In server mode pgAdmin 4 validates a path into the user's storage directory with...

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 path a second time after the check, so a folder named like an encoded "..", which a user can create, listed any directory on the host.
• Download, delete, rename and the file-exists check appended the path to the storage directory without a separator, so in a deployment where one user's storage directory name is a prefix of another's, another user's files could be read, deleted or moved; download also checked the path appended to itself.
• Creating a folder could create one in another user's directory.

The fix makes check_access_permission() return the path it checked and every caller act on that path, decodes folder paths once, and opens, deletes and renames files relative to directories…