Caching

Panther's real-time analysis engine examines events one-by-one and sometimes it's helpful to keep state across invocations. To accommodate stateful checks, rules can cache values by using built-in helper functions.

Importing the Helpers

The first step is to import the open source helper library:

import panther_oss_helpers

You may import specific functions:

from panther_oss_helpers import increment_counter

Counters

To implement a counter-based rule, use one or more of the following functions:

  • get_counter: Get the latest counter value

  • increment_counter: Add to the counter (default of 1)

  • reset_counter: Reset the counter to 0

  • set_key_expiration: Set the lifetime of the counter

The rule below provides a demonstration of using counters.

from panther_oss_helpers import increment_counter, set_key_expiration, reset_counter
def rule(event):
# Filter to only analyze AccessDenied calls
if event.get('errorCode') != 'AccessDenied':
return False
# Create our counter key, which should be fairly unique
key = '{}-AccessDeniedCounter'.format(event['userIdentity'].get('arn'))
# Increment the counter, and then check the current value
hourly_error_count = increment_counter(key)
if hourly_error_count == 1:
set_key_expiration(time.time() + 3600)
elif failure_hourly_count >= 10:
# If it exceeds our threshold, reset and then return an alert
reset_counter(key)
return True
return False

String Sets

To keep track of sets of strings, use the following functions:

  • get_string_set: Get the string set's current value

  • put_string_set: Overwrite a string set

  • add_to_string_set: Add one or more strings to a set

  • remove_from_string_set: Remove one or more strings from a set

  • reset_string_set: Empty the set

from panther_oss_helpers import add_to_string_set
def rule(event):
if event['eventName'] != 'AssumeRole':
return False
role_arn = event['requestParameters'].get('roleArn')
if not role_arn:
return False
role_arn_key = '{}-UniqueSourceIPs'.format(role_arn)
ip_addr = event['sourceIPAddress']
previously_seen_ips = get_string_set(role_arn_key)
# If this the only value, trust on first use
if len(previously_seen_ips) == 0:
add_to_string_set(role_arn_key, ip_addr)
return False
if ip_addr not in previously_seen_ips:
return True
return False

Testing

Currently, CLI testing does not support mocking function calls.