AWS CloudTrail is a robust service designed to strengthen oversight, adherence to regulations, and auditing within an AWS account.

With CloudTrail, you can track and log all activities and events that occur within your AWS environment. You can use CloudTrail to monitor, search, download, archive, analyze, and act on account activity throughout an AWS infrastructure.

Features of AWS CloudTrail

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CloudTrail records AWS Management Console actions, such as API calls, and other AWS service actions. This will include the activity within the last 90 days.

Configuring trails to deliver log files as well from multiple regions to a single S3 bucket is possible, which will provide a consolidated view of our account activity.  Additionally, CloudTrail supports logging across multiple AWS accounts.. This can be merged with Amazon CloudWatch Logs to allow real-time analysis of the account activity. 

There is also an option to create CloudWatch Alarms to be notified of specific API activity. These logs are saved in S3 and can be encrypted with AWS KMS keys.

Investigating Security Incidents

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CloudTrail provides detailed information that can help you understand the extent and effects of the event if a security incident occurred. We can investigate this event by adapting the below steps:

1. We can use CloudTrail Event History to identify unusual activities. We can look for unexpected API calls, changes in resource configurations, or activity from unknown IP addresses.
2. We have to analyze the event details, such as the time of the event, user identity, and source IP address. This information is important for understanding the specifics of the identified security incident.
3. Combine CloudTrail logs with other logs (e.g., VPC Flow Logs, CloudWatch Logs) to get a more detailed view of the incident. This will help you to trace the attack and to understand the extent and depth of the incident.
4. CloudTrail Insights automatically analyzes CloudTrail management events to detect unusual API activities in the AWS account. This will help to identify the incident quickly.
5. Set up CloudWatch Alarms to notify you of specific activities, such as changes to security groups, IAM roles, or unauthorized API calls. This will help you with rapid response and enable real-time monitoring.
6. Investigate the root cause by reviewing the sequence of events leading to the incident. Determine if there were any vulnerabilities or misconfigurations.

Resolving Security Incidents

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Once the investigation is completed, the next steps involve mitigation and remediation. It is very important to find appropriate resolution for any security incident occurred. This process involved the following steps:

1. Immediately isolate compromised resources to prevent further damage. This can include revoking credentials, stopping instances, or changing security group settings.
2. Apply additional security controls to prevent similar incidents in the future. This can be done by adding multi-factor authentication (MFA), using IAM roles with least privilege, and implementing network segmentation.
3. Review your AWS IAM policies, security group rules, and other configurations. Updated policies will help to close any security gaps.
4. Enable continuous monitoring and alerting for critical AWS services and resources. These services can be used for that: AWS Config, GuardDuty, and Security Hub for enhanced monitoring and threat detection.
5. After resolving the incident, conduct a thorough post-incident analysis to learn from the event. 

Best Practices for Using CloudTrail

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1. It is important to ensure that CloudTrail is enabled in all AWS regions to capture all the activity, even in regions you do not actively use.
2. Enable log file integrity validation to ensure that your CloudTrail logs are not tampered with.
3. For easier management and analysis, it is better to centralize your CloudTrail logs in a single S3 bucket.
4. Instead of depending on long-term access keys, it’s preferable to use IAM roles for accessing CloudTrail logs.
5. In order to encrypt CloudTrail logs for added security, use AWS KMS.

By using these tips and the features of AWS CloudTrail, you can easily investigate and fix security issues, keeping your AWS environment secure and compliant.