OSSEC - Custom rules example

August 08, 2016

Some 'rules' about rules

  • When parsing log, OSSEC will look at level 0 first, and then highest level -> lowest level
  • OSSEC will not produce alert for rules with level 0
  • It is best to put custom rules in local_rules.xml or other file to avoid being overwritten during upgrade
  • ossec-logtest is a very useful tool to test your rules & decoder

Example

Silencing certain rules

<rule id="100030" level="0">
  <if_sid>503,502</if_sid>
  <description>List of rules to be ignored.</description>
</rule>

OSSEC will not produce any alert when rule 502 and 503 is triggered


Ignore alert if rules triggered by certain IP

<rule id="100225" level="0">
  <if_sid>40101</if_sid>
  <srcip>127.0.0.1</srcip>
  <description>Ignore this</description>
</rule>

If rule 40101 triggered by 127.0.0.1, dont produce any alert


Ignore alert if contains certain strings

<rule id="100223" level="0">
  <if_sid>1002</if_sid>
  <match>terrorist|terror|femmefatale|heart-attack</match>
  <description>Ignore 1002 false positive</description>
</rule>

OSSEC is using OS_match/sregex syntax in <match>


Ignore alert if contains certain strings (using regex)

<rule id="100207" level="4">
  <if_sid>1002,1003</if_sid>
  <regex>^WordPress database error You have an error in your SQL syntax(\.*)functionName$</regex>
  <description>Unescaped SQL query, known issue</description>
</rule>

OSSEC is using OS_regex/regex syntax in <regex>


Trigger custom rule when certain field match certain value in cdb list

<rule id="100215" level="5">
  <if_sid>31101</if_sid>
  <list lookup="match_key" field="url">rules/badurl</list>
  <description>URL is in badurl</description>
</rule>

Trigger custom rule when certain rules is fired x time within n second from same srcip

<rule id="100216" level="10" frequency="4" timeframe="90">
  <if_matched_sid>100215</if_matched_sid>
  <same_source_ip />
  <description>Multiple badurl access </description>
  <description>from same source ip.</description>
  <group>web_scan,recon,</group>
</rule>

Overriding rules

<rule id="1003" level="13" overwrite="yes" maxsize="2000">
  <description>Non standard syslog message (size too large).</description>
</rule>

Original rule 1003 have 10245 as its maxsize. Using overwrite="yes" will make OSSEC overwrite certain field in original rule


Custom rule group

<group name="app_error">
  <rule id="100207" level="4">
    <if_sid>1002,1003</if_sid>
    <regex>^WordPress database error You have an error in your SQL syntax(\.*)functionName$</regex>
    <description>Unescaped SQL query, known issue</description>
  </rule>

  <rule id="100218" level="0">
    <if_sid>1003</if_sid>
    <match>WUID | WTB</match>
    <description>ignorance is bliss</description>
  </rule>
</group>