Comment 1 for bug 966734

In case it isn't clear about how this affects security, consider this case:
In a small environment without LDAP, NIS, etc, it's not unusual for the users to be created manually, and sometimes out of order, so their uids don't match between servers.

NFS Server: Has users alice (uid 1001), bob (uid 1002), craig (uid 1003)
NFS Client 1: Has users alice (uid 1001), bob (uid 1002), craig (uid 1003)
NFS Client 2: Has users alice (uid 1001), bob (uid 1003), craig (uid 1002)

Let's say each user's home directory is mounted from the server to the clients with nfs. A pretty normal situation.
On client machine #1, it would be working as expected.
On client machine #2, a sysadmin looking at the mounts would see that they were owned by the expected users.
ie. /home/alice is owned by alice, /home/bob is owned by bob, and /home/craig is owned by craig.

drwxr-x--- 2 alice users 4096 Mar 28 14:43 alice
drwxr-x--- 2 bob users 4096 Mar 30 10:43 bob
drwxr-x--- 2 craig users 4096 Mar 29 12:21 craig

However because the NFS id mapping is bugged, on client #2 with the non-matched UIDs, it will allow bob to execute something like this:
cat /home/craig/top_secret_document
echo "pwned" > /home/craig/.bashrc