Google Professional Security Operations Engineer Exam - Topic 4 Question 8 Discussion

Comprehensive and Detailed Explanation

The correct solution is Option D. The goal is to exclude events (i.e., stop false positives) when the principal.ip field contains any IP from the trusted 192.168.2.0/24 subnet.

The principal.ip field in UDM is a repeated field, meaning it can hold an array of values (e.g., ['1.2.3.4', '192.168.2.5']). YARA-L provides the any and all quantifiers to handle repeated fields.9

any $e.principal.ip: This checks if at least one IP in the array meets the condition.

all $e.principal.ip: This checks if every IP in the array meets the condition.

The function net.ip_in_range_cidr(...) returns true if an IP is in the specified range.

Therefore, the logic we need is: 'do not trigger this rule if any of the IPs in the principal.ip field are in the 192.168.2.0/24 range.'

This translates directly to the YARA-L syntax: not net.ip_in_range_cidr(any $e.principal.ip, '192.168.2.0/24')

Option B would only find events from that subnet.

Option A would only find events where all associated IPs are in that subnet.

Option C is the logical inverse of A and would incorrectly filter out events that might be malicious (e.g., ['1.2.3.4', '192.168.2.5'] would not be excluded because all IPs are not in the range).

Exact Extract from Google Security Operations Documents:

YARA-L 2.0 language syntax > Repeated fields and boolean expressions: When a boolean expression, such as a function call, is applied to a repeated field, you can use the any or all keywords to specify how the expression should be evaluated.10

any <repeated_field>: The expression evaluates to true if it is true for at least one of the values in the repeated field.

all <repeated_field>: The expression evaluates to true only if it is true for all of the values in the repeated field.

Functions > net.ip_in_range_cidr: The net.ip_in_range_cidr function is useful to bind rules to specific parts of the network.11 To exclude all private netblocks as defined in RFC1918, you can add a not to the start of the criteria:

and not (net.ip_in_range_cidr(any $e.principal.ip, '10.0.0.0/8') or net.ip_in_range_cidr(any $e.principal.ip, '172.16.0.0/12') or net.ip_in_range_cidr(any $e.principal.ip, '192.168.0.0/16'))

Google Cloud Documentation: Google Security Operations > Documentation > Detections > YARA-L 2.0 language syntax

Google Cloud Documentation: Google Security Operations > Documentation > Detections > YARA-L 2.0 functions > net.ip_in_range_cidr