Summary When a user with privilege of user creation creates a new user through the Admin UI and supplies a username containing path traversal sequences (for example ..\Nijat or ../Nijat), Grav writes the account YAML file to an unintended path outside user/accounts/. The written YAML can contain account fields such as email, fullname, twofasecret, and hashedpassword. In my tests, I was able to cause the Admin UI to write the following content into arbitrary .yaml files (including files like email.yaml, system.yaml, or other site YAML files like admin.yaml) — demonstrating arbitrary YAML write / overwrite via the Admin UI.

Example observed content written by the Admin UI (test data): username: ..\Nijat state: enabled email: EMAIL@gmail.com fullname: 'Nijat Alizada' language: en contenteditor: default twofaenabled: false twofasecret: RWVEIHC2AFVD6FCR6UHCO3DS4HWXKKDT avatar: { } hashedpassword: $2y$10$wl9Ktv3vUmDKCt8o6u2oOuRZr1I04OE0YZf2sJ1QcAherbNnk1XVC access: site: login: true

Steps to Reproduce 1. Log in to the Grav Admin UI as an administrator. 2. Create a new user with the following values (example): a. Username: ..\POC-TOKEN-2025-09-29 b. Fullname: POC-TOKEN-2025-09-29 c. Email: poc+2025-09-29@example.test d. Password: (any password) Observe that a YAML file containing the POC-TOKEN is written outside user/accounts/ (for example in the parent directory of user/accounts)

Impact 1. Config corruption / service disruption: Overwriting system.yaml, email.yaml, or plugin config files with attacker-controlled YAML (even if limited to fields present in account YAML) could break functionality, disable services, or cause misconfiguration requiring recovery from backups. 2. Account takeover, any user with create user privilege can modify other user's email and password by just creating a new user with the name "..\accounts\USERNAMEOFVICTIM"

Proof of Concept https://github.com/user-attachments/assets/cf503d74-f765-4031-8e22-71f6b3630847

Summary A privilege escalation vulnerability exists in Grav’s Admin plugin due to the absence of username uniqueness validation when creating users. A user with the create user permission can create a new account using the same username as an existing administrator account, set a new password/email, and then log in as that administrator. This effectively allows privilege escalation from limited user-manager permissions to full administrator access.

Steps to Reproduce 1. Make sure you have two accounts: an admin and a user with create user privilege 2. In the user account, navigate to /grav-admin/admin/accounts/users and click "Add" 3. Enter the name of the admin, complete registration and observe that the existing admin’s email is changed to the value you provided. 4. Log out from user account log in as admin with new credentials

Impact 1. Full admin takeover by any user with create user permission. 2. Ability to change admin credentials, install/remove plugins, read or modify site data, and execute any action available to an admin. 3. Severity: High/Critical.

PoC https://github.com/user-attachments/assets/3ab0a7d6-5055-41be-9e0e-2bd6ca359b37