Creating custom FIM rules

Wazuh includes out-of-the-box rules that trigger alerts on the creation, modification, or deletion of monitored files. The image below shows alerts for file addition, modification, and deletion.

FIM alerts

You can use custom Wazuh FIM rules to monitor changes to files and directories based on specific criteria such as filename, permissions, and content. For example, you can create a custom rule to detect changes to a critical system file or configuration file. Whenever a user or process modifies the file, Wazuh triggers a specific alert indicating the change and the details of the modification.

Creating custom FIM rules can extend the out-of-the-box detection capability of the Wazuh FIM module. This makes it easier to identify and respond to security incidents such as data breaches, insider threats, and other cyberattacks that involve file manipulation or modification.

This section shows you how to use the fields decoded from FIM events in custom rules. It explains what the decoded FIM events fields represent in the Wazuh alert fields.

Mapping FIM fields to Wazuh alerts

Fields are information that the Wazuh decoder extracts from events the Wazuh server receives. Each event type has specific fields. The decoder identifies them with a field name. The Wazuh server translates these fields into alert fields and sends them to the Wazuh indexer for storage.

FIM alerts: fields correspondence

The following table establishes a correspondence between the decoded FIM fields and their equivalent field in an alert.

FIM field

Alert field

Field description

file

path

File path in the current event

size

size_after

File size in the current event

hard_links

hard_links

List of hard links of the file

mode

mode

FIM event mode

perm

perm_after,win_perm_after

File permissions

uid

uid_after

User ID of the owner of the file

gid

gid_after

Group ID of the group that shares ownership of the file

uname

uname_after

User name of the owner of the file

gname

gname_after

Group name of the group that shares ownership of the file

md5

md5_after

MD5 hash of the file in the current event after changes

sha1

sha1_after

SHA1 hash of the file in the current event after changes

sha256

sha256_after

SHA256 hash of the file in the current event after changes

mtime

mtime_after

Timestamp of the file changes

inode

inode_after

Inode of the file in the current event

changed_content

diff

Reported changes on the file of the current event

changed_fields

changed_attributes

Changed fields in the file such as permissions, content, and other related attributes

win_attributes

attrs_after

File attributes such as hidden, read-only, and other related attributes

tag

tag

Custom tags to be added to one specific event

user_id

audit.user.id

The actual ID of the user that triggered the event

user_name

audit.user.name

The actual name of the user that triggered the event

group_id

audit.group.id

The actual group ID of the user that triggered the event

group_name

audit.group.name

The actual group name of the user that triggered the event

process_name

audit.process.name

The name of the process run by a user that triggered the event

process_id

audit.process.id

The ID of the process run by a user that triggered the event

ppid

audit.process.ppid

The parent ID of the process that triggered the event

effective_uid

audit.effective.user_id

Effective user ID used by the process triggering the event

effective_name

audit.effective_user.name

Effective user name used by the process triggering the event

parent_name

audit.process.parent_name

The process name of the parent of the process triggering the event

cwd

audit.process.cwd

Current work directory of the process triggering the event

parent_cwd

audit.process.parent_cwd

Current work directory of the parent process

audit_uid

audit.login_user.id

The ID of the user logged in to the system that triggered the event

audit_name

audit.login_user.name

The name of the user logged in to the system that triggered the event

arch

arch

Registry architecture (32 or 64 bits)

value_name

value_name

Registry value name

value_type

value_type

Registry value type

entry_type

entry_type

Registry entry type

Custom FIM rules examples

In the following examples, we demonstrate how you can customize the default FIM rules of Wazuh. You can see how to create custom rules with the decoded FIM fields and what their equivalent Wazuh alert fields are.

Trigger alerts when execute permission is added to a script

If a script contains malicious code, such as commands to delete or modify important files or data, then the execution of that code might result in serious damage or data loss. Therefore, you have to be careful when granting execute permission to shell scripts, and only do so for scripts that are trusted and thoroughly reviewed.

Wazuh already has an out-of-the-box rule that generates an alert when a file permission is modified. However, in this example, you can see how to create a custom FIM rule to further customize this alert.

Use case description

Endpoint

Description

Ubuntu 20.04

The FIM module monitors scripts in a directory on this endpoint to detect permission changes.

Wazuh server

Perform the following steps on the Wazuh server.

  1. Create a file fim_specialdir3.xml in the /var/ossec/etc/rules/ directory for the custom rule:

    # touch /var/ossec/etc/rules/fim_specialdir3.xml

  2. Add the following rule definition to /var/ossec/etc/rules/fim_specialdir3.xml. This rule triggers an alert when execute permission is added to a shell script in a monitored directory:

    <group name="syscheck">
  <rule id="100002" level="8">
    <if_sid>550</if_sid>
    <field name="file">.sh$</field>
    <field name="changed_fields">^permission$</field>
    <field name="perm" type="pcre2">\w\wx</field>
    <description>Execute permission added to shell script.</description>
    <mitre>
      <id>T1222.002</id>
    </mitre>
  </rule>
</group>

  3. Restart the Wazuh server to apply the configuration changes:

    # systemctl restart wazuh-manager

Ubuntu endpoint

Perform the following steps to configure the Wazuh FIM module to monitor the /specialdir3 directory.

  1. Create the /specialdir3 directory:

    # mkdir /specialdir3

  2. Edit the Wazuh agent /var/ossec/etc/ossec.conf configuration file and add the directory for monitoring:

    <syscheck>
   <directories realtime="yes">/specialdir3</directories>
</syscheck>

  3. Restart the Wazuh agent to apply the configuration changes:

    # systemctl restart wazuh-agent

Test the configuration

  1. Create a shell script file fim.sh in the monitored directory:

    # touch /specialdir3/fim.sh

  2. Add execute permission to the script:

    # chmod +x /specialdir3/fim.sh

Visualize the alert

Navigate to Modules > Integrity monitoring on the Wazuh dashboard to view the alert generated when the FIM module detects the addition of the execute permission.

Visualize the alert

You can see the alert fields that correspond to the decoded FIM fields in the alert data below:

{
  "_index": "wazuh-alerts-4.x-2023.03.02",
  "_type": "_doc",
  "_id": "dJbsooYBhj2oQFX8xGM5",
  "_version": 1,
  "_score": null,
  "_source": {
    "syscheck": {
      "perm_before": "r--r--r--",
      "uname_after": "root",
      "mtime_after": "2023-03-02T17:40:16",
      "size_after": "4",
      "gid_after": "0",
      "mode": "realtime",
      "path": "/specialdir3/fim.sh",
      "sha1_after": "084d24bbed96773031b898def2a3fb8c46134944",
      "changed_attributes": [
        "permission"
      ],
      "gname_after": "root",
      "uid_after": "0",
      "perm_after": "r-xr-xr-x",
      "event": "modified",
      "md5_after": "eb4585ad9fe0426781ed7c49252f8225",
      "sha256_after": "5040625b1fb6fa4af07226683f6e6003b29e5e70b16f8cfb24be7a752393f0ee",
      "inode_after": 1709981
    },
    "input": {
      "type": "log"
    },
    "agent": {
      "ip": "192.168.33.157",
      "name": "Ubuntu20.04",
      "id": "014"
    },
    "manager": {
      "name": "wazuh"
    },
    "rule": {
      "firedtimes": 2,
      "mail": false,
      "level": 8,
      "description": "Execute permission added to shell script.",
      "groups": [
        "syscheck"
      ],
      "mitre": {
        "technique": [
          "Linux and Mac File and Directory Permissions Modification"
        ],
        "id": [
          "T1222.002"
        ],
        "tactic": [
          "Defense Evasion"
        ]
      },
      "id": "100002"
    },
    "location": "syscheck",
    "decoder": {
      "name": "syscheck_integrity_changed"
    },
    "id": "1677770668.1123062",
    "full_log": "File '/specialdir3/fim.sh' modified\nMode: realtime\nChanged attributes: permission\nPermissions changed from 'r--r--r--' to 'r-xr-xr-x'\n",
    "timestamp": "2023-03-02T18:24:28.047+0300"
  },
  "fields": {
    "syscheck.mtime_after": [
      "2023-03-02T17:40:16.000Z"
    ],
    "timestamp": [
      "2023-03-02T15:24:28.047Z"
    ]
  },
  "highlight": {
    "agent.id": [
}

Trigger file deletion alerts

Deleting a file can result in loss of important data or system files when done accidentally or without authorization. If an attacker gains access to a system and deletes critical files, it can render the system unusable, causing data loss or downtime for the organization.

Wazuh has an out-of-the-box rule that generates an alert when a monitored file is deleted or when a file in a monitored directory is deleted. In this example, we create a custom FIM rule that triggers an alert indicating the user and the application that deleted the file.

Use case description

Endpoint

Description

Windows 10

The FIM module monitors a folder on this endpoint for file deletions.

Wazuh server

Perform the following steps on the Wazuh server.

  1. Create a file fim_win_test.xml in the /var/ossec/etc/rules/ directory:

    # touch /var/ossec/etc/rules/fim_win_test.xml

  2. Add the following rule definition to the /var/ossec/etc/rules/fim_win_test.xml file. This rule triggers alerts when a user deletes files with File Explorer. Replace <USER> with the username of your Windows endpoint:

    <group name="syscheck">
  <rule id="100003" level="8">
    <if_sid>553</if_sid>
    <field name="process_name">explorer.exe$</field>
    <field name="uname"><USER>$</field>
    <match>deleted</match>
    <description>The user "$(uname)" deleted a monitored file with  File Explorer</description>
    <mitre>
      <id>T1070.004</id>
      <id>T1485</id>
    </mitre>
  </rule>
</group>

  3. Restart the Wazuh server to apply the configuration changes:

    # systemctl restart wazuh-manager

Windows endpoint

Perform the following steps to configure the Wazuh FIM module to monitor file deletion in the C:\test directory.

  1. Create the C:\test directory on the endpoint:

    mkdir C:\test

  2. Edit the Wazuh agent C:\Program Files (x86)\ossec-agent\ossec.conf configuration file of the Wazuh agent. Add the C:\test directory for monitoring:

    <syscheck>
   <directories whodata="yes">C:\test</directories>
</syscheck>

  3. Restart the Wazuh agent using Powershell with administrator privilege to apply the changes:

    Restart-Service -Name wazuh

Test the configuration

  1. Create a text file with Notepad and save the file in the C:\test directory as hello.txt.

  2. Delete the hello.txt file with Windows File Explorer.

Visualize the alert

Navigate to Modules > Integrity monitoring on the Wazuh dashboard to view the alert generated when the FIM module detects the deletion of files in the monitored directory.

Deleted file alert

You can see the alert fields that correspond to the decoded FIM fields in the alert data below:

{
  "_index": "wazuh-alerts-4.x-2023.02.13",
  "_type": "_doc",
  "_id": "AJERS4YB6Ki-QqEQBORx",
  "_version": 1,
  "_score": null,
  "_source": {
    "syscheck": {
      "uname_after": "wazuh",
      "mtime_after": "2023-02-13T16:55:18",
      "size_after": "0",
      "win_perm_after": [
        {
          "allowed": [
            "DELETE",
            "READ_CONTROL",
            "WRITE_DAC",
            "WRITE_OWNER",
            "SYNCHRONIZE",
            "READ_DATA",
            "WRITE_DATA",
            "APPEND_DATA",
            "READ_EA",
            "WRITE_EA",
            "EXECUTE",
            "READ_ATTRIBUTES",
            "WRITE_ATTRIBUTES"
          ],
          "name": "Administrators"
        },
        {
          "allowed": [
            "DELETE",
            "READ_CONTROL",
            "WRITE_DAC",
            "WRITE_OWNER",
            "SYNCHRONIZE",
            "READ_DATA",
            "WRITE_DATA",
            "APPEND_DATA",
            "READ_EA",
            "WRITE_EA",
            "EXECUTE",
            "READ_ATTRIBUTES",
            "WRITE_ATTRIBUTES"
          ],
          "name": "SYSTEM"
        },
        {
          "allowed": [
            "READ_CONTROL",
            "SYNCHRONIZE",
            "READ_DATA",
            "READ_EA",
            "EXECUTE",
            "READ_ATTRIBUTES"
          ],
          "name": "Users"
        },
        {
          "allowed": [
            "DELETE",
            "READ_CONTROL",
            "SYNCHRONIZE",
            "READ_DATA",
            "WRITE_DATA",
            "APPEND_DATA",
            "READ_EA",
            "WRITE_EA",
            "EXECUTE",
            "READ_ATTRIBUTES",
            "WRITE_ATTRIBUTES"
          ],
          "name": "Authenticated Users"
        }
      ],
      "mode": "whodata",
      "path": "c:\\test\\hello.txt",
      "sha1_after": "da39a3ee5e6b4b0d3255bfef95601890afd80709",
      "audit": {
        "process": {
          "name": "C:\\Windows\\explorer.exe",
          "id": "7480"
        },
        "user": {
          "name": "wazuh",
          "id": "S-1-5-21-3321418754-1060537631-2258373948-1002"
        }
      },
      "attrs_after": [
        "ARCHIVE"
      ],
      "uid_after": "S-1-5-21-3321418754-1060537631-2258373948-1002",
      "event": "deleted",
      "md5_after": "d41d8cd98f00b204e9800998ecf8427e",
      "sha256_after": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
    },
    "input": {
      "type": "log"
    },
    "agent": {
      "ip": "192.168.33.129",
      "name": "Windows",
      "id": "010"
    },
    "manager": {
      "name": "wazuh"
    },
    "rule": {
      "firedtimes": 1,
      "mail": false,
      "level": 8,
      "description": "The user \"wazuh\" deleted a monitored file with  File Explorer",
      "groups": [
        "syscheck"
      ],
      "mitre": {
        "technique": [
          "File Deletion",
          "Data Destruction"
        ],
        "id": [
          "T1070.004",
          "T1485"
        ],
        "tactic": [
          "Defense Evasion",
          "Impact"
        ]
      },
      "id": "100003"
    },
    "location": "syscheck",
    "decoder": {
      "name": "syscheck_deleted"
    },
    "id": "1676296648.685388",
    "full_log": "File 'c:\\test\\hello.txt' deleted\nMode: whodata\n",
    "timestamp": "2023-02-13T16:57:28.958+0300"
  },
  "fields": {
    "syscheck.mtime_after": [
      "2023-02-13T16:55:18.000Z"
    ],
    "timestamp": [
      "2023-02-13T13:57:28.958Z"
    ]
  },
}

Change alert severity for sensitive files

With a custom rule, you can alter the level of an FIM alert when detecting changes to a specific file or file pattern. In the following example, the custom rule raises the FIM alert level to 12 when a user or process modifies a critical file.

Use case description

Endpoint

Description

macOS Monterey

The FIM module monitors a file on this endpoint and raises the alert severity when it’s modified.

Wazuh server

Perform the following steps on the Wazuh server.

  1. Create a file fim_alert.xml in the /var/ossec/etc/rules/ directory on the Wazuh server:

    # touch /var/ossec/etc/rules/fim_alert.xml

  2. Add the following rule definition to the /var/ossec/etc/rules/fim_alert.xml file. This rule raises the FIM alert level to 12 when a user or process modifies a critical file:

    <group name="syscheck">
<rule id="100005" level="12">
  <if_sid>550</if_sid>
  <field name="file">customer_details.rtf</field>
  <match>modified</match>
  <description>Customer details file modified!</description>
</rule>
</group>

  3. Restart the Wazuh server to apply the configuration changes:

    # systemctl restart wazuh-manager

macOS endpoint

  1. Use TextEdit to create a file customer_details.rtf. Then, save it in the Documents directory.

  2. Edit the Wazuh agent /Library/Ossec/etc/ossec.conf configuration file and add the customer_details.rtf file for monitoring:

    <syscheck>
  <frequency>300</frequency>
  <directories>/Users/*/Documents/customer_details.rtf</directories>
</syscheck>

  3. Restart the Wazuh agent to apply the configuration changes:

    /Library/Ossec/bin/wazuh-control restart

Test the configuration

  1. Add text to the customer_details.rtf file with TextEdit. Wait for 5 minutes. This is the time configured for the FIM scan.

Visualize the alert

Navigate to Modules > Integrity monitoring on the Wazuh dashboard to view the alert. In this example, you can see an alert with a severity of 12 on the Wazuh dashboard when the FIM module detects changes in the monitored file.

Severity of 12 alert