"Apache httpd 1.3.37, and 2.0.59 and 2.2.4 with the Prefork MPM module,
allows local users to cause a denial of service by modifying the
worker_score and process_score arrays to reference an arbitrary
process ID, which is sent a SIGUSR1 signal from the master process,
"SIGUSR1 killer.""
Fixed upstream by
http://svn.apache.org/viewvc?view=rev&rev=547987
Note that you'd need to have given local users the ability to run scripts (such
as php, cgi) in order for them to kill arbitrary processes.