rsync contains an off by one stack overflow flaw. This flaw can result in a NULL byte written past the end of the stack variable. More information is located here: http://c-skills.blogspot.com/2007/08/cve-2007-4091.html The patch is here: http://www.suse.de/%7Ekrahmer/rsync-2.6.9-fname-obo.diff
This flaw does not affect Red Hat Enterprise Linux 2.1, 3, or 4 due to the version of rsync. This flaw does exist in Red Hat Enterprise Linux 5, but due to the nature of the flaw, the use of -fstack-protector when building the binary renders this flaw unexploitable.
It should be noted that when running rsync in daemon mode, the daemon spawn a worker process to handle the connection. This flaw would not be a denial of service.