[New Rule] Kubernetes Unusual Decision by User Agent

[Aegrah]

Summary

This rule detects unusual request responses in Kubernetes audit logs through the use of the "new_terms" rule type. In production environments, default API requests are typically made by system components or trusted users, which are expected to have a consistent user agent and allowed response annotations. By monitoring for anomalies in the username and response annotations, this rule helps identify potential unauthorized access or misconfigurations in the Kubernetes environment.

Telemetry

User agents, usernames, and decisions are typically consistent in production environments, as all API requests should be allowed. This rule leverages the new_terms rule type to find requests that deviate from this pattern.

If this rule were to be noisy, I can exclude certain service account behavior + commonly hit API endpoints.

[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.

[terrancedejesus]

Did we intentionally exclude from rule object?

[Aegrah]

That is a good question. I copy pasted the template from the other k8s rules. I just did a check in the kubernetes repository, and the from field is nowhere present.

@imays11 do you happen to know why this is not set? Is this related to timestamps in the audit log file system?

[w0rk3r]

The default value (if not specified) for it is now-6m afaik

[terrancedejesus]

I dont know if it's applicable but may want to ensure user_agent is not "" instead as * will include these.

[Aegrah]

I want to filter out cases where the user_agent.original field does not exist, as this will break the logic based on the new_terms value; generally speaking, the user_agent.original is supplied; having an empty string there is (as far as I know based on my research) suspicious, as that is not common by default system behavior.

The empty user_agent.original example above is based on reconnaissance activity from a network scanner.

[w0rk3r]

Why we are filtering for the user_agent.original field and not "kubernetes.audit.annotations.authorization_k8s_io/decision", "kubernetes.audit.user.username"?

[Aegrah]

Because these two fields are always present, I figured it does not require filtering, while the user agent string can be empty in certain cases.

[imays11]

Do you have any logs we can view in the shared stack?