Before correction, the copy_file_range system call checked only for the CAP_READ and CAP_WRITE capabilities on the input and output file descriptors, respectively. Using an offset is logically equivalent to seeking, and the system call must additionally require the CAP_SEEK capability. This incorrect privilege check enabled sandboxed processes with only read or write but no seek capability on a file descriptor to read data from or write data to an arbitrary location within the file corresponding to that file descriptor.
The product attempts to drop privileges but does not check or incorrectly checks to see if the drop succeeded.