In server mode pgAdmin 4 validates a path into the user's storage directory with check_access_permission(), which builds the absolute path with get_abs_path(). Several callers then acted on a path they built differently, so the path checked and the path used could name different files: Opening a file in the Query Tool, the Google client secret and, on Windows, Import/Export Data, Backup, Restore and saving a file joined the path to the storage directory after stripping leading slashes and backslashes. On POSIX a path checked as a name inside storage could become an absolute path once stripped, so any file the pgAdmin account can read could be opened; on Windows a path naming a drive discarded the storage directory, so these could read, or write, outside it. Listing a folder decoded the…