Anyhow Alex Kirk responded with a more efficient rule and Richard thought i might like to post it here for the benefit of all of those who read this blog and who want to protect their databases from this issue. All credit goes to Alex Kirk. Here is the rule:
alert tcp $EXTERNAL_NET any -> $SQL_SERVERS $ORACLE_PORTS (msg:"ORACLE
AUTH_ALTER_SESSION exploit attempt"; flow:to_server,established;
content:"AUTH_ALTER_SESSION"; nocase;
pcre:"/AUTH_ALTER_SESSION(?!.{1,10}ALTER\s+SESSION\s+SET)/smi";
reference:http://www.imperva.com/application_defense_center/papers/oracle-dbms-01172006.html;
classtype:attempted-admin; )
tag:session,5,packets; )