# using setresgid() for safely dropping utmp group membership which were needed
# for makesocketpath() to create the user directory under /var/run/tmux which is
# only writeable for processes that have setgid utmp.
--- a/tmux.c
+++ b/tmux.c
@@ -235,6 +235,7 @@
 	struct keylist	*keylist;
 	char		*s, *path, *label, *home, **var;
 	int	 	 opt, flags, quiet, keys;
+	u_int		gid;
 
 #if defined(DEBUG) && defined(__OpenBSD__)
 	malloc_options = (char *) "AFGJPX";
@@ -483,6 +484,12 @@
 			}
 		}
 	}
+	gid = getgid();
+	/* drop unnecessary privileges which were needed for makesocketpath() 
+	 * to create the user directory under /var/run/tmux which is only 
+	 * writeable for processes that have setgid utmp. */
+ 	if (setresgid(gid, gid, gid) != 0)
+ 		return (NULL);
 	if (label != NULL)
 		xfree(label);
 	if (realpath(path, socket_path) == NULL)