Grav CMS CVE-2026-42609

Summary

A business logic vulnerability in the Grav Admin Panel allows a low-privileged user (with only user creation permissions) to overwrite existing accounts, including the primary administrator. By creating a new user with a username that already exists, the system updates the existing account's metadata and permissions instead of rejecting the request. This leads to a Denial of Service (DoS) on administrative functions and Privilege De-escalation of the root account.

Details

The vulnerability stems from an insecure "Create or Update" logic within the user management module. When the admin-addon handles a user creation request, it does not strictly validate whether the username is already taken by a higher-privileged account. Instead of returning a "409 Conflict" or a validation error, the application logic proceeds to overwrite the existing user configuration file (e.g., user/accounts/root0.yaml) with the new, lower-privileged data provided by the attacker. Because the attacker cannot assign higher permissions to themselves (due to existing fixes), the result is that the targeted account (the original Admin/Root) has its access levels wiped or replaced by the attacker's input, effectively locking the real administrator out of the system.

PoC

1. Log in as a Super User (e.g., root0) and create a low-privileged user (e.g., adminuser).
2. Assign adminuser the following specific permissions:

admin.login admin.users.list admin.users.read admin.users.create

1. Log out and log back in as adminuser.
2. Navigate to User Accounts -> Add.
3. Fill in the form with the following details:

Username: root0 (The exact username of the Super User) Email: anything@grav.f Fullname: Fake Root0

1. Click Save.
2. Observe that the account is successfully "created".
3. The original administrative permissions are gone, and the account is now restricted.

PoC video

https://github.com/user-attachments/assets/047cb44e-0279-402b-b4fb-12bf5d427a5e

Impact

This is a Privilege De-escalation and Account Disruption vulnerability. Who is impacted: Any Grav installation where a non-admin user is granted permission to create other users. Consequence: An attacker can effectively disable all administrative accounts on the platform, leading to a complete loss of management control over the CMS.

---

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: UserObject::save already had a uniqueness guard (commit 19c2f8da7, November 2025) that blocks the PoC. This release tightens that guard:

1. strpos($key, '@@') → str_contains($key, '@@'). The previous form was falsy when the transient-key marker was at position 0 (e.g. @@hash), silently bypassing the check. str_contains returns a proper boolean.
2. The instanceof FileStorage gate was dropped so the uniqueness check runs for any FlexStorageInterface backend - not just the default file-per-user YAML one.

A low-privileged user with admin.users.create can no longer disrupt a super-admin account by submitting that admin's username through the "add user" form.

Files:

• system/src/Grav/Common/Flex/Types/Users/UserObject.php.
• tests/unit/Grav/Common/Security/UserOverwriteSecurityTest.php - 3 tests pinning the PoC, the @@-prefix edge case, and pass-through for free usernames.