Link race in pgAdmin 4 lets file access escape storage directory

In server mode pgAdmin 4 validates a path a user gives for a file in their storage directory with check_access_permission() and then opened it by name. Several places opened it with a plain open() or handed the path to a utility that opened it later: the servers.json export and import and the import file listing, and the files given to pg_dump, pg_dumpall, pg_restore and psql by Backup, Restore and Import/Export Data. A symbolic link swapped in at the file or at any parent directory between the check and the open redirected the operation outside the storage directory, so a file could be written, overwritten or read, or loaded into a table, as the operating-system account running pgAdmin. A hard link to an outside file, which O_NOFOLLOW does not detect, and a FIFO, which could hang a request, had the same effect. CVE-2026-86861 fixed this for the File Manager's save_file; these were the remaining paths. Exploitation requires creating such a link or FIFO inside the user's storage directory. pgAdmin offers no way to do so, and the per-user storage directories belong to the pgAdmin account, so this requires write access to the storage backend as a different operating-system user, for example a shared or network-mounted storage directory, and winning the race. The fix opens each file by walking down from the storage directory one component at a time with O_NOFOLLOW, accepting only a regular file with one link, and passes the utilities the open descriptor (/dev/fd/N) on Linux. Error messages no longer include the server path. This issue affects pgAdmin 4: before 9.19.