Grav vulnerable to Path traversal / arbitrary YAML write via user creation leading to Account Takeover / System Corruption

Grav is a file-based Web platform. Prior to 1.8.0-beta.27, 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, twofa_secret, and hashed_password. This vulnerability is fixed in 1.8.0-beta.27.