You're staring at a CloudTrail alert: three files deleted from your S3 bucket in 13 seconds. Your team needs to determine if this was authorized, who did it, and what else the actor accessed. But you face a choice that affects how quickly you'll contain this incident and how much of your annual security budget you'll spend getting there.
The decision isn't whether CloudTrail analysis matters. AU-2 (Audit Events) under NIST SP 800-53 Rev 5 and AC.2.007 under NIST SP 800-171 Rev 2 already require you to review audit records for unauthorized activity. The question is whether your team will build internal capability or rely on third-party tools to meet those obligations.
The Decision You Are Facing
Your organization needs a repeatable process for investigating CloudTrail events that meet your authorization boundary requirements. You can:
Path A: Build internal capability using native AWS tools and custom queries
Path B: Purchase a security information and event management (SIEM) platform with pre-built CloudTrail integrations
Path C: Engage a managed detection and response (MDR) provider
Each path carries different cost structures, staffing requirements, and response timelines. Your authorization package under the Risk Management Framework will document whichever approach you select, so this isn't a decision you can defer or revisit quarterly without triggering a significant change review.
Key Factors That Affect Your Choice
Your authorization baseline drives minimum response capabilities. If you're pursuing FedRAMP Moderate or CMMC Level 2, you must demonstrate continuous monitoring (CA-7) and incident response (IR-4) capabilities. The control implementation doesn't specify tools, but your System Security Plan must document how you'll meet the required response timeline, typically detection within hours, not days.
Staff expertise determines build feasibility. Analyzing the reconnaissance pattern requires understanding that a ListBuckets call at 14:31:22 UTC followed by targeted deletions at 14:45:12 UTC indicates scripted reconnaissance, not random access. If your team recognizes assumed role sessions, cross-account access patterns, and HTTP status codes without consulting documentation, you have the foundation for an internal build. If those terms require explanation, you're looking at months of training before you can reliably interpret CloudTrail evidence.
Event volume affects tool selection. An organization processing 50,000 CloudTrail events daily faces different economics than one handling 5 million. Native AWS CloudWatch Logs Insights handles moderate volumes without additional licensing costs. High-volume environments require either significant query optimization work or purpose-built security analytics platforms that aggregate and index events for sub-second search.
Compliance scope influences documentation burden. FedRAMP authorizations require you to document your security architecture, including how log aggregation, analysis, and alerting function within your boundary. If you build internally, you own the System Security Plan sections describing your CloudTrail analysis methodology, query library, and analyst training program. Third-party tools shift some documentation burden to the vendor's inheritance documentation, but you still document the integration points and your organization's use of the platform.
Path A: Build Internal Capability
Choose this path when:
- Your team already operates CloudWatch or AWS Security Hub for other monitoring requirements
- You have security engineers who write and maintain detection queries
- Your event volume stays below 100,000 daily events per account
- You need full control over query logic to meet specific compliance interpretations
- Your authorization boundary prohibits third-party security tool access to CloudTrail data
You'll implement CloudTrail analysis through CloudWatch Logs Insights or Athena queries against S3-stored logs. Your team writes queries that identify patterns like the cross-account access scenario: filtering for AssumedRole user identities, correlating ListBuckets reconnaissance with subsequent DeleteObject operations, and flagging session names that don't match your organization's naming conventions.
Specific implementation requirements:
Create a query library covering the event types your assessor will test during AU-2 validation. At minimum: failed authentication attempts (AC-7), privilege changes (AC-2), and data access outside normal patterns (AU-12). Document each query's detection logic in your Continuous Monitoring Plan.
Establish response runbooks that map CloudTrail event types to IR-4 incident categories. When your query detects three DeleteObject operations within 13 seconds from an assumed role, your runbook must specify who responds, what evidence gets preserved, and when you escalate to your Authorizing Official.
Train your security operations team to interpret CloudTrail's JSON structure. They need to distinguish between principalId (the unique role identifier), sessionName (the attacker-controlled label), and sourceIPAddress (origin of the API call) without referring to AWS documentation during an active incident.
This path fails when:
Your team treats CloudTrail analysis as a side project rather than a core security function. Building internal capability means ongoing maintenance as AWS introduces new services and API actions. If you can't commit staff time to updating queries quarterly and training analysts on new attack patterns, your detection capability will degrade within six months.
Path B: Purchase a SIEM Platform
Choose this path when:
- You're already consolidating logs from multiple sources (application logs, network flows, CloudTrail)
- Your compliance scope requires correlation across cloud and on-premises environments
- You need pre-built content packs for common CloudTrail attack patterns
- Your assessor expects commercial-grade security tooling as evidence of due diligence
- You have budget for both licensing and the security engineer who'll administer the platform
Modern SIEM platforms ingest CloudTrail logs, normalize the JSON structure, and provide pre-built dashboards for reconnaissance patterns, privilege escalation attempts, and data exfiltration indicators. You're purchasing detection content developed by the vendor's threat intelligence team, which typically includes the cross-account access patterns and timing analysis.
Specific implementation requirements:
Configure CloudTrail to deliver logs to an S3 bucket the SIEM can access, or stream events through CloudWatch Logs with a subscription filter. Your System Security Plan documents this data flow and the encryption protecting logs in transit (SC-8) and at rest (SC-28).
Customize the vendor's default rules to match your environment. A generic "unusual API call volume" alert generates noise in a DevOps environment with frequent deployments. You'll tune thresholds, whitelist known automation accounts, and add context like your organization's approved session naming conventions.
Train your team on the SIEM's query language and investigation workflow. Commercial platforms abstract CloudTrail's raw JSON, but your analysts still need to pivot from an alert to the underlying evidence when documenting an incident for your Authorizing Official.
This path fails when:
You treat the SIEM as a compliance checkbox rather than an operational tool. Purchasing the platform satisfies CA-7's requirement for automated monitoring, but if your team never investigates the alerts or tunes the detection rules, you're documenting a control that doesn't function as described. Assessors test operational effectiveness, not license procurement.
Path C: Engage an MDR Provider
Choose this path when:
- You lack in-house security operations staff
- Your organization needs 24/7 monitoring to meet continuous monitoring requirements
- You want a provider who'll escalate validated incidents rather than raw alerts
- Your budget supports managed services but not full-time security engineering headcount
- Your compliance framework permits third-party access to CloudTrail data under appropriate agreements
MDR providers deploy their own tooling in your environment, monitor CloudTrail events, and escalate validated incidents to your team. You're outsourcing the analysis burden while retaining incident response ownership.
Specific implementation requirements:
Establish a clear scope in your MDR contract: which AWS accounts they'll monitor, what event types trigger escalation, and how they'll deliver evidence for your incident documentation. Your System Security Plan must reference this agreement and document how the provider's monitoring satisfies your CA-7 obligations.
Grant the provider read-only access to CloudTrail logs through a dedicated IAM role. Document this external connection in your authorization boundary and ensure the provider's own security posture meets your Authorizing Official's risk tolerance. If they're FedRAMP Authorized, reference their authorization package in your inheritance documentation.
Maintain internal capability to validate the provider's findings. When they escalate an incident involving the reconnaissance and deletion pattern, your team must independently verify the CloudTrail evidence before declaring a confirmed breach. You own the accuracy of what you report to your Authorizing Official.
This path fails when:
You assume the MDR provider owns your incident response obligations. They detect and escalate; you contain, eradicate, and recover per IR-4. If your team can't interpret the CloudTrail evidence they provide or execute your incident response plan without their guidance, you haven't met the control's intent.
Summary Matrix
| Factor | Build Internal | Purchase SIEM | Engage MDR |
|---|---|---|---|
| Minimum staff expertise | Security engineer who writes queries | Security engineer who configures rules | Security coordinator who validates escalations |
| Upfront cost | Staff time only | $50K, $500K+ licensing | Contract negotiation |
| Ongoing cost | Staff time for maintenance | Annual licensing + admin time | Monthly/annual service fees |
| Response timeline | Depends on query coverage | Depends on rule tuning | Provider's SLA (typically <1 hour) |
| Control documentation burden | Full SSP sections for custom capability | Vendor inheritance + integration docs | Service agreement + validation procedures |
| Best fit for | Mature security teams, moderate volume | Multi-source log correlation needs | Limited in-house security staff |
Your choice determines whether you're explaining custom CloudTrail queries to your assessor or demonstrating how your SIEM's detection rules satisfy AU-2. Both approaches meet the requirement if you can show operational effectiveness. Neither approach works if you can't interpret the evidence when your Authorizing Official asks what happened during that 13-second window at 14:45:12 UTC.
