[New Rule] AWS Lambda Function Invoked Cross-Account

[bryans3c]

Pull Request

Issue link(s):

• https://github.com/elastic/ia-trade-team/issues/920

Summary - What I changed

Add detection of an AWS Lambda function invoked by a principal whose AWS account differs from the account that owns the function (a cross-account invocation). The caller's account is parsed from the invoking principal's ARN and compared to
the function account. Adversaries who have been granted invoke permission on a function from an external account, or who
operate from a separate attacker-controlled account, can use cross-account invocation to execute functions or retrieve the data they return. This is the data-plane counterpart to detecting the cross-account grant itself, and relies on AWS Lambda data event logging, which is not enabled by default.

How To Test

Query & data can be assessed in TRADE stack and other telemetry stacks.

Checklist

• Added a label for the type of pr: bug, enhancement, schema, maintenance, Rule: New, Rule: Deprecation, Rule: Tuning, Hunt: New, or Hunt: Tuning so guidelines can be generated
• Added the meta:rapid-merge label if planning to merge within 24 hours
• Secret and sensitive material has been managed correctly
• Automated testing was updated or added to match the most common scenarios
• Documentation and comments were added for features that require explanation

Contributor checklist

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?

[github-actions]

Rule: New - Guidelines

These guidelines serve as a reminder set of considerations when proposing a new rule.

Documentation and Context

• Detailed description of the rule.
• List any new fields required in ECS/data sources.
• Link related issues or PRs.
• Include references.

Rule Metadata Checks

• creation_date matches the date of creation PR initially merged.
• min_stack_version should support the widest stack versions.
• name and description should be descriptive and not include typos.
• query should be inclusive, not overly exclusive, considering performance for diverse environments. Non ecs fields should be added to non-ecs-schema.json if not available in an integration.
• min_stack_comments and min_stack_version should be included if the rule is only compatible starting from a specific stack version.
• index pattern should be neither too specific nor too vague, ensuring it accurately matches the relevant data stream (e.g., use logs-endpoint.process-* for process data).
• integration should align with the index. If the integration is newly introduced, ensure the manifest, schemas, and new_rule.yaml template are updated.
• setup should include the necessary steps to configure the integration.
• note should include any additional information (e.g. Triage and analysis investigation guides, timeline templates).
• tags should be relevant to the threat and align/added to the EXPECTED_RULE_TAGS in the definitions.py file.
• threat, techniques, and subtechniques should map to ATT&CK always if possible.

New BBR Rules

• building_block_type should be included if the rule is a building block and the rule should be located in the rules_building_block folder.
• bypass_bbr_timing should be included if adding custom lookback timing to the rule.

Testing and Validation

• Provide evidence of testing and detecting the expected threat.
• Check for existence of coverage to prevent duplication.

[Copilot]

Pull request overview

This PR adds a new AWS detection rule to identify cross-account AWS Lambda function invocations using CloudTrail Lambda data events, highlighting scenarios where the invoking principal’s account differs from the function owner’s account.

Changes:

• Introduces a new ES|QL rule that parses the caller account from aws.cloudtrail.user_identity.arn and the function owner account from aws.cloudtrail.request_parameters.
• Aggregates matching invocations and enriches the alert with invocation count and source IPs.
• Adds an investigation guide and setup instructions emphasizing the need to enable Lambda data event logging.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 3 comments.