Without pam_xauth, when xauth is enabled and a user uses the su(1) command to assume another user's privileges, that user is no longer able to access the original user's X display because the new user does not have the key needed to access the display. pam_xauth solves the problem by forwarding the key from the user running su (the source user) to the user whose identity the source user is assuming (the target user) when the session is created, and destroying the key when the session is torn down.
pam_xauth will only forward keys if xauth can list a key connected to the $DISPLAY environment variable.
Primitive access control is provided by ~/.xauth/export in the invoking user's home directory and ~/.xauth/import in the target user's home directory.
If a user has a ~/.xauth/import file, the user will only receive cookies from users listed in the file. If there is no ~/.xauth/import file, the user will accept cookies from any other user.
If a user has a .xauth/export file, the user will only forward cookies to users listed in the file. If there is no ~/.xauth/export file, and the invoking user is not root, the user will forward cookies to any other user. If there is no ~/.xauth/export file, and the invoking user is root, the user will not forward cookies to other users.
Add the following line to /etc/pam.d/su to forward xauth keys between users when calling su:
session optional pam_xauth.so
pam_xauth will work only if it is used from a setuid application in which the getuid() call returns the id of the user running the application, and for which PAM can supply the name of the account that the user is attempting to assume. The typical application of this type is su(1). The application must call both pam_open_session() and pam_close_session() with the ruid set to the uid of the calling user and the euid set to root, and must have provided as the PAM_USER item the name of the target user.
pam_xauth calls xauth(1) as the source user to extract the key for $DISPLAY, then calls xauth as the target user to merge the key into the a temporary database and later remove the database.