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78 lines
2.8 KiB
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78 lines
2.8 KiB
ReStructuredText
Making sense out of Alerts
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When alert happens it's important to figure out what it means. Is it
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serious? Relevant? A false positive?
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To find out more about the rule that fired, it's always a good idea to
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look at the actual rule.
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The first thing to look at in a rule is the description that follows
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the "msg" keyword. Lets consider an example:
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::
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msg:"ET SCAN sipscan probe";
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The "ET" indicates the rule came from the Emerging Threats
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project. "SCAN" indicates the purpose of the rule is to match on some
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form of scanning. Following that a more or less detailed description
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is given.
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Most rules contain some pointers to more information in the form of
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the "reference" keyword.
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Consider the following example rule:
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::
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alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS \
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(msg:"ET CURRENT_EVENTS Adobe 0day Shovelware"; \
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flow:established,to_server; content:"GET "; nocase; depth:4; \
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content:!"|0d 0a|Referer\:"; nocase; \
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uricontent:"/ppp/listdir.php?dir="; \
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pcre:"/\/[a-z]{2}\/[a-z]{4}01\/ppp\/listdir\.php\?dir=/U"; \
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classtype:trojan-activity; \
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reference:url,isc.sans.org/diary.html?storyid=7747; \
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reference:url,doc.emergingthreats.net/2010496; \
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reference:url,www.emergingthreats.net/cgi-bin/cvsweb.cgi/sigs/CURRENT_EVENTS/CURRENT_Adobe; \
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sid:2010496; rev:2;)
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In this rule the reference keyword indicates 3 url's to visit for more
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information:
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::
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isc.sans.org/diary.html?storyid=7747
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doc.emergingthreats.net/2010496
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www.emergingthreats.net/cgi-bin/cvsweb.cgi/sigs/CURRENT_EVENTS/CURRENT_Adobe
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Some rules contain a reference like: "reference:cve,2009-3958;" should
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allow you to find info about the specific CVE using your favorite
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search engine.
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It's not always straight forward and sometimes not all of that
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information is available publicly. Usually asking about it on the
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signature support channel helps a lot then.
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In :doc:`../rule-management/suricata-update` more information on the rule
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sources and their documentation and support methods can be found.
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In many cases, looking at just the alert and the packet that triggered
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it won't be enough to be conclusive. When using the default Eve settings
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a lot of metadata will be added to the alert.
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For example, if a rule fired that indicates your web application is
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attacked, looking at the metadata might reveal that the web
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application replied with 404 not found. This will usually mean the
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attack failed. Usually, not always.
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Not every protocol leads to metadata generation, so when running an
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IDS engine like Suricata, it's often recommended to combine it with
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full packet capture. Using tools like Evebox, Sguil or Snorby, the
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full TCP session or UDP flow can be inspected.
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Obviously there is a lot more to Incidence Response, but this should
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get you started.
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