It was reported [1] that keepalived wrote its PID files in /var/run as world-writable files.  This could allow an unprivileged user to change the referenced PID in the PID file, and upon a service restart or stop of keepalived, would kill the other process.

The keepalived daemonize code explicitly sets umask(0) which is the root cause of this flaw, so PID file creation should probably contain an explicit permissions settings.

For instance, as root:

# service keepalived status
keepalived (pid  9634) is running...
# cat /var/run/keepalived.pid 
9634
# ls -al /var/run/keepalived.pid 
-rw-rw-rw- 1 root root 5 May 10 15:13 /var/run/keepalived.pid

Then as an unprivileged user:

% echo $(ps ax|grep sshd|grep -v grep|cut -d ' ' -f 2) >/var/run/keepalived.pid 
% cat /var/run/keepalived.pid 
1638

Back to root:

# service keepalived status
keepalived (pid  1638) is running...
# service keepalived stop
Stopping keepalived:                                       [  OK  ]
# service sshd status
openssh-daemon is stopped
# ps ax|grep sshd|grep -v grep
# ps ax|grep keepalived|grep -v grep
 9634 ?        Ss     0:00 /usr/sbin/keepalived -D
 9636 ?        S      0:00 /usr/sbin/keepalived -D
 9637 ?        S      0:00 /usr/sbin/keepalived -D

[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=626281