When mounting a remote filesystem using NFS, the kernel did not sanitize remotely provided filenames for the path separator character, "/". This allows readdir(3) and related functions to return filesystem entries with names containing additional path components.
The lack of validation described above gives rise to a confused deputy problem. For example, a program copying files from an NFS mount could be tricked into copying from outside the intended source directory, and/or to a location outside the intended destination directory.