6. Making sense out of AlertsΒΆ

When alert happens it’s important to figure out what it means. Is it serious? Relevant? A false positive?

To find out more about the rule that fired, it’s always a good idea to look at the actual rule.

The first thing to look at in a rule is the description that follows the “msg” keyword. Lets consider an example:

msg:"ET SCAN sipscan probe";

The “ET” indicates the rule came from the Emerging Threats project. “SCAN” indicates the purpose of the rule is to match on some form of scanning. Following that a more or less detailed description is given.

Most rules contain some pointers to more information in the form of the “reference” keyword.

Consider the following example rule:

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS \
  (msg:"ET CURRENT_EVENTS Adobe 0day Shovelware"; \
  flow:established,to_server; content:"GET "; nocase; depth:4; \
  content:!"|0d 0a|Referer\:"; nocase; \
  uricontent:"/ppp/listdir.php?dir="; \
  pcre:"/\/[a-z]{2}\/[a-z]{4}01\/ppp\/listdir\.php\?dir=/U"; \
  classtype:trojan-activity; \
  reference:url,isc.sans.org/diary.html?storyid=7747; \
  reference:url,doc.emergingthreats.net/2010496; \
  reference:url,www.emergingthreats.net/cgi-bin/cvsweb.cgi/sigs/CURRENT_EVENTS/CURRENT_Adobe; \
  sid:2010496; rev:2;)

In this rule the reference keyword indicates 3 url’s to visit for more information:


Some rules contain a reference like: “reference:cve,2009-3958;” should allow you to find info about the specific CVE using your favourite search engine.

It’s not always straight forward and sometimes not all of that information is available publicly. Usually asking about it on the signature support lists helps a lot then.

For the Emerging Threats list this is: http://lists.emergingthreats.net/mailman/listinfo/emerging-sigs

For the VRT ruleset: https://lists.sourceforge.net/lists/listinfo/snort-sigs

In many cases, looking at just the alert and the packet that triggered it won’t be enough to be conclusive. When running an IDS engine like Suricata, it’s always recommended to combine it with full packet capturing. Using tools like Sguil or Snorby, the full TCP session or UDP flow can be inspected.

For example, if a rule fired that indicates your web application is attacked, looking at the full TCP session might reveal that the web application replied with 404 not found. This will usually mean the attack failed. Usually, not always.

Obviously there is a lot more to Incidence Response, but this should get you started.