CVE-2026-42609

Description

### 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 3. Log out and log back in as adminuser. 4. Navigate to User Accounts -> Add. 5. Fill in the form with the following details: Username: root0 (The exact username of the Super User) Email: `[email protected]` Fullname: Fake Root0 7. Click Save. 8. Observe that the account is successfully "created". 9. 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`](https://github.com/getgrav/grav/commit/d904efc33) — will ship in **2.0.0-beta.2**. **What changed:** `UserObject::save` already had a uniqueness guard (commit [`19c2f8da7`](https://github.com/getgrav/grav/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`](https://github.com/getgrav/grav/blob/2.0/system/src/Grav/Common/Flex/Types/Users/UserObject.php). - [`tests/unit/Grav/Common/Security/UserOverwriteSecurityTest.php`](https://github.com/getgrav/grav/blob/2.0/tests/unit/Grav/Common/Security/UserOverwriteSecurityTest.php) — 3 tests pinning the PoC, the `@@`-prefix edge case, and pass-through for free usernames.