sigma-rules/rules/integrations/aws/initial_access_aws_api_unusual_asn.toml

[metadata]
creation_date = "2026/04/21"
integration = ["aws"]
maturity = "production"
updated_date = "2026/04/21"

[rule]
author = ["Elastic"]
description = """
Surfaces an AWS identity whose successful API traffic is dominated by a small set of large cloud-provider source AS
organization labels, yet also shows a very small share of traffic from other AS organization names—including at least one
sensitive control-plane, credential, storage, or model-invocation action on that uncommon network path with recent
activity from the uncommon path. The intent is to highlight disproportionate “baseline” cloud egress versus sparse use
from rarer networks on the same principal, a shape that can appear when automation or CI credentials are reused or
pivoted outside their usual hosted-cloud footprint.
"""
false_positives = [
    """
    Global employees on VPNs, split DNS or proxy paths that change AS labels, regional carrier rebrands, or mobile
    hotspots can produce a small non-cloud AS share on the same IAM user as hyperscaler- or SaaS-classified traffic.
    Corporate travel, emergency break-glass from a home ISP, and multi-region runners may also widen AS diversity without
    malice. Tune thresholds, add account or principal allowlists, or narrow the sensitive-action list after baseline review.
    """,
]
from = "now-7d"
interval = "1h"
language = "esql"
license = "Elastic License v2"
name = "AWS Rare Source AS Organization Activity"
note = """## Triage and analysis

### Investigating AWS Rare Source AS Organization Activity After High Cloud-Provider Volume

The rule aggregates roughly seven days of successful CloudTrail per `user.name` and `aws.cloudtrail.user_identity.type`.
It expects a **high count** of events whose GeoIP AS organization matches a short allowlist of large cloud/SaaS providers,
**at least one** event from a different AS organization, a **low ratio** of uncommon-network events to all events, few
**distinct** uncommon AS labels, and **recent** uncommon-network timestamps. It further requires at least one
**sensitive** API from an uncommon network (see query `event.action` list).

#### Possible investigation steps

- Compare `Esql.src_asn_values` to `Esql.user_agent_values` and map each `source.ip` (from raw CloudTrail) to expected
  admin paths, pipelines, or offices.
- Pivot on `user.name` and `aws.cloudtrail.user_identity.access_key_id` (from underlying events) for IAM, STS, S3, and
  Secrets Manager activity around `Esql.most_recent_low_asn_day`.
- Confirm whether the identity is meant for automation only; if so, rare human ISP ASNs warrant higher scrutiny.
- Review `Esql.untrusted_suspicious_actions` for the mix of discovery versus privilege-changing APIs.

### False positive analysis

- **Threshold sensitivity**: Raise `Esql.trusted_cloud_event_count` or Lower `Esql.rare_asn_ratio` and `Esql.untrusted_event_count` if legitimate rare-ASN
  noise persists.
- **MongoDB / other allowlist labels**: Extend `is_trusted_cloud` if your approved automation consistently appears under
  another legal-entity string.

### Response and remediation

- If abuse is plausible: rotate credentials for the principal, enforce OIDC or short-lived keys for automation, and
  tighten IAM and data-plane permissions.

### Additional information

- [CloudTrail user identity](https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudtrail-event-reference-user-identity.html)
"""
references = [
    "https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudtrail-event-reference.html",
]
risk_score = 73
rule_id = "d1b37c0b-4f8b-4cfb-9a1d-639bf8c028b7"
severity = "high"
tags = [
    "Domain: Cloud",
    "Data Source: AWS",
    "Data Source: Amazon Web Services",
    "Data Source: AWS CloudTrail",
    "Use Case: Threat Detection",
    "Tactic: Initial Access",
    "Resources: Investigation Guide",
]
timestamp_override = "event.ingested"
type = "esql"

query = '''
FROM logs-aws.cloudtrail-*
| WHERE event.dataset == "aws.cloudtrail"
  AND event.outcome == "success"
  AND source.as.organization.name IS NOT NULL
  AND user.name IS NOT NULL

| EVAL is_trusted_cloud = CASE(
    source.as.organization.name LIKE "Amazon*" OR
    source.as.organization.name == "Google LLC" OR
    source.as.organization.name == "Microsoft Corporation" OR
	source.as.organization.name == "MongoDB, Inc.",
    true, false
  )

| EVAL is_suspicious_action = CASE(
    event.action IN (
      "GetCallerIdentity", "GetAccountSummary", "ListAccountAliases",
      "GetSecretValue", "ListSecrets", "DescribeSecret",
      "GetParameter", "GetParameters", "GetParametersByPath",
      "AssumeRole", "AssumeRoleWithWebIdentity", "AssumeRoleWithSAML",
      "AttachUserPolicy", "AttachRolePolicy",
      "PutUserPolicy", "PutRolePolicy",
      "CreateAccessKey", "UpdateAccessKey",
      "CreateUser", "CreateLoginProfile",
      "UpdateLoginProfile", "AddUserToGroup",
      "GetObject", "ListBuckets", "ListObjects", "ListObjectsV2",
      "InvokeModel", "InvokeModelWithResponseStream", "Converse"
    ), true, false
  )

// Single aggregation — full event count preserved for ratio logic
// suspicious action tracking is additive on top
| STATS
    Esql.total_events_all_asns = COUNT(*),
    Esql.count_distinct_asns = COUNT_DISTINCT(source.as.organization.name),
    Esql.src_asn_values = VALUES(source.as.organization.name),
	Esql.user_agent_values = VALUES(user_agent.original),
	Esql.related_users = VALUES(user.changes.name),
	Esql.source_ip_values = VALUES(source.address),
    Esql.has_trusted_cloud_asn = MAX(is_trusted_cloud),
    Esql.trusted_cloud_event_count = SUM(CASE(is_trusted_cloud == true, 1, 0)),
    Esql.untrusted_event_count = SUM(CASE(is_trusted_cloud == false, 1, 0)),
    // Suspicious action visibility from untrusted ASNs — informational only, not a filter
    Esql.untrusted_suspicious_count  = SUM(CASE(
        is_trusted_cloud == false AND is_suspicious_action == true, 1, 0
      )),
    Esql.untrusted_suspicious_actions = VALUES(CASE(
        is_trusted_cloud == false AND is_suspicious_action == true,
        event.action, null
      )),
    Esql.most_recent_low_asn_day   = MAX(CASE(
        is_trusted_cloud == false, @timestamp, null
      ))
    BY user.name, aws.cloudtrail.user_identity.type

| EVAL Esql.rare_asn_ratio = TO_DOUBLE(Esql.untrusted_event_count) / TO_DOUBLE(Esql.total_events_all_asns), 
       Esql.unique_action_from_untrusted_asn = MV_COUNT(Esql.untrusted_suspicious_actions)

// Detection thresholds — unchanged, full event counts drive the logic
| WHERE Esql.has_trusted_cloud_asn == true
  AND Esql.untrusted_event_count >= 1
  AND Esql.trusted_cloud_event_count >= 100
  AND Esql.rare_asn_ratio <= 0.01
  AND Esql.unique_action_from_untrusted_asn >= 2 
  AND Esql.count_distinct_asns <= 5
  AND Esql.most_recent_low_asn_day >= NOW() - 1 hour

| KEEP user.name,
       aws.cloudtrail.user_identity.type,
       Esql.*
'''

[rule.investigation_fields]
field_names = [
    "user.name",
    "aws.cloudtrail.user_identity.type",
    "Esql.*"
]



[[rule.threat]]
framework = "MITRE ATT&CK"

[[rule.threat.technique]]
id = "T1078"
name = "Valid Accounts"
reference = "https://attack.mitre.org/techniques/T1078/"

[[rule.threat.technique.subtechnique]]
id = "T1078.004"
name = "Cloud Accounts"
reference = "https://attack.mitre.org/techniques/T1078/004/"

[rule.threat.tactic]
id = "TA0001"
name = "Initial Access"
reference = "https://attack.mitre.org/tactics/TA0001/"