Custom rules and decoders

Adding new decoders and rules

Check out this example on how to create new decoders and rules. The following log corresponds to a program called example:

Dec 25 20:45:02 MyHost example[12345]: User 'admin' logged from '192.168.1.100'

1. Add a new decoder to /var/ossec/etc/decoders/local_decoder.xml to decode the log information:

   <decoder name="example">
     <program_name>^example</program_name>
   </decoder>
   
   <decoder name="example">
     <parent>example</parent>
     <regex>User '(\w+)' logged from '(\d+.\d+.\d+.\d+)'</regex>
     <order>user, srcip</order>
   </decoder>
   

2. Add the following rule to /var/ossec/etc/rules/local_rules.xml.

   <group name="custom_rules_example,">
     <rule id="100010" level="0">
       <program_name>example</program_name>
       <description>User logged</description>
     </rule>
   </group>
   

3. Run /var/ossec/bin/wazuh-logtest. Input the example log above to test the decoder and rule:

   Type one log per line
   
   Dec 25 20:45:02 MyHost example[12345]: User 'admin' logged from '192.168.1.100'
   
   **Phase 1: Completed pre-decoding.
           full event: 'Dec 25 20:45:02 MyHost example[12345]: User 'admin' logged from '192.168.1.100''
           timestamp: 'Dec 25 20:45:02'
           hostname: 'MyHost'
           program_name: 'example'
   
   **Phase 2: Completed decoding.
           name: 'example'
           dstuser: 'admin'
           srcip: '192.168.1.100'
   
   **Phase 3: Completed filtering (rules).
           id: '100010'
           level: '0'
           description: 'User logged'
           groups: '['custom_rules_example']'
           firedtimes: '1'
           mail: 'False'
   

   To test your rules and decoders using wazuh-logtest, it's enough to save the changes made to the decoder and rule files. However, you need to restart the Wazuh manager to generate alerts based on these changes.

4. Restart the Wazuh manager to load the updated rules and decoders:

   SystemdSysV init

   # systemctl restart wazuh-manager
   

Changing an existing rule

You can change the default Wazuh rules. To do so, we recommend copying the rules to a file in the /var/ossec/etc/rules/ directory, making the necessary changes, and adding the overwrite="yes" tag to the modified rules. These steps guarantee that your changes won't be lost during updates.

Here's an example on how to change the level value of the SSH rule 5710 from 5 to 10.

1. Open the rule file /var/ossec/ruleset/rules/0095-sshd_rules.xml.

2. Find and copy the rule definition for rule id 5710:

   <group name="syslog,sshd,">
     ...
     <rule id="5710" level="5">
       <if_sid>5700</if_sid>
       <match>illegal user|invalid user</match>
       <description>sshd: Attempt to login using a non-existent user</description>
       <mitre>
         <id>T1110</id>
       </mitre>
       <group>invalid_login,authentication_failed,pci_dss_10.2.4,pci_dss_10.2.5,pci_dss_10.6.1,gpg13_7.1,gdpr_IV_35.7.d,gdpr_IV_32.2,hipaa_164.312.b,nist_800_53_AU.14,nist_800_53_AC.7,nist_800_53_AU.6,tsc_CC6.1,tsc_CC6.8,tsc_CC7.2,tsc_CC7.3,</group>
     </rule>
     ...
   </group>
   

3. Paste the copied rule definition into /var/ossec/etc/rules/local_rules.xml. Modify the level value, and add overwrite="yes" to indicate that this rule overwrites an already defined rule.

   <group name="syslog,sshd,">
     <rule id="5710" level="10" overwrite="yes">
       <if_sid>5700</if_sid>
       <match>illegal user|invalid user</match>
       <description>sshd: Attempt to login using a non-existent user</description>
       <mitre>
         <id>T1110</id>
       </mitre>
       <group>invalid_login,authentication_failed,pci_dss_10.2.4,pci_dss_10.2.5,pci_dss_10.6.1,gpg13_7.1,gdpr_IV_35.7.d,gdpr_IV_32.2,hipaa_164.312.b,nist_800_53_AU.14,nist_800_53_AC.7,nist_800_53_AU.6,tsc_CC6.1,tsc_CC6.8,tsc_CC7.2,tsc_CC7.3,</group>
     </rule>
   </group>
   

   Warning

   To maintain consistency between loaded rules, currently it's not possible to overwrite the if_sid, if_group, if_level, if_matched_sid, and if_matched_group labels. These tags are ignored when they are in an overwrite rule, keeping the original values.

4. Restart the Wazuh manager to load the updated rules:

   SystemdSysV init

   # systemctl restart wazuh-manager
   

Changing an existing decoder

To change a default decoder, you can rewrite its file in the /var/ossec/etc/decoders directory, make the changes, and exclude the original decoder file from the loading list.

For example, if you want to customize decoders in the 0310-ssh_decoders.xml file, follow these steps:

1. Copy the decoder file /var/ossec/ruleset/decoders/0310-ssh_decoders.xml to the user folder /var/ossec/etc/decoders. This keeps the changes you make when updating to a newer version.

2. Edit the /var/ossec/etc/ossec.conf configuration file. Set the <decoder_exclude> tag to exclude the original ruleset/decoders/0310-ssh_decoders.xml decoder file from the loading list. With this configuration, Wazuh loads the decoder file located in the user folder and not the file in the default folder.

   <ruleset>
     <!-- Default ruleset -->
     <decoder_dir>ruleset/decoders</decoder_dir>
     <rule_dir>ruleset/rules</rule_dir>
     <rule_exclude>0215-policy_rules.xml</rule_exclude>
     <list>etc/lists/audit-keys</list>
   
     <!-- User-defined ruleset -->
     <decoder_dir>etc/decoders</decoder_dir>
     <rule_dir>etc/rules</rule_dir>
     <decoder_exclude>ruleset/decoders/0310-ssh_decoders.xml</decoder_exclude>
   </ruleset>
   

3. Make changes to /var/ossec/etc/decoders/0310-ssh_decoders.xml.

4. Restart the Wazuh manager so the changes take effect:

   SystemdSysV init

   # systemctl restart wazuh-manager
   

   Warning

   Since you're excluding the original decoder file, you don't benefit from any updates it might get. Your custom file remains unchanged during upgrades so consider applying relevant changes manually.