Rules Syntax

In this section, xml labels used to configure rules are listed.

Available options

rule

<rule> is the label that starts the block that defines a rule. In this section the different options to this label are explained.

level

Definition

Specifies the level of the rule. Alerts and responses use this value.

Allowed values

0 to 16

id

Definition

Specifies the ID of the rule.

Allowed values

Any number from 1 to 999999

maxsize

Definition

Specifies the maximum size of the event.

Allowed values

Any number from 1 to 9999

frequency

Definition

Number of times the rule must have matched before firing.

Allowed values

Any number from 2 to 9999

timeframe

Definition

The timeframe in seconds. This option is intended to be used with the frequency option.

Allowed values

Any number from 1 to 99999

ignore

Definition

The time (in seconds) to ignore this rule after firing it (to avoid floods).

Allowed values

Any number from 1 to 999999

overwrite

Definition

Used to supersede an OSSEC rule with local changes.

Allowed values

yes, no

noalert

Definition

Not trigger any alert if the rule matches.

Allowed values

Attribute with no value

Example:

<!--- Rule definition -->
<rule id="100001" maxsize="300" level="3">
  ...
</rule>

In this example, the rule is assigned with the ID 100001, a maximum size of each event of 300 characters and the rule level in 3.

match

Any string to match against the log event.

Default Value

n/a

Allowed values

Any sregex expression

Example:

<rule id="100001" maxsize="300" level="3">
  <if_sid>100020</if_sid>
  <match>Queue flood!</match>
  <description> Flooded events queue.</description>
</rule>

If the rule matches the id 100200 that contains the Queue flood! phrase in it, rule activates and sends an event.

regex

Any regex to match against the log event.

Default Value

n/a

Allowed values

Any regex expression

Example:

regex is used to find a variety of strings in a rule. For example, if we want to match any valid IP:

<rule id="100001" level="3">
  <if_sid>10050</if_sid>
  <regex>^(\d+.\d+.\d+.\d+)$</regex>
  <description>Matches any valid IP</description>
</rule>

decoded_as

Default Value

n/a

Allowed values

Any decoder name

category

Selects in which rule decoding category the rule should be included: ids, syslog, firewall, web-log, squid or windows.

Default Value

n/a

Allowed values

Any category

field

Any OS_Regex to be compared to a field extracted by the decoder.

name

Specifies the name of the field extracted by the decoder.

srcip

Any IP address or CIDR block to be compared to an IP decoded as srcip. Use “!” to negate it.

Default Value

n/a

Allowed values

Any srcip

dstip

Any IP address or CIDR block to be compared to an IP decoded as dstip. Use “!” to negate it.

Default Value

n/a

Allowed values

Any dstip

extra_data

Any string that is decoded into the extra_data field.

Default Value

n/a

Allowed values

Any string.

user

Any username (decoded as the username).

Default Value

n/a

Allowed values

Any sregex expression

program_name

Program name is decoded from syslog process name.

Default Value

n/a

Allowed values

Any sregex expression

hostname

Any hostname (decoded as the syslog hostname) or log file.

Default Value

n/a

Allowed values

Any sregex expression

time

Time that the event was generated.

Default Value

n/a

Allowed values

Any time range (hh:mm-hh:mm, hh:mm am-hh:mm pm, hh-hh, hh am-hh pm)

weekday

Week day that the event was generated.

Default Value

n/a

Allowed values

monday - sunday, weekdays, weekends

id

Any ID (decoded as the ID).

Default Value

n/a

Allowed values

Any sregex expression

url

Any URL (decoded as the URL).

Default Value

n/a

Allowed values

Any sregex expression

location

New in version 3.5.0.

The event extended location of the incoming event.

Default Value

n/a

Allowed values

Any sregex expression

The location identifies the origin of the input. If the event comes from an agent, its name and registered IP (as it was added) is appended to the location.

Example of a location for a log pulled from “/var/log/syslog” in an agent with name “dbserver” and registered with IP “any”:

(dbserver) any->/var/log/syslog

The following components use a static location:

Component

Location

Windows Eventchannel

EventChannel

Windows Eventlog

WinEvtLog

FIM (Syscheck)

syscheck

Rootcheck

rootcheck

Syscollector

syscollector

Vuln Detector

vulnerability-detector

Azure Logs

azure-logs

AWS S3 integration

aws-s3

Docker integration

Wazuh-Docker

Osquery integration

osquery

OpenSCAP integration

open-scap

CIS-CAT integration

wodle_cis-cat

action

Any action (decoded as the ACTION).

Default Value

n/a

Allowed values

Any String.

if_sid

Matches if the ID has matched.

Default Value

n/a

Allowed values

Any rule id

if_group

Matches if the group has matched before.

Default Value

n/a

Allowed values

Any Group

if_level

Matches if the level has matched before.

Default Value

n/a

Allowed values

Any level from 1 to 16

if_matched_sid

Matches if an alert of the defined ID has been triggered in a set number of seconds.

This option is used in conjunction with frequency and timeframe.

Default Value

n/a

Allowed values

Any rule id

Note

Rules at level 0 are discarded immediately and will not be used with the if_matched_rules. The level must be at least 1, but the <no_log> option can be added to the rule to make sure it does not get logged.

if_matched_group

Matches if an alert of the defined group has been triggered in a set number of seconds.

This option is used in conjunction with frequency and timeframe.

Default Value

n/a

Allowed values

Any Group

if_fts

Makes the decoder that processed the event to take the fts line into consideration.

Example of use

<if_fts />

same_id

Specifies that the decoded id must be the same. This option is used in conjunction with frequency and timeframe.

Example of use

<same_id />

same_source_ip

Specifies that the decoded source ip must be the same. This option is used in conjunction with frequency and timeframe.

Example of use

<same_source_ip />

same_src_port

Specifies that the decoded source port must be the same. This option is used in conjunction with frequency and timeframe.

Example of use

<same_src_port />

same_dst_port

Specifies that the decoded destination port must be the same. This option is used in conjunction with frequency and timeframe.

Example of use

<same_dst_port />

same_location

Specifies that the location must be the same. This option is used in conjunction with frequency and timeframe.

Example of use

<same_location />

same_user

Specifies that the decoded user must be the same. This option is used in conjunction with frequency and timeframe.

Example of use

<same_user />

same_field

New in version 3.9.0.

Specifies that the decoded field must be the same as the previous one. This option is used in conjunction with frequency and timeframe.

Example of use

<same_field />

As an example of this option, check this rule:

<rule id="100001" level="3">
  <if_sid>221</if_sid>
  <field name="netinfo.iface.name">ens33</field>
  <description>Testing interface alert</description>
</rule>

<rule id="100002" level="7" frequency="3" timeframe="300">
  <if_matched_sid>100001</if_matched_sid>
  <same_field>netinfo.iface.mac</same_field>
  <description>Testing options for correlating repeated fields</description>
</rule>

Note

Rule 100002 will trigger when the last three events had the same netinfo.iface.mac address.

not_same_field

New in version 3.9.0.

Specifies that the decoded field must be different than the previous one. This option is used in conjunction with frequency and timeframe.

Example of use

<not_same_field />

As an example of this option, check this rule:

<rule id="100001" level="3">
  <if_sid>221</if_sid>
  <field name="netinfo.iface.name">ens33</field>
  <description>Testing interface alert</description>
</rule>

<rule id="100002" level="7" frequency="3" timeframe="300">
  <if_matched_sid>100001</if_matched_sid>
  <not_same_field>netinfo.iface.mac</not_same_field>
  <description>Testing options for correlating repeated fields</description>
</rule>

Note

Rule 100002 will trigger when the last three events do not have the same netinfo.iface.mac address.

different_url

Specifies that the decoded url must be different. This option is used in conjunction with frequency and timeframe.

Example of use

<different_url />

different_srcgeoip

Specifies that the source geoip location must be different. This option is used in conjunction with frequency and timeframe.

Example of use

<different_srcgeoip />

Example:

As an example to this last options, check this rule:

<rule id=100005 level="0">
  <match> Could not open /home </match>
  <same_user />
  <different_srcgeoip />
  <same_dst_port />
</rule>

That rule filters when the same user tries to open file /home but returns an error, on a different ip and using same port.

description

Used to add a description to a rule so it makes more clear and readable its funcionality. This option apports more readable information for the users, so is usually added to the rules.

Default Value

n/a

Allowed values

Any string

Examples:

<rule id="100009" level="1">
  ...
  <regex>^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$</regex>
  <description> Rule to match IPs </description>
</rule>

<rule id="100015" level="2">
  ...
  <description> A timeout occured. </description>
</rule>

<rule id="100035" level="4">
  ...
  <description> File missing. Root acces unrestricted. </description>
</rule>

Since Wazuh version 3.3 it is possible to include any decoded field (static or dynamic) to the description message. You can use the following syntax: $(field_name) to add a field to the description.

Example:

<rule id="100005" level="8">
  <match>illegal user|invalid user</match>
  <description>sshd: Attempt to login using a non-existent user from IP $(attempt_ip)</description>
  <options>no_log</options>
</rule>

list

Perform a CDB lookup using an ossec list. This is a fast on disk database which will always find keys within two seeks of the file.

Default Value

n/a

Allowed values

Path to the CDB file to be used for lookup from the OSSEC directory.Must also be included in the ossec.conf file.

Attribute

Description

field

key in the CDB: srcip, srcport, dstip, dstport, extra_data, user, url, id, hostname, program_name, status, action, dynamic field.

lookup

match_key

key to search within the cdb and will match if they key is present. Default.

not_match_key

key to search and will match if it is not present in the database.

match_key_value

searched for in the cdb. It will be compared with regex from attribute check_value.

address_match_key

IP and the key to search within the cdb and will match if they key is present.

not_address_match_key

IP the key to search and will match if it IS NOT present in the database

address_match_key_value

IP to search in the cdb. It will be compared with regex from attribute check_value.

check_value

regex for matching on the value pulled out of the cdb when using types: address_match_key_value, match_key_value

info

Extra information may be added through the following attributes:

Default Value

n/a

Allowed values

Any string

Attribute

Allowed values

Description

type

text

This is the default when no type is selected. Additional,information about the alert/event.

link

Link to more information about the alert/event.

cve

The CVE Number related to this alert/event.

ovsdb

The osvdb id related to this alert/event.

options

Additional rule options.

Attribute

Description

alert_by_email

Always alert by email.

no_email_alert

Never alert by email.

no_log

Do not log this alert.

no_full_log

Do not include the full_log field in the alert.

no_counter

Omit field rule.firedtimes in the JSON alert.

Example:

<rule id="9800" level="8">
  <match>illegal user|invalid user</match>
  <description>sshd: Attempt to login using a non-existent user</description>
  <options>no_log</options>
</rule>

Note

Use one <options> tag for each option you want to add.

check_diff

Used to determine when the output of a command changes.

Example of use

<check_diff />

group

Add additional groups to the alert. Groups are optional tags added to alerts.

They can be used by other rules by using if_group or if_matched_group, or by alert parsing tools to categorize alerts.

Groups are variables that define a behaviour. When an alert includes that group label, this behaviour will occur.

Example:

<rule id="3801" level="4">
  <description>Group for rules related with spam.</description>
  <group>spam,</group>
</rule>

Now, every rule with the line <group>spam,</group> will be included in that group.

It’s a very useful label to keep the rules ordered.

Default Value

n/a

Allowed values

Any String

status

Declares the actual status of a rule.

Default Value

n/a

Allowed values

started, aborted, succedeed, failed, lost…

var

Defines a variable that may be used in any place of the same file.

Attribute

Value

name

Name for the variable.

Example:

 <var name="joe_folder">/home/joe/</var>

  <group name="local,">

    <rule id="100001" level="5">
      <if_sid>550</if_sid>
      <field name="file">^$joe_folder</field>
      <description>A Joe's file was modified.</description>
      <group>ossec,pci_dss_10.6.1,gpg13_10.1,gdpr_IV_35.7.d,</group>
    </rule>

</group>

BAD_WORDS

<var name=”BAD_WORDS”>error|warning|failure</var>

BAD_WORDS is a very used use case of <var> option.

Is used to include many words in the same variable. Later, this variable can be matched into the decoders to check if any of those words are in a caught event.

Example:

<var name="BAD_WORDS">error|warning|failure</var>

<group name="syslog,errors,">
  <rule id="XXXX" level="2">
    <match>$BAD_WORDS</match>
    <description>Error found.</description>
  </rule>
</group>