[New Rule] AWS AssumeRoleWithWebIdentity from unusual source ASN

[terrancedejesus]

Summary

Adds AWS STS AssumeRole with Web Identity from Unusual ASN (initial_access_sts_assume_role_web_identity_new_source_asn.toml), a New Terms rule on successful AssumeRoleWithWebIdentity when aws.cloudtrail.resources.arn + source.as.number appear together for the first time within the history window (now-10d lookback, now-6m rule interval). Requires ASN enrichment (source.as.number) and populated role resource ARN.

Threat

• Valid cloud accounts / alternate auth material (MITRE T1078.004, T1550.001): reuse of OIDC or web-identity federation where a JWT or equivalent token is exercised from networks you have not previously seen for that same target IAM role (e.g. VPN, VPS, or replay off normal CI egress).
• Complements static-key and tooling-specific detections after supply-chain or credential theft (see references).

Gap addressed

Existing rules cover AssumeRole (IAM user/service patterns), role chaining, MFA device novelties, and OIDC provider creation, but not first-time federation ASN surface per assumed role for web identity. This targets post-exfil replay where API calls still succeed and egress ASN diverges from the org’s learned CI/IdP baseline.

Limits

• In-runner-only abuse (same ASN as legitimate jobs) is not the primary signal; pair with behavioral enumeration rules.
• False positives: new runner regions, VPN/proxy shifts, GeoIP/ASN updates, new roles without history. tune allowlists and window as needed.

References

• AssumeRoleWithWebIdentity
• IAM OIDC providers
• Kudelski — Trivy supply-chain compromise

[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]

Suggested change

	with exceptions on `aws.cloudtrail.resources.arn`, `source.as.organization.name`, or expected ASNs; extend the
	with exceptions on `user.id`, `source.as.organization.name`, or expected ASNs; extend the

[terrancedejesus]

Suggested change

	value = ["aws.cloudtrail.resources.arn", "source.as.number"]
	value = ["user.id", "source.as.number"]

[terrancedejesus]

Closing this pull request. Choosing not to ship detection as it strays from the main behaviors reported for TeamPCP in that long-term creds were used for post compromise.