[New Rule] Adding Coverage for `AWS S3 Unauthenticated Bucket Access …

…by Rare Source` (#4315)

* adding new rule 'AWS S3 Unauthenticated Object Retrieval by Rare Source'

* adjusted logic to capture multiple event calls

* updated verbiage

* updated MITRE mappings

* fixing date

(cherry picked from commit fb13b89)

rules/integrations/aws/collection_s3_unauthenticated_bucket_access_by_rare_source.toml

@@ -0,0 +1,180 @@
+[metadata]
+creation_date = "2024/12/17"
+integration = ["aws"]
+maturity = "production"
+updated_date = "2025/01/20"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies AWS CloudTrail events where an unauthenticated source is attempting to access an S3 bucket. This activity may
+indicate a misconfigured S3 bucket policy that allows public access to the bucket, potentially exposing sensitive data
+to unauthorized users. Adversaries can specify `--no-sign-request` in the AWS CLI to retrieve objects from an S3 bucket
+without authentication. This is a [New
+Terms](https://www.elastic.co/guide/en/security/current/rules-ui-create.html#create-new-terms-rule) rule, which means it
+will only trigger once for each unique value of the `source.address` field that has not been seen making this API
+request within the last 7 days. This field contains the IP address of the source making the request.
+"""
+from = "now-9m"
+index = ["filebeat-*", "logs-aws.cloudtrail*"]
+language = "kuery"
+license = "Elastic License v2"
+name = "AWS S3 Unauthenticated Bucket Access by Rare Source"
+note = """## Investigating AWS S3 Unauthenticated Bucket Access by Rare Source
+
+This rule detects requests to an AWS S3 bucket by an unauthenticated source, which could indicate a misconfigured bucket policy allowing public access. Adversaries can exploit this misconfiguration by using tools or AWS CLI options like `--no-sign-request` to access bucket contents.
+
+The rule triggers when an unauthenticated IP address retrieves an object, and that IP has not been seen in the last 7 days.
+
+### Possible Investigation Steps
+
+1. **Identify the Source of the Request**:
+    - Review the `source.address` field to determine the IP address of the request source.
+    - Check `source.geo` fields for geographic details of the originating IP address.
+    - Analyze the `user_agent.original` field to identify the client or tool used (e.g., `Python Requests`, `aws-cli`, browser).
+
+2. **Review the Accessed Bucket and Object**:
+    - Analyze the `aws.cloudtrail.resources.arn` field to identify the S3 bucket and object being accessed.
+    - Inspect `aws.cloudtrail.request_parameters` for bucket name and object key to determine which file was retrieved.
+    - Review the `even.action` field to identify which API call was made (e.g., `GetObject`, `ListObjects`, `PutObject`, `ListBucket`).
+
+3. **Validate the Source IP and Context**:
+    - Determine if the IP address (`source.address`) has any prior activity in your environment.
+    - Correlate the IP with threat intelligence or blocklist databases to check for malicious indicators.
+    - Review CloudTrail logs for other activities originating from the same IP.
+
+4. **Analyze the S3 Bucket Configuration**:
+    - Review the S3 bucket's Access Control List (ACL) and bucket policy to check for misconfigurations allowing public or unauthenticated access.
+    - Look for overly permissive settings, such as `Principal: *` or `Effect: Allow` rules that expose the bucket.
+
+5. **Investigate Additional Activity**:
+    - Check if there are subsequent actions, such as:
+        - **Additional `GetObject` API calls**: Indicating further data exfiltration.
+        - **ListObjects requests**: Attempting to enumerate the bucket's contents.
+    - Correlate events within the same timeframe to identify related suspicious activity.
+
+6. **Assess the Data Exposed**:
+    - Identify the retrieved object(s) and analyze their content to assess potential data exposure.
+    - Determine if the file contains sensitive information, such as credentials, intellectual property, or PII.
+
+### False Positive Analysis
+
+- **Public Buckets by Design**: Some S3 buckets may intentionally allow public access. Verify with the bucket owner if the access was expected.
+- **Automated Tools**: Security scanners or legitimate services may generate `GetObject` events to validate bucket configurations.
+
+### Response and Remediation
+
+1. **Immediate Action**:
+    - Restrict or remove public access to the affected S3 bucket.
+    - Update the bucket policy to ensure access is restricted to trusted principals.
+    - Enable **S3 Block Public Access** settings to prevent unintended public access.
+
+2. **Monitoring and Detection**:
+    - Enable detailed logging and monitoring for all S3 bucket activities.
+    - Configure real-time alerts for unauthenticated `GetObject` or `ListObjects` events on sensitive S3 buckets.
+
+3. **Security Audits**:
+    - Regularly audit S3 bucket policies and ACLs to ensure they adhere to AWS security best practices.
+    - Use AWS tools like **Trusted Advisor** or **Access Analyzer** to identify and address misconfigurations.
+
+4. **Investigate for Data Exfiltration**:
+    - Analyze historical CloudTrail logs to determine if other sensitive files were accessed or exfiltrated.
+    - Assess the scope of the exposure and initiate further response if sensitive data was compromised.
+
+### Additional Resources
+
+- [AWS Documentation: S3 Bucket Policy Best Practices](https://docs.aws.amazon.com/AmazonS3/latest/userguide/example-bucket-policies.html)
+- [AWS S3 Block Public Access](https://docs.aws.amazon.com/AmazonS3/latest/userguide/access-control-block-public-access.html)
+"""
+references = [
+    "https://hackingthe.cloud/aws/exploitation/Misconfigured_Resource-Based_Policies/exploting_public_resources_attack_playbook/",
+]
+risk_score = 47
+rule_id = "59bf26c2-bcbe-11ef-a215-f661ea17fbce"
+severity = "medium"
+tags = [
+    "Domain: Cloud",
+    "Data Source: AWS",
+    "Data Source: Amazon Web Services",
+    "Data Source: Amazon S3",
+    "Use Case: Asset Visibility",
+    "Resources: Investigation Guide",
+    "Tactic: Collection",
+]
+timestamp_override = "event.ingested"
+type = "new_terms"
+
+query = '''
+event.dataset: "aws.cloudtrail"
+    and event.provider: "s3.amazonaws.com"
+    and event.action: (
+        "GetObject" or
+        "PutObject" or
+        "ListObjects" or
+        "DeleteObject" or
+        "ListBucket")
+    and event.outcome: "success"
+    and aws.cloudtrail.user_identity.type: ("AWSAccount" or "Unknown")
+    and cloud.account.id: "anonymous"
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1530"
+name = "Data from Cloud Storage"
+reference = "https://attack.mitre.org/techniques/T1530/"
+
+
+[rule.threat.tactic]
+id = "TA0009"
+name = "Collection"
+reference = "https://attack.mitre.org/tactics/TA0009/"
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1619"
+name = "Cloud Storage Object Discovery"
+reference = "https://attack.mitre.org/techniques/T1619/"
+
+
+[rule.threat.tactic]
+id = "TA0007"
+name = "Discovery"
+reference = "https://attack.mitre.org/tactics/TA0007/"
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1485"
+name = "Data Destruction"
+reference = "https://attack.mitre.org/techniques/T1485/"
+
+
+[rule.threat.tactic]
+id = "TA0040"
+name = "Impact"
+reference = "https://attack.mitre.org/tactics/TA0040/"
+
+[rule.investigation_fields]
+field_names = [
+    "@timestamp",
+    "cloud.account.id",
+    "aws.cloudtrail.user_identity.type",
+    "source.address",
+    "user_agent.original",
+    "aws.cloudtrail.resources.arn",
+    "event.action",
+    "event.outcome",
+    "cloud.region",
+    "aws.cloudtrail.request_parameters",
+]
+
+[rule.new_terms]
+field = "new_terms_fields"
+value = ["source.address"]
+[[rule.new_terms.history_window_start]]
+field = "history_window_start"
+value = "now-7d"
+
+