Creating PyPanther Detections

Overview

PyPanther Detections are in closed beta starting with Panther version 1.108. Please share any bug reports and feature requests with your Panther support team.

You can import Panther-managed PyPanther Detections and make your own modifications, as well as create your own custom ones. After you've created detections, you can register, test, and upload them.

Before writing PyPanther Detections, you’ll need to set up your environment. See Getting started using PyPanther Detections.

High-level guidelines for creating PyPanther Detections

When working with PyPanther Detections:

• A main.py file controls your entire detection configuration, outlining which rules to register, override configurations, and custom rule definitions. You can either:

  • (Recommended) Define your detections in various other files/folders, then import them into main.py (learn more in the PyPanther Detections Style Guide)

  • Define your detections in main.py

• A PyPanther rule is defined in a single Python file. Within it, you can import Panther-managed (or your own custom) PyPanther rules and specify overrides. A single Python file can define multiple detections.

• All PyPanther rules subclass the pypanther Rule class or a parent class of type Rule.

• Rules must be registered to be tested and uploaded to your Panther instance.

• All event object functions currently available in v1 detections are available in PyPanther Detections. These include: get(), deep_get(), deep_walk(), and udm().

• All alert functions available in Python (v1) detections are available in PyPanther Detections, such as title() and severity(). See Rule auxiliary/alerting function reference.

• Use the pypanther type-ahead hints in your IDE, like searching for available rules or viewing properties of classes.

Writing a custom PyPanther Detection

A "custom" PyPanther rule is one that you write completely from scratch—i.e., one that isn't built from a Panther-managed rule. Custom PyPanther rules are defined in a Python class that subclasses the pypanther Rule class. In this class, you must:

• Define a rule() function

  • (Optional) Define any of the other alert functions, like title() or destinations()

• Define certain attributes, such as log_types

  • (Optional) Define additional attributes, such as threshold or dedup_period_minutes

The id attribute is only required for a rule if you plan to register it.

See the Rule property reference section for a full list of required and optional fields.

from pypanther import Rule, Severity, LogType

# Custom rule for a Panther-supported log type
class MyCloudTrailRule(Rule):
    id = "MyCloudTrailRule"
    tests = True
    log_types = [LogType.AWS_CLOUDTRAIL]
    default_severity = Severity.MEDIUM
    threshold = 50
    dedup_period_minutes = 1

    def rule(self, event) -> bool:
        return (
	    event.get("eventType") == "AssumeRole" and 
	        400 <= int(event.get("errorCode", 0)) <= 413
	)

Importing Panther-managed rules

Panther-managed rules can be imported directly or using the get_panther_rules() function.

Panther-managed rules currently all have a -prototype suffix (e.g., AWS.Root.Activity-prototype). This is temporary, and will be removed in the future.

You may want to import Panther-managed rules (into main.py or another file) to either register them individually as-is, set overrides on them, or subclass them. You can import Panther-managed rules directly (from the pypanther.rules module) or by using the get_panther_rules() function.

To import a Panther-managed rule directly using the rules module, you would use a statement like:

from pypanther.rules.github import GitHubAdvancedSecurityChange

The get_panther_rules() function can filter on any Rule class attribute, such as default_severity, log_types, or tag. When filtering, keys use AND logic, and values use OR logic.

Get all Panther-managed rules using get_panther_rules():

from pypanther import get_panther_rules

all_rules = get_panther_rules()

Get Panther-managed rules with certain severities using get_panther_rules():

from pypanther import get_panther_rules, Severity

important_rules = get_panther_rules(
    default_severity=[
        Severity.HIGH,
        Severity.CRITICAL,
    ]
)

Get Panther-managed rules for certain log types using get_panther_rules():

from pypanther import get_panther_rules, LogType

cloudtrail_okta_rules = get_panther_rules(
    log_types=[
        LogType.AWS_CLOUDTRAIL,
        LogType.OKTA_SYSTEM_LOG
    ]
)

Get Panther-managed rules that meet multiple criteria using get_panther_rules():

from pypanther import get_panther_rules, Severity

rules_i_care_about = get_panther_rules(
    enabled=True,
    default_severity=[Severity.CRITICAL, Severity.HIGH],
    tags=["Configuration Required"],
)

See Using list comprehension for an example of how to use get_panther_rules() with advanced Python.

Once you’ve imported a Panther-managed rule, you can modify it using overrides or inheritance.

Applying overrides on existing rules

If your objective is to modify a rule's logic, it's recommended to use include/exclude filters instead of overriding the rule() function itself. Learn more in Use filters instead of overriding rule().

When making overrides on Panther-managed detections, it's recommended to:

• Outside of main.py, store all of your overrides in apply_overrides() functions.

• In main.py, call pypanther's apply_overrides() to apply each of your apply_overrides() functions.

Learn more about apply_overrides() in PyPanther Detections Style Guide.

Overriding single attributes

You can override a single rule attribute in a one-line statement using the override() function:

from pypanther import Severity
from pypanther.rules.aws_cloudtrail import AWSCloudTrailCreated

# Set rule severity to High
AWSCloudTrailCreated.override(default_severity=Severity.HIGH)

Overriding multiple attributes with the override function

It’s also possible to make multi-attribute overrides with the override() function:

from pypanther import Severity
from pypanther.rules.box import BoxContentWorkflowPolicyViolation

BoxContentWorkflowPolicyViolation.override(
    default_severity=Severity.HIGH,
    default_runbook="Check if other internal users hit this same violation"
)

Applying overrides on multiple PyPanther rules

To apply overrides on multiple rules at once, iterate over the collection using a for loop.

This could be useful, for example, when updating a certain attribute for all rules associated to a certain LogType.

from pypanther import get_panther_rules, Severity, LogType

rules = get_panther_rules(
    enabled=True,
    default_severity=[Severity.CRITICAL, Severity.HIGH],
    tags=["Configuration Required"],
)

# Define the destination [override](<https://docs.panther.com/detections/rules/python#destinations>)
def aws_cloudtrail_destinations(self, event):
    if event.get('recipientAccountId') == 112233445566:
        # Name or UUID of a configured Slack Destination
        return ['AWS Notifications']
    # Suppress the alert, doesn't deliver
    return ['SKIP']

# Appending to the Panther-managed rules' destinations and tags using extend()
for rule in rules:
    rule.extend(
        default_destinations=["Slack #security"],
        tags=["Production"],
    )
		
    # Updating the destinations method for CloudTrail rules
    if LogType.AWS_CLOUDTRAIL in rule.log_types:
        rule.destinations = aws_cloudtrail_destinations

Extending list attributes on existing rules

When making modifications to an existing rule, you might want to add items to a list-type rule attribute (like tags, tests, include_filters, or exclude_filters) while preserving the existing list.

Instead of overriding the attribute (using one of the methods in Applying overrides on existing rules), which would replace the existing list value, use the pypanther extend() function to append new values to the list attribute.

from pypanther import Severity
from pypanther.rules.box_rules.box_policy_violation import BoxContentWorkflowPolicyViolation

BoxContentWorkflowPolicyViolation.extend(
    tags=["Production"],
    include_filters=[lambda event: event.get("env") == "prod"]
)

# Because BoxContentWorkflowPolicyViolation already has a "Box" tag, it will now 
# have two tags: "Production" and "Box"

# Because BoxContentWorkflowPolicyViolation does not already define include_filters, 
# it will now have the single filter above

Creating include or exclude filters

PyPanther Detection filters let you exclude certain events from being evaluated by a rule. Filters are designed to be applied on top of existing rule logic (likely for Panther-managed PyPanther Detections you are importing).

Each filter defines logic that is run before the rule() function, and the outcome of the filter determines whether or not the event should go on to be evaluated by the rule.

Common use cases for filters include:

• To target only certain environments, like prod

• To exclude events that are known false positives, due to a misconfiguration or other non-malicious scenario

There are two types of filters:

• include_filters: If the filter returns True for an event, the event is evaluated by rule()

• exclude_filters: If the filter returns True for an event, the event is dismissed (i.e., not evaluated by rule())

The Rule base class has include_filters and exclude_filters attributes, which each contain a list of functions that will be evaluated against the log event.

Examples as standalone functions:

from pypanther.rules.github import GitHubAdvancedSecurityChange

# Include only repos that are production
prod_repos = ["prod_repo1", "prod_repo2"]
GitHubAdvancedSecurityChange.extend(
    include_filters=[lambda e: e.get("repo") in prod_repos]
)

from pypanther.rules.box import BoxContentWorkflowPolicyViolation

# Exclude accounts that are for development
dev_repos = ["dev_repo1", "dev_repo2"]
GitHubAdvancedSecurityChange.extend(
    exclude_filters=[lambda e: e.get("repo") in dev_repos]
)

Example as part of an inherited rule definition:

from pypanther.wrap import include, exclude
from pypanther.rules.github import GitHubAdvancedSecurityChange

class GitHubAdvancedSecurityChangeOverride(GitHubAdvancedSecurityChange):
    id = "GitHubAdvancedSecurityChangeOverride"
    # In a real-world scenario, likely only one of the below would be necessary
    # Extend parent rule's include_filters
    include_filters = super().include_filters.append(lambda e: e.get("repo") in prod_repos)
    # Override parent rule's exclude_filters
    exclude_filters = [lambda e: e.get("repo") in dev_repos]
    ...

Filters can also be reused with a for loop to be applied to multiple rules:

from pypanther.wrap import include

rules = [
    # Panther-managed and custom rules
    ...
]

def prod_filter(event):
    return event.get("repo") in prod_repos
		
for rule in rules:
    rule.extend(include_filters=[prod_filter])

Ensuring necessary fields are set on configuration-required rules

Panther-managed rules that require some customer configuration before they are uploaded into a Panther environment may include a validate_config() function, which defines one or more conditions that must be met for the rule to pass the test command (and function properly).

Most commonly, validate_config() verifies that some class variable, such as an allowlist or denylist, has been assigned a value. If the requirements included in validate_config() are not met, an exception will be raised when the pypanther test command is run (if the rule is registered).

Example:

class ValidateMyRule(Rule):
    id = "Validate.MyRule"
    log_types = [LogType.PANTHER_AUDIT]
    default_severity = Severity.HIGH

    allowed_domains: list[str] = []

    def rule(self, event):
        return event.get("domain") not in self.allowed_domains

    @classmethod 
    def validate_config(cls):
        assert (
            len(cls.allowed_domains) > 0
        ), "The allowed_domains field on your rule must be populated"

In this example, if allowed_domains is not assigned a non-empty list, an assertion error will be thrown during pypanther test.

To set this value, you can use a statement like:

ValidateMyRule.allowed_domains = ["example.com"]

Creating PyPanther rules with inheritance

You can use inheritance to create rules that are subclasses of other rules (that are Panther-managed or custom).

It’s recommended to use inheritance when you’re creating a collection of rules that all share a number of characteristics—for example, rule() function logic, property values, class variables, or helper functions. In this case, it’s useful to create a base rule that all related rules inherit from.

For example, it may be useful to create a base rule for each LogType, from which all rules for that LogType are extended.

Inheritance is commonly used with filters—i.e., subclassed rules can define additional criteria an event must meet in order to be processed by the rule.

If you don’t plan to register a base rule, it’s not required to provide it an id property.

Example:

# Custom rule for a custom log type — the parent rule
class HostIDSBaseRule(Rule):
    # id not necessary since we're not uploading this parent rule
    log_types = ['Custom.HostIDS']
    default_severity = Severity.HIGH
    threshold = 1
    dedup_period_minutes = 6 * 60 # 6 hours

    def rule(self, event) -> bool:
        return event.get('event_name') == 'confirmed_compromise'
    
    def host_user_lookup(self, hostname):
	return 'groot'
		
    def title(self, event): -> str:
	return f"Confirmed compromise from hostname {event.get('hostname')}"
		
    def alert_context(self, event):
	user = self.host_user_lookup(event.get('hostName') 
	return {
	    'hostname': event['hostName'],
	    'time': event['p_event_time'],
	    'user': user,
	}

# Inherited rule #1

class IDSCommandAndControl(HostIDSBaseRule):
    id = 'IDSCommandAndControl'
    threshold = 19
    
    # Filter on event_type (in addition to base rule function)
    include_filters = [lambda e: e.get('event_type') == 'c2']
    
    def title(self, event):
	return f"Confirmed c2 on host {event.get('hostname')}"
	
    # From parent rule, inherits rule(), alert_context(), log_types, severity, and dedup_period_minutes
    # From pypanther Rule, inherits other fields (like Enabled)

# Inherited rule #2

class IDSCommandAndControl(HostIDSBaseRule):
    id = 'HostIDSMalware'
    threshold = 2
    default_severity = Severity.CRITICAL
    
    # Filter on event_type (in addition to base rule function)
    include_filters = [lambda e: e.get('event_type') == 'malware']

    def title(self, event):
	return f"Confirmed malware on host {event.get('hostname')}"
    
    # From parent rule, inherits rule(), alert_context(), log_types, and dedup_period_minutes
    # From pypanther Rule, inherits other fields (like enabled)

Using advanced Python

Because PyPanther rules are fully defined in Python, you can use its full expressiveness when customizing your detections.

Calling super()

For more advanced use cases, you can supplement the logic in functions defined by the parent rule.

# Creating an inherited rule
class MyRule(AnExistingRule):
    def alert_context(self, event):
	# Preserve the parent rule's alert context and extend with a new field
	context = super().alert_context(event)
	context["new field"] = "new_value"
	return context
				
    def severity(self, event):
	# Conditionally increment the parent rule's severity
	if event.get("env") == "prod":
	    return Severity.CRITICAL
	return super().severity(event)

Using list comprehension

You can use Python’s list comprehension functionality to create a new list based on an existing list with condensed syntax. This may be particularly useful when you want to filter a list of detections fetched using get_panther_rules().

# Collection of rules that have more than one LogType
rules = [rule for rule in get_panther_rules() if len(rule.log_types) > 1]
					
# Collection of rules that do not require configuration
rules = [rule for rule in get_panther_rules() if "Configuration Required" not in rule.tags]