6. Making sense out of Alerts

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:

isc.sans.org/diary.html?storyid=7747
doc.emergingthreats.net/2010496
www.emergingthreats.net/cgi-bin/cvsweb.cgi/sigs/CURRENT_EVENTS/CURRENT_Adobe

Some rules contain a reference like: “reference:cve,2009-3958;” should allow you to find info about the specific CVE using your favorite 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 channel helps a lot then.

In Rule Management with Suricata-Update more information on the rule sources and their documentation and support methods can be found.

In many cases, looking at just the alert and the packet that triggered it won’t be enough to be conclusive. When using the default Eve settings a lot of metadata will be added to the alert.

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

Not every protocol leads to metadata generation, so when running an IDS engine like Suricata, it’s often recommended to combine it with full packet capture. Using tools like Evebox, Sguil or Snorby, the full TCP session or UDP flow can be inspected.

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