.2

1.3.0-rc.3

1.3.0-rc.4

1.3.0-rc.5

1.3.0

1.3.1

1.3.2

1.3.3

1.3.4

1.3.5

1.3.6

1.3.7

1.3.8

1.3.9

1.3.10

1.4.0-beta.1

1.4.0-beta.2

1.4.0-beta.3

1.4.0-rc.1

1.4.0-rc.2

1.4.0

1.4.1

1.4.2

1.4.3

1.4.4

1.4.5

1.4.6

1.4.7

1.4.8

1.5.0-beta.1

1.5.0-beta.2

1.5.0-rc.1

1.5.0

1.5.1

1.5.2

1.5.3

1.5.4

1.5.5

1.5.6

1.5.7

1.5.8

1.5.9

1.5.10

1.6.0-beta.1

1.6.0-beta.2

1.6.0-beta.3

1.6.0-beta.4

1.6.0-beta.5

1.6.0-beta.6

1.6.0-beta.7

1.6.0-beta.8

1.6.0-rc.1

1.6.0-rc.2

1.6.0-rc.3

1.6.0-rc.4

1.6.0

1.6.1

1.6.2

1.6.3

1.6.4

1.6.5

1.6.6

1.6.7

1.6.8

1.6.9

1.6.10

1.6.11

1.6.12

1.6.13

1.6.14

1.6.15

1.6.16

1.6.17

1.6.18

1.6.19

1.6.20

1.6.21

1.6.22

1.6.23

1.6.24

1.6.25

1.6.26

1.6.27

1.6.28

1.6.29

1.6.30

1.6.31

1.7.0-beta.1

1.7.0-beta.2

1.7.0-beta.3

1.7.0-beta.4

1.7.0-beta.5

1.7.0-beta.6

1.7.0-beta.7

1.7.0-beta.8

1.7.0-beta.9

1.7.0-beta.10

1.7.0-rc.1

1.7.0-rc.2

1.7.0-rc.3

1.7.0-rc.4

1.7.0-rc.5

1.7.0-rc.6

1.7.0-rc.7

1.7.0-rc.8

1.7.0-rc.9

1.7.0-rc.10

1.7.0-rc.11

1.7.0-rc.12

1.7.0-rc.13

1.7.0-rc.14

1.7.0-rc.15

1.7.0-rc.16

1.7.0-rc.17

1.7.0-rc.18

1.7.0-rc.19

1.7.0-rc.20

1.7.0

1.7.1

1.7.3

1.7.4

1.7.5

1.7.6

1.7.7

1.7.8

1.7.9

1.7.10

1.7.12

1.7.13

1.7.14

1.7.15

1.7.16

1.7.17

1.7.18

1.7.19

1.7.20

1.7.21

1.7.22

1.7.23

1.7.24

1.7.25

1.7.26

1.7.26.1

1.7.27

1.7.27.1

1.7.28

1.7.29

1.7.29.1

1.7.30

1.7.31

1.7.32

1.7.33

1.7.34

1.7.35

1.7.36

1.7.37

1.7.37.1

1.7.38

1.7.39

1.7.39.1

1.7.39.2

1.7.39.3

1.7.39.4

1.7.40

1.7.41

1.7.41.1

1.7.41.2

1.7.42

1.7.42.1

1.7.42.2

1.7.42.3

1.7.43

1.7.44

1.7.45

1.7.46

1.7.47

1.7.48

1.7.49

1.7.49.1

1.7.49.2

1.7.49.3

1.7.49.4

1.7.49.5

1.7.51

1.7.52

1.7.53

1.7.53.1

1.7.53.2

1.7.53.3

1.8.0-beta.1

1.8.0-beta.2

1.8.0-beta.3

1.8.0-beta.4

1.8.0-beta.5

1.8.0-beta.6

1.8.0-beta.7

1.8.0-beta.8

1.8.0-beta.9

1.8.0-beta.10

1.8.0-beta.11

1.8.0-beta.12

1.8.0-beta.13

1.8.0-beta.14

1.8.0-beta.15

1.8.0-beta.16

1.8.0-beta.17

1.8.0-beta.18

1.8.0-beta.19

1.8.0-beta.20

1.8.0-beta.21

1.8.0-beta.22

1.8.0-beta.23

1.8.0-beta.24

1.8.0-beta.25

1.8.0-beta.26

1.8.0-beta.27

1.8.0-beta.28

1.8.0-beta.29

2.*

2.0.0-beta.1

Database specific

source

"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/05/GHSA-hmcx-ch82-3fv2/GHSA-hmcx-ch82-3fv2.json"